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International Journal of Molecular Sciences logoLink to International Journal of Molecular Sciences
. 2017 Dec 7;18(12):2434. doi: 10.3390/ijms18122434

Secondary Metabolic Profiles of Two Cultivars of Piper nigrum (Black Pepper) Resulting from Infection by Fusarium solani f. sp. piperis

Shirlley F M da Luz 1, Lydia F Yamaguchi 2, Massuo J Kato 2, Oriel F de Lemos 3, Luciana P Xavier 1, José Guilherme S Maia 4, Alessandra de R Ramos 5, William N Setzer 6, Joyce Kelly do R da Silva 1,*
PMCID: PMC5751101  PMID: 29215548

Abstract

Bragantina and Cingapura are the main black pepper (Piper nigrum L.) cultivars and the Pará state is the largest producer in Brazil with about 90% of national production, representing the third largest production in the world. The infection of Fusarium solani f. sp. piperis, the causal agent of Fusarium disease in black pepper, was monitored on the cultivars Bragantina (susceptible) and Cingapura (tolerant), during 45 days’ post infection (dpi). Gas Chromatography-Mass spectrometry (GC-MS) analysis of the volatile concentrates of both cultivars showed that the Bragantina responded with the production of higher contents of α-bisabolol at 21 dpi and a decrease of elemol, mostly at 30 dpi; while Cingapura displayed an decrease of δ-elemene production, except at 15 dpi. The phenolic content determined by the Folin Ciocalteu method showed an increase in the leaves of plants inoculated at 7 dpi (Bragantina) and 7–15 dpi (Cingapura); in the roots, the infection caused a phenolic content decrease in Bragantina cultivar at 45 dpi and an increase in the Cingapura cultivar at 15, 30 and 45 dpi. High Performance Liquid Chromatography-Mass spectrometry (HPLC-MS) analysis of the root extracts showed a qualitative variation of alkamides during infection. The results indicated that there is a possible relationship between secondary metabolites and tolerance against phytopathogens.

Keywords: plant-pathogen interaction, essential oils, sesquiterpenes, α-bisabolol, δ-elemene, alkamides

1. Introduction

Black pepper (Piper nigrum L.) is by far one of the most important spices and has been used for centuries in food, to treat many ailments, and as cosmeceutical products. It is cultivated in Asian countries such as Malaysia, India, Indonesia, Thailand, Vietnam, China and Sri Lanka, in addition to Madagascar and Brazil [1,2]. In Brazil, the cultivation of P. nigrum started in the 17th century and since 1933, it has been economically explored by Japanese immigrants [3]. Pará state is the largest producer in Brazil, with about 90% of national production, representing the third largest production in the world. Bragantina and Cingapura are the main black pepper cultivars in Brazil. The Bragantina cultivar is a hybrid obtained from South India, also known as “Panniyur”, exhibiting large leaves, heart-shaped and long spikes with an approximate length of 14.0 cm. The Cingapura cultivar, also known as “Kuching”, has small and narrow leaves, short spikes on average of 7.0 cm length. The average production per plant is 3 kg and 2.5 kg of black pepper for Bragantina and Cingapura cultivars, respectively. The amount of the amide piperine, the pungent principle of black pepper, is higher in the fruits of Cingapura (69.1%) cultivar than Bragantina (41.6%) cultivar [4,5].

The phytopathogens Phytophthora capsici (India) and Fusarium solani f. sp. piperis (Brazil) are the causative agents of root rot or Fusarium disease (fusariosis) to the crop of black pepper [6]. The first symptoms of fusariosis start at the roots or branches and promote falling leaves, root rot, and plant death [3]. Pathogens that cause wilting, such as Fusarium solani f. sp. piperis, have a hemibiotrophic life cycle. This and other similar pathogens initially colonize living tissue and, at a later stage, colonize dead tissue [7].

Despite the studies for controlling fusariosis in black pepper, there are no reports on secondary metabolites involved in this plant-pathogen interaction. In general, the plant presents specific defense mechanisms to respond to the attack of pathogens and herbivores, which include the mediation of terpenoid and polyphenolic compounds in the biologic systems [8]. Metabolomics provides a means for comparison between different species, and it can assist in identification of key genes and enzymes involved in various physiological processes, such as biotic and abiotic stresses [9]. The approach is emerging as a promising means of development of biomarkers for the early diagnosis of diseases, interaction mechanism, identification of resistance and susceptibility in various cultures [10]. For example, the infection of mango (Mangifera indica) by Fusarium moniliforme var. subglutinans has increased the production of phenolic compounds [11]; and the volatile compounds of wheat (Triticum aestivum) have been mentioned as the specific biomarkers involved in resistance against Fusarium graminearum infection [12]. Study of metabolomic profiles involved in host-pathogen interaction has revealed significant differences in the composition of secondary metabolites in leaves and roots, indicating that these organs employ distinct chemical defense systems [13]. The analysis by Gas Chromatography-Mass Spectrometry (GC-MS) of the volatile compounds emitted in the interaction between eucalyptus (Eucalyptus globulus) and the pathogen Teratosphaeria nubilosa identified about 40 biomarkers that can diagnose the disease before the observation of symptoms [14].

The understanding of these functions in the defense system of plants is important for obtaining resistant cultivars by biotechnological breeding [15]. Thus, the aim of this study was to identify the volatile constituents produced by the cultivars of Bragantina and Cingapura, during infection by F. solani f. sp. piperis, in order to contribute to the discovery of potentially useful secondary metabolites in the black pepper defense system.

2. Results

2.1. Evaluation of Susceptibility and Tolerance Of Black Pepper Cultivars after Inoculation with Fusarium solani f. sp. piperis

Progression of the host-pathogen interaction was monitored visually in the periods of 7, 15, 21, 30 and 45 days’ post infection (dpi) (Figure 1). For both cultivars inoculated, no visible foliar symptoms were observed at 7 and 15 dpi. The first symptoms of Fusarium disease were observed at 21 dpi, when the leaves turned yellow in the susceptible Bragantina cultivar (Figure 1, 21 dpi). Disease symptoms progressed rapidly in the Bragantina cultivar, with the partial necrosis of leaves and the total drying of branches (Figure 1, 30 and 45 dpi). On the other hand, the plants of Cingapura cultivar did not display foliar symptoms at all during the 45-day period.

Figure 1.

Figure 1

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Visual evaluation of Fusariosis symptoms in P. nigrum cultivars during inoculation by F. solani f. sp. piperis. dpi: days post infection.

2.2. Profiles of Volatile Compounds of the Black Pepper Cultivars

The P. nigrum leaves, from the Bragantina and Cingapura cultivars, after Fusarium infection, were submitted to simultaneous distillation and extraction to obtain their volatile concentrates. Individual components were identified by comparison of both mass spectrum and gas chromatography (GC) retention data with authentic compounds, previously analyzed and stored in the data system. Commercial libraries, which contained retention indices and mass spectra of volatile compounds commonly found in essential oils were used [16,17]. The GC and GC-MS analysis of the volatile concentrates from inoculated plants (IP) and control plants (CP) allowed the identification of 30 constituents in Bragantina cultivar and 54 components in Cingapura cultivar (Table 1 and Table 2). In the volatile concentrates of Bragantina cultivar, the identified constituents presented an average of 91.6%, with the predominance of oxygenated sesquiterpenes (IP, 77.4%; CP, 82.0%), where α-bisabolol (IP, 56.3%; CP, 51.5%), elemol (IP, 12.6%; CP, 24.0%) were the main compounds, followed by a smaller amount of germacrene D (IP, 3.6%; CP, 4.4%), a sesquiterpene hydrocarbon. For volatile concentrates of Cingapura cultivar the identified constituents showed a median composition of 95.2%, with prevalence of sesquiterpene hydrocarbons (IP, 76.2%; CP, 72.6%), where δ-elemene (IP, 57.1%; CP, 53.3%), α-zingiberene (IP, 4.3%; CP, 5.1%) and β-caryophyllene (IP, 4.2%; CP, 4.7%) were the major components, followed by muurola-4,10(14)-dien-1-β-ol (IP, 4.1%; CP, 4.9%), an oxygenated sesquiterpene.

Table 1.

Volatile compounds (%) of the leaves of Bragantina cultivar: inoculated plants (IP) and control plants (CP).

Bragantina Cultivar 7 dpi 15 dpi 21 dpi 30 dpi 45 dpi
Constituents RIa RIb IP CP IP CP IP CP IP CP IP CP
3E-Hexenol 845 844 0.3 0.5 0.3 + 0.1 0.6 * 0.7 * 0.2 *
Limonene 1022 1024 0.1 0.1 0.1 0.1 0.1
E-β-Ocimene 1029 1044 1.2 + 0.1 0.5 1.1 0.8 1.0 + 0.5
Linalool 1091 1095 0.5 0.7 0.3 0.4 0.4 0.3 0.3 + 0.1
Pregeijerene 1269 1285 0.3 0.5 0.2 0.2 0.3 0.2 0.2
2-Undecanone 1285 1293 0.1 * 0.1 0.1 0.1 0.1
δ-Elemene 1325 1335 0.1 0.1 0.1 0.1 0.1
α-Ylangene 1359 1373 0.1 * 0.1 0.1 0.1
β-Bourbonene 1374 1387 0.1 *
β-Elemene 1380 1389 0.2 + 0.1 0.2 0.2 0.1
β-Caryophyllene 1407 1417 1.0 + 0.4 1.1 1.2 0.7 0.9 0.9 0.9 0.6 0.8
γ-Elemene 1418 1434 0.1 0.1 0.2 + 0.1 0.1
Geranyl acetone 1440 1453 0.1 0.1 0.1 0.1 0.1 *
E-β-Farnesene 1443 1454 0.7 + 0.2 0.8 0.8 0.7 0.7 0.7 0.5
Germacrene D 1470 1484 4.2 + 1.4 5.7 5.1 3.4 4.3 2.2 − 6.9 2.5 4.1
β-Selinene 1476 1489 0.1 0.1 0.1 − 0.2 0.5 * 0.2 0.2
α-Zingiberene 1483 1493 0.2 + 0.1 0.2 − 0.4 0.2 0.9 * 0.9 0.7
α-Muurolene 1490 1500 0.1 * 0.1 0.1 0.4 * 0.2 + 0.1
β-Bisabolene 1497 1505 3.6 * 1.1 4.3 4.1 4.2 4.2 6.0 + 3.6 3.0 2.8
β-Sesquiphellandrene 1514 1521 0.1 * 0.3 0.4 0.7 * 0.6 0.5
E-γ-Bisabolene 1517 1529 0.1 0.1 2.0
Elemol 1535 1548 19.0 27.3 24.4 22.3 15.3 28.4 0.7 − 32.9 3.4 − 9.2
Germacrene B 1545 1559 0.3 * 0.3 0.1 0.1
E-Nerolidol 1551 1561 1.8 1.7 1.5 2.0 1.7 1.2 1.1 1.2 1.0
Caryophyllene oxide 1569 1582 0.2 0.2 0.1 * 0.1
Longiborneol 1591 1599 0.1 0.1 0.1 * 0.1 0.1
β-Eudesmol 1640 1649 0.3 0.3 0.3 0.3 10.7 * 14.9 12.2
α-Bisabolol 1679 1685 51.4 53.9 44.8 47.9 64.3 51.4 60.1 49.8 60.7 54.6
(2Z,6Z)-Farnesol 1696 1698 1.6 * 1.9 1.5 0.5 1.5
(2Z,6E)-Farnesol 1704 1722 3.2 * 3.5 3.2 1.1 1.5 2.5
Monoterpene hydrocarbons 1.4 0.3 0.1 0.7 1.1 0.8 1.2 0.6
Oxygenated monoterpenes 0.9 1.2 0.6 0.7 0.4 0.6 0.6 0.4
Sesquiterpene hydrocarbons 10.9 3.7 13.2 12.8 9.0 10.7 11.6 13.4 8.8 10.2
Oxygenated sesquiterpenes 77.6 83.5 76.3 77.2 81.3 82.9 71.5 85.3 80.3 81.2
Others 0.3 0.5 0.3 0.1 0.6 0.7 0.2
Total 91.1 89.2 90.5 91.5 92.4 95.0 83.8 98.7 91.1 92.4
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RIa: Retention index calculated; RIb: Retention Index of Literature [17]; * Compounds produced only by inoculated plants; + Compounds that increased more than 50% in inoculated plants, − Compounds that reduced more than 50% in inoculated plants.

Table 2.

Volatile compounds (%) of the leaves of Cingapura cultivar: inoculated plants (IP) and control plants (CP).

Cingapura Cultivar 7 dpi 15 dpi 21 dpi 30 dpi 45 dpi
Constituents RIa RIb IP CP IP CP IP CP IP CP IP CP
3E-Hexenol 846 844 1.6 * 0.3 * 0.2 + 0.1
α-Pinene 928 932 0.5 * 0.1 0.1
β-Pinene 970 974 0.1
Myrcene 983 988 0.1
δ-2-Carene 1004 1001 0.2 * 0.1 *
Limonene 1023 1024 0.2 *
E-β-Ocimene 1040 1044 0.4 * 0.2 0.2
Linalool 1092 1095 2.6 2.0 1.0 1.4 1.2 − 2.5 1.5 + 0.6 0.5 0.8
Methyl citronellate 1250 1257 0.1
Carvenone 1252 1255 0.1 * 0.1 0.1 0.1 0.1
2-Undecanone 1285 1293 0.1
δ-Elemene 1322 1335 45.9 57.6 73.6 + 48.7 48.8 55.1 61.2 57.1 55.8 48.1
δ-Cubebene 1337 0.6 *
α-Cubebene 1365 1345 0.4 − 2.1 0.4 0.3 0.6 * 0.5 0.9 0.7
α-Copaene 1365 1374 1.0 *
α-Ylangene 1367 1373 1.2 * 0.8 + 0.4 1.1 * 1.2 1.2 1.4
β-Elemene 1379 1389 2.7 4.1 2.1 1.6 3.1 3.0 3.2 2.7 3.1 3.9
α-Gurjunene 1396 1409 0.7 * 0.4 0.3 0.8 0.8 0.7 1.0 1.1
β-Caryophyllene 1407 1417 3.0 3.8 2.3 3.1 6.6 8.7 3.9 3.0 5.4 4.9
6,9-Guaiadiene 1430 1442 0.1 * 0.5 * 0.7 1.2
α-Humulene 1443 1452 0.6 − 2.2 0.2 0.5 0.9 * 0.8 * 0.9 *
Thujopsadiene 1449 1465 0.3 * 0.3 0.5 * 0.4 * 0.3 *
allo-Aromadendrene 1461 1458 0.1 * 0.1 * 0.5 * 0.4 *
9-epi-E-Caryophyllene 1465 1464 0.2 * 0.1 − 0.2 0.4
Germacrene D 1470 1484 0.6 * 0.6 + 0.3 0.6 * 0.7 0.6 0.5 0.7
β-Selinene 1476 1489 2.2 4.2 1.8 1.3 3.3 2.9 3.4 2.5 3.6 4.4
α-Zingiberene 1482 1493 2.9 − 6.9 2.9 2.3 5.2 5.0 5.2 4.3 5.5 6.8
Z- Cycloisolongifol-5-ol 1498 1513 0.3
epi-Cubebol 1503 1493 0.3 * 0.1 − 0.7 0.9 * 0.3 0.4
δ-Amorphene 1507 1511 0.7 * 0.3 * 0.7 + 0.4 0.4 0.7
γ-Vetivenene 1534 1531 0.2 * 0.1 0.1
γ-Elemene 1545 1465 0.1
E-Nerolidol 1549 1561 2.9 4.8 1.3 − 8.5 3.9 6.6 2.7 4.0 2.7 2.2
Germacrene D-4-ol 1564 1574 0.2 * 0.1 0.3 0.3 * 0.4 0.4 0.5
Caryophyllene oxide 1576 1582 6.6 6.3 2.9 3.9 3.6 3.8
Spathulenol 1577 1577 3.1 6.1 6.1 3.6 4.5 0.3 *
β-Copaen-4α-ol 1595 1590 2.8 1.5 * 1.7 1.7 0.8 *
β-Atlantol 1596 1608 3.4 *
Ledol 1602 1602 0.4
1,10-di-epi-Cubenol 1607 1618 1.6 * 0.6 − 1.2 1.0 * 0.8 1.3 1.1
Muurola-4,10(14)-dien-1β-ol 1613 1630 4.3 5.9 2.0 − 4.9 6.5 5.5 3.7 5.3 3.8 3.1
α-Cadinol 1642 1652 0.5 * 0.2 − 0.4 0.4 * 0.3 * 0.3 0.5
allo-Aromadendrene epoxide 1649 1639 0.2 * 0.1 * 0.1 − 0.2
Khusinol 1659 1679 0.7 * 0.3 − 0.6 0.4 * 0.2 0.5
Germacra-4(15),5,10(14)-trien-1α-ol 1672 1685 0.2 * 0.1 0.3 0.1
2Z,6Z-Farnesal 1684 1684 0.2 * 0.1 * 0.1 0.1
2E, 6Z-Farnesal 1696 1713 0.6
2Z,6E-Farnesal 1696 1715 0.2 * 0.4 *
Eudesma-4,11-dien-2α-ol 1704 1704 1.2 * 0.6 − 1.9 0.9 * 1.2 1.1 1.1 − 2.2
E, E, E-7,10,13-Hexadecatrienal 1718 1824 0.8 *
iso-Aromadendrene epoxide 1719 1719 0.6 * 0.3 − 0.6 0.4 0.5 0.3 0.5
2E,6E-Farnesal 1725 1725 0.4 * 1.0 0.8 0.7 0.8 − 2.0
Monoterpene hydrocarbons 1.3 0.4 0.7
Oxygenated monoterpenes 2.7 2.0 1.1 1.7 1.2 2.5 1.5 0.6 0.6 0.9
Sesquiterpene hydrocarbons 61.8 80.6 86.1 59.5 72.0 74.7 81.9 73.7 79.0 74.4
Oxygenated sesquiterpenes 23.3 17.0 8.7 31.7 22.8 18.2 16.0 19.5 14.8 17.2
Others 1.6 0.3 0.2 0.1
Total 90.7 99.9 96.4 93.4 95.7 95.4 99.4 93.8 94.6 92.6
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RIa: Retention index calculated; RIb: Retention Index of Literature [17]. * Compounds produced only by inoculated plants, + Compounds that increased more than 50% in inoculated plants, − Compounds that reduced more than 50% in inoculated plants.

2.3. Variation in Volatile Profiles of the Black Pepper Cultivars, after Fusarium Infection

The volatile concentrates of Bragantina cultivar were dominated by oxygenated sesquiterpenes (IP, 77.4%; CP, 82.0%), followed by sesquiterpene hydrocarbons (IP, 10.7%; CP, 10.2%) as minor compounds (Table 1, Figure 2). The production of oxygenated sesquiterpenes showed a drastic decrease (71.5%) in the inoculated plants, just at 30 dpi, which can be associated with the progress of Fusarium disease during that cultivation period. In contrast, the Cingapura cultivar showed sesquiterpene hydrocarbons as main compounds (IP, 76.2%; CP, 72.6%) and oxygenated sesquiterpenes in minor proportions (IP, 17.1%; CP, 20.7%) (Table 2, Figure 2). An increase in production of sesquiterpene hydrocarbons was observed with inoculated plants at 15, 30 and 45 dpi. On the other hand, the sesquiterpene hydrocarbons content was equivalent in the inoculated and control plants at the 21 dpi.

Figure 2.

Figure 2

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Sesquiterpenes variation in P. nigrum cultivars during infection by F. solani f. sp. piperis. (A) Sesquiterpenes hydrocarbons; (B) Oxygenated sesquiterpenes; (a) inoculated plants; (b) control plants. dpi: days post infection.

Inoculated plants (IP) of Bragantina cultivar showed quantitative difference for α-bisabolol and elemol, the main compounds, when compared to control plants (CP) for the duration of cultivation (from 7 to 45 dpi). The production of α-bisabolol was higher at 21 dpi (IP, 64.3%; CP, 51.4%), 30 dpi (IP, 60.1%; CP, 49.8%), and 45 dpi (IP, 60.7%; CP, 54.6%). With respect to elemol, however, the situation was reversed. There was a reduction of its percentage in the inoculated plants when compared to the control plants: 7 dpi (IP, 19.0%; CP, 27.3%), 21 dpi (IP, 15.3%; CP, 28.4%), 30 dpi (IP, 0.7%; CP, 32.9%), and 45 dpi (IP, 3.4%; CP, 9.2%). The content of minor compounds also showed significant variation in the inoculated plant, after the appearance of the symptoms of Fusarium disease. For example, increase of β-bisabolene and E-β-ocimene, decrease of germacrene D, variation for the isomers β-, γ- and δ-elemene, as well as a large production of β-eudesmol at 30 dpi (10.7%) and 45 dpi (14.9%). Other alterations can also be observed in the inoculated Bragantina cultivar. E-β-Ocimene (1.2%), β-caryophyllene (1.0%), germacrene D (4.2%), β-bisabolene (3.6%) (2Z,6Z)-farnesol (1.6%) and (2Z,6E)-farnesol (3.2%) increased above 1%.

Similarly, inoculated plants of Cingapura cultivar also showed qualitative and quantitative differences in the volatile profiles, due to infection, particularly for δ-elemene and β-caryophyllene, their main compounds. δ-Elemene production was higher at 15 dpi (IP, 73.6%; CP, 48.7%), 30 dpi (IP, 61.2%; CP, 57.1%) and 45 dpi (IP, 55.8%; CP, 48.1%). However, β-caryophyllene increased only at 30 dpi (IP, 3.9%; CP, 3.0%) and 45 dpi (IP, 5.4%; CP, 4.9%). The sesquiterpenes α-ylangene (1.2%), β-atlantol (3.4%), 1,10-di-epi-cubebol (1.6%) and eudesma-4,11-dien-2α-ol (1.2%) were produced above 1%, only by inoculated plants at 7 dpi. E-Nerolidol showed a decrease at 7 dpi (IP, 2.7%; CP, 4.8%), 15 dpi (IP, 1.3%; CP, 8.5%), 21 dpi (IP, 3.9%; CP, 6.6%) and 30 dpi (IP, 2.7%; CP, 4.0%).

2.4. Variation in Total Phenolic Compounds Profiles of the Black Pepper Cultivars, after the Fusarium Infection

The variation in total phenolic compounds was evaluated in the leaves and roots of Bragantina and Cingapura cultivars, after the inoculation, and expressed as milligrams of gallic acid equivalents per gram of gram of extract (mg GAE·g−1). Bragantina cultivar leaves showed no significant variation, while the Cingapura cultivar presented a significant production in the inoculated plants at 7 dpi, which can be associated as a response to the infection in the early days (Figure 3). Root total phenolic compounds showed different behavior in comparison to the leaves. In the Bragantina cultivar (susceptible to Fusarium) the only significant change was observed at 45 dpi, which was a dramatic decrease of total phenolic compounds in the inoculated plants, when compared to control plants (IP, 39.9 mg GAE·g−1; CP, 91.9 mg GAE·g−1). On the other hand, Cingapura cultivar (tolerant to Fusarium) produced higher content of total phenolic compounds in the inoculated plants at 15 (IP, 91.9 mg GAE·g−1; CP, 39.9 mg GAE·g−1), 30 (IP, 75.8 mg GAE·g−1; CP, 42.6 mg GAE·g−1) and 45 dpi (IP, 173.8 mg GAE·g−1; CP, 108.7 mg GAE·g−1) (Figure 3). Conversely, there was a decrease in total phenolic compounds at 21 dpi (IP, 23.2 mg GAE·g−1; CP, 74.1 mg GAE·g−1), which could be related to the beginning of the development cycle of Fusarium, since the first symptoms for the susceptible cultivars were observed in this period.

Figure 3.

Figure 3

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Phenolic content variation in P. nigrum cultivars during infection by F. solani f. sp. piperis. (A) Leaves (B) Roots, mg GAE·g−1 (mg of gallic acid equivalent per gram of extract). A, a, B, b, C, c, D, d, E, e, F, f, G, g, H, h, I, i, J, j values with different letters are statistically different at the p < 0.05 level (Tukey’s test).

2.5. Chemical Profiles of the Root Extracts from Bragantina and Cingapura Cultivars, after Fusarium Infection

High Performance Liquid Chromatography-Mass Spectrometry (HPLC-MS) analysis of root extracts of Bragantina and Cingapura cultivars resulted in the identification of various alkamide compounds (Figure 4). Individual compounds were identified by comparison of their mass spectra with those previously reported for P. nigrum samples. The alkamides compounds, such as piperine (6R), pellitorine/(2E,4E)-N-isobutyldecadienamide (8aR/8bR), pipyaqubine/(2E,4E,13Z)-N-isobutyl-octadeca-2,4,13-trienamide (9aR/9bR), (2E,4E)-N-isobutyloctadeca-2,4-dienamide/pipericine (14aR/14bR), (2E,4E,10E)-N-11-(3,4,methylenedyoxiphenyl)undecatrienoylpiperidine 12R and guineesine (13R) were identified in both cultivars. The inoculated Bragantina cultivar produced the compounds piperamine/piperlonguminine (2aR/2bR) and pipyaqubine/(2E,4E,13Z) -N-isobutyl-octadeca-2,4,13-trienamide (9aR/9bR) only at 7 dpi and 15 dpi. Higher qualitative difference can be observed at 21 dpi, in which the alkamides (2E,8E)-N-9-(3,4-methylenedioxyphenyl)nonadienoylpiperidine (1R), 1-(eicosa-2E,4E,15Z-trienyl)pyrrolidine/1-(eicosa-2E,4E,14Z-trienyl)pyrrolidine (3aR/3bR), pipyaqubine/(2E,4E,13Z)-N-isobutyl-octadeca-2,4,13-trienamide (9aR/9bR) and (2E,4E)-N-isobutyl-eicosadienamide (15R) were absent. The chemical profile of Cingapura cultivar did not display qualitative changes, which could be attributed to its tolerance. The compounds (2E,4E)-N-isobutyloctadeca-2,4-dienamide/pipericine (14aR/14bR) and pipyaqubine/(2E,4E,13Z) -N-isobutyl-octadeca-2,4,13-trienamide (9aR/9bR) were identified in the inoculated plants at 7 dpi and 15 dpi, respectively (Table 3 and Table 4).

Figure 4.

Figure 4

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Identified compounds in roots of P. nigrum cv. Bragantina and Cingapura during infection with F. solani f. sp. piperis.

Table 3.

Chemical profile of root extracts of Bragantina cultivar: inoculated plants (IP) and control plants (CP).

RT Alkamides MW MF 7 dpi 15 dpi 21 dpi 30 dpi 45 dpi
(min) (Figure 4) (Molecular Weight) (Molecular Formula) IP CP IP CP IP CP IP CP IP CP
17.1 1R 341.4402 C21H27NO3 X X − X − X
18.8 2aR/2bR 273.3243 C16H19NO3 + X X
19.7 3aR/3bR 359.5827 C24H41NO − X X X
20.5 4R 339.4246 C21H25NO3 X X − X
21.4 5R 381.5035 C24H31NO3 X X X X X X X X
22.2 6R 285.3350 C17H19NO3 X X X X X X X X X X
22.6 7R 452.8303 C31H64O +
23.3 8aR/8bR 224.3587 C14H26NO X X X X X X X X X X
23.8 9aR/9bR 333.5457 C22H39NO X X +
24.1 10R 311.3720 C19H21NO3 +
25.6 11aR/11bR 361.5983 C24H43NO − X X X
26.2 12R 367.4772 C23H29NO3 X X X X X X X X X X
26.8 13R 383.5191 C24H33NO3 X X X X X X X X X X
27.7 9aR/9bR 333.5457 C22H39NO X X + − X X X X X
28.3 14aR/14bR 335.5613 C22H41NO X X X X X X X X X X
29.0 15aR/15bR 363.6139 C24H45NO − X
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RT: Retention time; MW: Molecular weight; MF: Molecular formula; Alkamides in bold represent changes in function of the infection; (X): Alkamides present in inoculated and control plants; (−) Alkamides present in control plants and absent in inoculated plants; (+) Alkamide present only in inoculated plants.

Table 4.

Chemical profile of root extracts of Cingapura cultivar: inoculated plants (IP) and control plants (CP).

RT Alkamides MW MF 7 dpi 15 dpi 21 dpi 30 dpi 45 dpi
(min) (Figure 4) (Molecular Weight) (Molecular Formula) IP CP IP CP IP CP IP CP IP CP
22.2 6R 285.3350 C17H19NO3 X X X X X X − X X X
23.3 8aR/8bR 224.3587 C14H26NO X X X X X X X X X X
23.9 9aR/9bR 333.5457 C22H39NO X X − X X X − X X X
24.0 14aR/14bR 335.5613 C22H41NO + X X − X X X
24.1 9aR/9bR 333.5457 C22H39NO X X X X − X X X
24.4 16R 235.3616 C15H25NO X X X X X X − X X X
26.3 12R 367.4772 C23H29NO3 X X X X X X X X X X
26.8 13R 383.5191 C24H33NO3 X X X X X X X X X X
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RT: Retention time; MW: Molecular weight; MF: Molecular formula; Alkamides in bold represent changes in function of the infection; (X): Alkamides present in inoculated plants and control plants; (−): Alkamides present in control plants and absent in inoculated plants; (+): Alkamides present only in inoculated plants.

2.6. PCA Analysis

The components first Principal Component (PC1) and second Principal Component (PC2). displayed the patterns found in Bragantina (58.8%) and Cingapura (45.2%) cultivars. Bragantina cultivar showed a positive PC1 correlation mostly with elemol (0.351), E-nerolidol (0.307), germacrene D (0.243) and (2Z,6E)-farnesol (0.232) and its including the samples C7, C15, C21, C30, I7 and I15. Compounds with more positive loadings in PC2 were β-caryophyllene (0.417), germacrene D (0.347), β-bisabolene (0.313), (2Z,6E)-farnesol (0.307) and grouped the samples C15, C30, C45, I15 and I30 (Figure 5A). PC1 of the Cingapura cultivar showed higher positive correlation with isoaromadendrene epoxide (0.293), eudesma-4,11-dien-2α-ol (0.279), 1,10-di-epi-cubenol (0.256), germacrene D (0.255) and α-gurjunene (0.229). PC2 of the Cingapura cultivar presented a positive correlation with (2E,6Z)-farnesal (0.269), E-nerolidol (0.266), linalool (0.246), α-gurjunene (0.229) and α-pinene (0.228) (Figure 5B).

Figure 5.

Figure 5

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Bidimensional plot of first two components obtained by Principal Component Analysis (PCA) of P. nigrum cultivars based on Essential Oil (EO) chemical composition. Inoculated plants: I7, I15, I21, I30 and I45. Control Plants: C7, C15, C21, C30 and C45. (A) Bragantina; and (B) Cingapura. PC1: First Principal Component, PC2: Second Principal Component.

3. Discussion

For populations of an agricultural crop and its associated pathogens, it has been seen that the genetic variation for its resistance and infectivity is ubiquitous [18,19]. Although P. nigrum cultivars are not completely resistant to Fusarium, no contrary evidence has been reported yet [20].

The main compounds that make up the aroma of black pepper are α- and β-pinene, sabinene, myrcene, limonene and β-caryophyllene, followed by minute quantities of δ-3-carene, β-phellandrene, elemicin, muurolol, cubenol and bulsenol [21,22]. Normally, the P. nigrum cultivars have shown variability in their essential oil constituents [23,24,25]. For example, the main compounds identified in 26 different cultivars of black pepper collected in Kerala, India, were elemol and germacrene D in the leaf oils, and β-caryophyllene in the fruit oils [26].

The essential oil of Cingapura cultivar was dominated by sesquiterpenes hydrocarbons (61.8–86.1%), as δ-elemene (57.6%) and β-caryophyllene (3.8%), in contrast with a specimen collected in Cameroon that showed low concentrations of δ-elemene (2.6%) and β-caryophyllene (7.3%) [21]. Inoculated plants activate signal transduction cascades mediated by plant hormones, which lead to higher expression of genes related to defense [27]. There are many chemical compounds involved in interactions between plants, which include aldehydes and low molecular weight alcohols such as 2E-hexenal, 3Z-hexen-1-ol, terpenes such as myrcene and ocimene consisting of mixtures of E-β-ocimene, Z-β-ocimene and allo-ocimene [28].

The response to infections of Bragantina and Cingapura cultivars promotes the production of 3E-hexenol, which is considered a volatile produced in green leaves in response to a herbivore attack. Cingapura inoculated plants produced 3E-hexenol (7 and 15 dpi), which indicated a response in the early stages. However, in the Bragantina cultivar their emission was observed from 21 to 45 dpi only in inoculated plants. In the oxidation of linoleic acid by the action of lipoxygenase enzymes (LOX), 3Z-hexenal is the first product formed followed by isomerization reactions resulting in the formation of 2E-hexenal. These aldehydes are converted to the corresponding alcohols by dehydrogenase action [29,30].

Among minor compounds, E-β-ocimene was frequent and showed higher concentrations in Bragantina inoculated plants at 7, 21 and 45 dpi and Cingapura plants at 7 dpi. This compound has been reported as important in plant-plant interactions, pathogen resistance and changes in abiotic factors [31]. The emission of E-β-ocimene was correlated during interaction of tobacco and peanut, inoculated by Pseudomonas syringae and Sclerotium rolfsii, respectively [32,33,34,35,36]. The plants present direct and indirect defense responses when they are damaged by herbivores, or inoculated by fungal and bacterial pathogens [37]. The significant change in the production of α-bisabolol observed in Bragantina cultivar at 21 dpi and δ-elemene in Cingapura cultivar at 15 dpi may be a response to plant-pathogen interactions, since some sesquiterpenes are produced as phytoalexins by the plant defense system [38]. PCA analysis of volatile compounds from each cultivar discriminated the cultivar in two groups. For the Bragantina cultivar, the negative loadings in the PC1 were observed for inoculated samples after 21 dpi, such as I30, I45 and I21, except the sample C45. However, the groups in the Cingapura samples did not display a response after the infection.

Oxygenated sesquiterpenes were dominant in Bragantina cultivar (susceptible) and sesquiterpene hydrocarbons in Cingapura cultivar (tolerant). The Bragantina cultivar did not display a direct influence of oxygenated sesquiterpenes during the infection. In general, sesquiterpene levels decreased in inoculated plants as initial response to disease (from 7 to 15 dpi) and after 21 dpi, this difference decreased. However, in Cingapura cultivar, sesquiterpene hydrocarbons showed a direct correlation with infection, because a high production was observed in early stages (7–15 dpi). From 30 dpi, a decrease in this difference was observed (Figure 2). The increase in concentration of sesquiterpene hydrocarbons was determined as a marker of infection by Fusarium in corn. Inoculated grains produced high concentrations of β-selinene, α-selinene, β-macrocarpene, β-bisabolene and trichodiene during infection by F. graminearum, F. verticillioides and F. subglutinans, while uninoculated grains showed high levels of cycloisosativene and α-ylangene [39]. These results are in contrast with the earlier reports on response of two lines of P. nigrum inoculated by Phytophthora capsici, which the higher accumulation of phenolic compounds in the roots and stem were observed with the susceptible line (Sreekara) in comparison to the tolerant (04-P24) [40].

Plants submitted to stress displace the energy invested in primary metabolism to secondary metabolism and consequently there is a reduction in the growth rate, reproduction and competitiveness. Subsequently, the synthesis of secondary metabolites is altered for survival and results in regulation of carbon flow between primary and secondary metabolism [41]. Resistance to pests or diseases of certain species could be related to the concentration and variety of phenolic compounds [42,43]. Due to their toxicity, compounds such as flavonoids and hydroxycinnamic acids act as passive or inducing barriers against microorganisms and herbivores [43,44].

The infection by Colletotrichum nymphaea, the causal agent of anthracnose in strawberry (Fragaria sp.) in two cultivars “Elsanta” (susceptible) and “Honeoye” (resistant) promoted an increase in the concentration of phenolic compounds as ellagic acid derivatives, flavonols and flavan-3-ols [45]. Furthermore, multivariate analysis of phenolic compounds of different mango cultivars (Mangifera indica) allowed the identification of biomarkers in the groups as tolerant and susceptible to Fusarium infection. The larger levels of phenolic compounds were observed in more tolerant cultivars “Nam docmai” and “Roza” [46]. These results support the hypothesis that phenolic compounds play an important role in host-resistance in inoculated tissue during plant-pathogen interactions [43]. Despite the variation in total phenolic contents in the roots of both cultivars, they displayed a wide diversity of alkamides. Additionally, it is important to emphasize the low selectivity of phenolic compounds by Folin-Ciocalteu assay, once it displays false positives to non-phenolic reducing species, such as vitamin C, amino acids and thiol derivatives [47]. In addition, many nitrogen-containing compounds showed considerable reactivity toward Folin Ciocalteu reagent. These classes include hydrazines, hydroxylamines, guanidines, tertiary amines, aromatic amines, pyrroles, and indoles [48].

The plant with the highest levels of alkamides (AAs) are found in species of Asteraceae, Piperaceae, Solanaceae and Rutaceae [49,50]. There are numerous studies on antimicrobial activities of AAs by different test methods in vitro, e.g., dilution, disk diffusion and bioautography [51]. The antifungal activity of AAs has been reported against Cladosporium species [52,53]. In plants, these compounds play an important role in seedling development in physiological or stress conditions [54]. Kim et al. (2010) [55] described two hypothetical mechanisms of their protective role: (1) NAE (N-acylethanolamines) accumulation could modulate the level of other lipids (e.g., phosphatidic acid) in response to pathogens or (2) NAEs might interfere with the quorum sensing mechanism (i.e., inter-pathogen communication to coordinate their activities) of pathogen.

4. Materials and Methods

4.1. Plant Materials

Plants of black pepper, from Brangatina and Cingapura cultivars, were provided by Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA)—Campus of Amazônia Oriental, city of Belém, state of Pará, Brazil. Twenty-eight plants of each cultivar, with three months of cultivation, were used in the experiment.

4.2. Fungal Strains and Inoculation on Black Pepper

The origin and identification of the fungal strain used to infection experiments are not clear, nor reference given. F. solani f. sp. piperis isolates were collected from black pepper commercial planting located in Baião city (Pará State). The fungal identification was performed based on morphological characteristics of conidia and microconidia in the inoculant solution, which present an oval shape and fusoid, respectively. The fungus was cultured on potato dextrose agar (PDA) for 10 days, at 27 °C, before starting the experiment. The fungi spores were collected in sterile water and the inoculant solution was prepared at a concentration of 1.43 × 105 spores/mL. The roots of each plant of black pepper cultivars were inoculated with the spore solution according to Meireles, while plants of the control group were treated with distilled water [7]. The Fusarium infection and the production of secondary metabolites were monitored at 7, 15, 21, 30 and 45 days post inoculation (dpi). After the experimentation, the phytopathogen was again isolated from the roots of the black pepper cultivars under study.

4.3. Volatile Concentrates and Extracts Preparation

The volatile constituents were extracted from the leaves (3 g) of each treated plant by simultaneous distillation-extraction process using a Likens-Nickerson apparatus and n-pentane (3 mL) as solvent. After 2 h, the organic fraction was removed. Fresh leaves and roots (5 g) were extracted by percolation (96 h) with ethyl acetate and the residual solvent was removed under reduced pressure.

4.4. Volatile Concentrates Analysis

An aliquot (1 μL) of organic fraction of each sample was submitted for analysis of volatile components. The analyses were carried on a GC-MS Thermo Focus DSQ II (Thermo Fisher Scientific, Austin, TX, USA), under the following conditions: DB-5ms (30 m × 0.25 mm; 0.25 mm film thickness, Agilent, Santa Clara, CA, USA) fused-silica capillary column; programmed temperature, 60–240 °C (3 °C/min); injector temperature, 250 °C; carrier gas, helium, adjusted to a linear velocity of 32 cm/s (measured at 100 °C); injection type, split (2 μL); split flow was adjusted to yield 20:1 ratio; septum sweep was a constant 10 mL/min; EIMS (Electron Impact Mass Spectrometry), electron energy, 70 eV; temperature of the ion source and connection parts, 200 °C. The quantitative data regarding the volatile constituents were obtained by peak area normalization using a FOCUS GC/FID (Thermo Fisher Scientific, Austin, TX, USA) operated under similar conditions of the GC-MS, except the carrier gas, which was nitrogen. The retention indices were calculated for all the volatile constituents using a homologous series of n-alkanes (C8–C32, Sigma-Aldrich, St. Louis, MO, USA) [56].

4.5. Total Phenolics Content

The amount of total phenolics (TP) of the ethyl acetate extract was determined using the Folin-Ciocalteu procedure (Sigma-Aldrich, St. Louis, MO, USA) [57,58]. The extracts were dissolved in methanol at an initial concentration of 40 mg/mL and then diluted in water. Aliquots (500 μL) of the aqueous solution was mixed with 250 μL of Folin-Ciocalteu reagent (1.0 N, Sigma-Aldrich, St. Louis, MO, USA) and 1250 μL of sodium carbonate (75 g/L). The absorbance was measured after 30 min, at 760 nm and 25 °C (UV-Vis spectrophotometer, Biosystems RA2708, Costa Brava, Barcelona, Spain). The experimental calibration curve was prepared using gallic acid at concentrations of zero, 0.5, 1, 2, 4, 6, 8 and 10 mg/L, which were submitted to the same procedure. The total phenolics content are expressed as gallic acid equivalents (GAE) in milligrams per gram of extract (mg GAE/g).

4.6. Extract Samples Analysis

The ethyl acetate extracts were analyzed by a liquid phase chromatograph coupled to a mass spectrometer detector (HPLC-ESI-MS, Shimadzu, Tokyo, Japan), which was equipped with analytical pumps LC-20AD (HPLC-ESI-MS, Shimadzu, Tokyo, Japan) (2), sampler SIL-20AHT (HPLC-ESI-MS, Shimadzu, Tokyo, Japan), detector UV/Vis SPD-20A (HPLC-ESI-MS, Shimadzu, Tokyo, Japan), oven CTO-20A (HPLC-ESI-MS, Shimadzu, Tokyo, Japan) and controller CBM-20A (HPLC-ESI-MS, Shimadzu, Tokyo, Japan). The mobile phase was methanol (MeOH) (plus 0.1% formic acid) and water (plus 0.1% formic acid) in a gradient of 30% of MeOH maintained for 2 min, then, from 2 to 25 min the percentage of MeOH was increased to 100% and kept for an additional 5 min. The column was Luna 5u PFP (2) 100 A, 150 × 2 mm (Phenomenex, Torrance, CA, USA). The UV detector was kept in 254 and 280 nm, the oven was in 40 °C. The flow was 200 µL/min and it was injected directly in the mass spectrometer. The mass spectrometer Bruker micrOTOF-QII (Bruker, Billerica, MA, USA) was in positive mode with nebulizer and dry gas (N2, 8 L/min), at 4 Bar. The dry temperature was 200 °C and the collision and quadrupole energy was 12 and 6 eV, respectively. The funnels RF1 and RF2 were set to 200 Vpp.

4.7. Statistical Analysis

Samples were assayed in triplicate in phenolic content assay and the results are shown as means ± standard deviation. Analysis of variance was conducted and the differences between variables were tested for significance by one-way analysis of variance (ANOVA) with Tukey’s post hoc test using GraphPad version 5.0. (GraphPad Software, San Diego, CA, USA) Differences at p < 0.05 were considered statistically significant. The identified volatile compounds were used as variables in the Principal Component Analysis (PCA) to classify and group the inoculated and control plants. These data were analyzed using the Minitab software (free version, Minitab software, State College, PA, USA).

Acknowledgments

The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (Brazilian Council for Scientific and Technological Development) for financial support.

Author Contributions

Joyce Kelly do R. da Silva and Alessandra de R. Ramos conceived and designed the experiments; Shirlley F. M. da Luz, Luciana P. Xavier and Lydia F. Yamaguchi performed the experiments; Shirlley F. M. da Luz., J.K.d.R.d.S., Joyce Kelly do R. da Silva, José Guilherme S. Maia, Massuo J. Kato and Oriel F. de Lemos analyzed the data; Joyce Kelly do R. da Silva, José Guilherme S. Maia, Massuo J. Kato, Oriel F. de Lemos and William N. Setzer contributed reagents/materials/analysis tools; Shirlley F. M. da Luz, Joyce Kelly do R. da Silva, Alessandra de R. Ramos, William N. Setzer and José Guilherme S. Maia wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data and in the decision to publish the results.

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