Plant peptides in plant defense responses

ABSTRACT

Plant peptides secreted as signal molecular to trigger cell-to-cell signaling are indispensable for plant growth and defense processes. Preciously, it is regraded some plant peptides function in plant growth and development, whereas others regulate defense response in plant-microbe interactions. However, this prejudice is got rid due to more and more evidence showed growth-related plant peptides also exhibit bifunctional roles in plant defense response against different microbial pathogens. Here we provide a mini-review of reported types of plant peptides, including their basic information, reported receptor ligands, and especially direct or indirect roles in plant immune responses.

Introduction

As important growth niche and nutrition supplier for diverse microbial pathogens, plants have to evolve sophisticated strategies to protect themselves against biotic stresses. Typically, plants sense pathogen signal by recognizing microbe-associated molecules and launch diverse immune responses. These microbe-associated molecules include pathogen-associated molecular patterns sensed by pattern recognition receptors initiating pattern-triggered immunity (PTI) and effectors recognized by resistance (R) proteins activating effector-triggered immunity. On the other hand, plants biosynthesize specific endogenous factors as damage-associated molecular patterns (DAMPs) and transport into apoplast also sensed by PRRs to induce PTI. In contrast, pathogens sometimes take advantage of plant endogenous factors to promote disease development.¹ In recent years, several studies revealed the bifacial roles of plant peptides as these types of endogenous factors in plant immunity to influence plant-microbe interaction.

Generally, plant peptides are arbitrarily referred to proteins at length of small than 100 amino acids.² Majority of plant peptides are derived from a longer precursor with unknown biological function. In Arabidopsis, more than 1000 genes encoding potent plant peptides.³ According to the biosynthesis and structure properties, plant peptides can be roughly divided into three groups: (1) small posttranslationally modified peptides, (2) cysteine-rich peptides, and (3) others. Up to now, the majority researches of plant peptides are focus on their roles as new type of phytohormone in steering growth and development, including cell proliferation, pollen tube growth, organ senescence, etc.⁴ However, in recent years, multiple functions of plant peptides were revealed in manipulating plant immune response, although our understanding of the immune roles of plant peptide is still limited at present. Here, we briefly summarize some typical plant peptides and their responses in plant immunity (Table 1), aiming at boosting the knowledge on the immune function and signaling of plant peptides in future.

Table 1.

Plant peptides in plant defense responses.

Peptides with modifications	Receptor(s)	Length	Modification	Susceptibility	Resistance	Ref.
CLV3/CLEs	CLV1/2, BAM1/2/3, etc	12–13 aa	glycosylation, hydroxylation	R. solanacearum,	B.cinerea,	9, 10
				H. arabidopsidis,	P. cucumerina
				H. schachtii
HYPSYS	n.d.	15–20 aa	hydroxylation	n.f.	H. armigera,	14, 15
					S. litura
IDA/IDLs	HAE, HSL2	n.d	hydroxylation	P. syringae	n.f.	18
PSK	PSKR1/2	5 aa	sulfation	P. syringae,	B. cinerea, A. brassicicola	6, 22–25
				F. oxysporum
PSY1	PSY1R	18 aa	hydroxylation, sulfation,	P. syringae,	A. brassicicola	22, 23, 25
			glycosylation	F. oxysporum
Cysteine-rich peptides	Receptor(s)	Size	Cys Residues (No.)	Susceptibility	Resistance	Ref.
PDFs	n.d.	~ 5 kDa	8 (4–6)	n.f.	Fusarium spp.,	28–30
					B.cinerea,
					V. dahlia,
					P. carotovorum
RALFs	FER, ANX1/2	~ 5 kDa	4	P. syringae (ANX1),	P. syringae (FER)	35–37
				F. oxysporum
EPF/EPFLs	ERECTA, TMM	~ 5 kDa	6–8	n.f.	P. cucumerina,	40–45
					V. longisporum,
					M. oryzae,
					R. solanacearum, P. irregular
Others	Receptor(s)	Length	Specialty	Susceptibility	Resistance	Ref.
SYSTEMIN	SYR1	18 aa	Non Cys-rich and	n.f.	B. cinerea,	1, 47
			modifications		G. clarum
PEPs	PEPR1/2	23–36 aa	Non Cys-rich and	n.f.	P. syringae,	51, 52
			modifications		P. irregular,
					C. heterostrophus,
					C. graminicola

n.d., not detected; n.f., not found yet.

Small posttranslationally modified peptides

In this group, plant small posttranslationally modified (PTM) peptides are constitutive of a maximum of 20 amino acids with no or few Cys residue. Typically, the Pro residues in small PTM peptides are modified through hydroxylation, glycosylation, and Tyr and Cys sulfation also is consist in this type peptides.⁵ These posttranslationally modifications likely control biological activity, proteolytic processing of small PTM peptides, and binding affinity to their receptors.^5,6 Approximate 7 peptide families of small PTM peptides have been characterized, while only several reported have direct or indirect evidence functions in plant immune regulation. In this section, 5 canonical small PTM peptides will be briefly introduced with their roles in plant immune responses.

CLE (CLAVATA3/EMBRYO SURROUNDING REGION) peptide family is a well-study example in plant small PTM peptides. CLE genes are conserved in land plant species including total 32 orthologs in Arabidopsis and 15 orthologs in tomato.^7,8 The mature CLE peptides consist of 12–13 amino acid with hydroxylation at two Pro residues, and meanwhile the second Pro^Hyp is also glycosylated by three residues of _L-arabinose.² Moreover, peptide-receptor relationships in CLE peptide family are multiple, since one CLE peptide can recognize several receptors with different affinity, while one receptor kinase also can bind to different peptides. For example, CLV1 and CLV2, the leucine-rich repeat receptor-like kinase/protein (LRR-RLK/RLP), act as the receptors of CLV3 peptide involved in negatively regulating plant defense contributing to Ralstonia solanacearum and Hyaloperonospora arabidopsidis in Arabidopsis.⁹ In addition, cyst nematodes (Heterodera schachtii) secrete and utilize CLE-like effectors for successful plant infection dependent on host CLV2 receptor.¹⁰ In contract, Arabidopsis CLV1 positively enhances plant defense in response to two necrotrophic fungi, Botrytis cinerea and Plectrosphaerella cucumerina.⁹ However, whether CLV3 peptide itself can activated plant innate immunity still debated.^11–13

HYPSYS (HYDROXYPROLINE-RICH GLYCOPEPIDE SYSTEMINS) peptides are also glycopeptides consist of 15 to 20 amino acid with several possible Pro^Hyp residues. HYPSYS peptides are mainly found within the Solanaceae family formed from large preproproteins. HYPSYS has an important role in plant local and systemic defenses against insect herbivores like Helicoverpa armigera larvae by activating protease inhibitors abundance in tomato.¹⁴ Except for Solanaceae family, HYPSYS expressed in sweet potato (Ipomoea batatas) also suppress Spodoptera litura growth through enhancing lignin biosynthesis.¹⁵

IDA (INFLORESCENCE DEFICIENT IN ABSCISSION) and IDL (IDA-LIKE) peptides has not been identified in structure, but are supposed to be derived from a preproprotein with C-terminal conserved motif (EPIP) containing several Pro^Hyp residues, which are essential to IDA activity in biological processes.¹⁶ As the LRR-RLK type proteins, HEASE (HAE) and HAE-LIKE2 (HSL2) act as the receptors of IDA/IDL identified in Arabidopsis.¹⁷ In Pseudomonas syringae-Arabidopsis interaction, IDL6 suppresses a broad amount of stress-induced gene expression and increases polygalacturonase activity impacting pectin degradation, which suggests P. syringae manipulates the IDL6-HAE/HSL2 signaling to enhance its infection.¹⁸

PSK (PHYTOSULFOKINE) is pentapeptide in the sequence of YIYTQ containing two sulfated Tyr residues, which is essential for its growth and immunity activity.^6,19 A LRR-RLK type receptor, PSKRs have been identified as PSK receptor in Arabidopsis, carrot and tomato.^6,20,21 In Arabidopsis pskr1 mutants, hosts respectively exhibited more susceptible to the fungal pathogen Alternaria brassicicola.²² Moreover, PSK as a novel damage-associated molecular pattern, initiates cytosolic Ca²⁺ flux through PSKR1 to activate auxin-dependent plant immune response against B. cinerea in tomato plants.⁶ However, PSK-PSKR signaling displays opposite role in attenuating plant pattern-triggered immunity against P. syringae.^23,24

PSY1 (PLANT PEPTIDE CONTAINING SULFATED TYROSINE1) is another Tyr sulfated peptide like PSK. Except for Tyr sulfation, PSY1 posttranslational modification also contains hydroxylation and glycosylation in Pro residues. PSY1 RECEPTOR (PSY1R) identified as a PSY1 receptor also belongs to LRR-RLK. As far, the antagonistic role of PSY1-PSY1R in plant immune response against biotrophic and necrotrophic pathogens are quite similar to PSK-PSKR1 signaling in plant immunity.^22,23 In addition, it is reported PSY1 signaling similar to PSK signaling induces Arabidopsis susceptibility to Fusarium oxysporum probably because the F. oxysporum effector suppress the negative feedback regulation of PSY1R.²⁵

Cysteine-rich peptides

Cysteine-rich peptides are characterized according to a Cys-rich domain containing 2 to 16 Cys residues, which the numbers are considerably altered based on both peptide length and plant species. The majority of studied Cys-rich peptides are regarded as antimicrobial peptides in plant-microbe interactions, however there are still some peptides mainly involved in plant growth and development also exhibiting the regulating roles in plant immune responses. In this section, we introduce several representative Cysteine-rich peptides and their functions in plant immune response, especially recent results in those of previous lack reported antimicrobial activity members.

PDFs (PLANT DEFENSINS), around 5 kDa small peptides, seem the best studied Cysteine-rich peptides as well as antimicrobial peptides.² PDFs are widely distributed in angiosperm and defined as one of the largest and diverse pathogenesis-related protein family.²⁶ In Arabidopsis genome, it contains approximate 285 PDF and defensin-like genes.²⁷ Majority of Arabidopsis PDFs have eight Cys residues except for some contain only four to six. PDFs in diverse model plants and crops exhibited their involvement in the innate immune responses to fungal pathogens including Fusarium spp., Botrytis cinerea, and Verticillium dahlia, and bacteria pathogens like Pectobacterium carotovorum.^28–30 PDFs exhibited antimicrobial activity not only because they bind host intracellular targets triggering defense signaling like cell death induction, but they also are able to interact with various fungal sphingolipids and phospholipids.²

RALF (RAPID ALKALINIZATION FACTOR) peptides are originated from the C-terminal end of a preproprotein with about 5 kDa in mature status. RALFs get the name after they initiating a rapid pH increase in tobacco cell cultures.³¹ Usually, RALFs contain four conserved Cys residues which are able to establish two disulfide bridges.³² In 2014, the first RALF receptor identified was FERONIA (FER), which belongs to the Catharanthus roseus receptor-like kinase (Cr-RLK) family.³³ Previous studies were primarily reported RALF as a novel plant peptide hormone involved in plant growth and development regulation, including cell elongation in root cells, decreasing nodulation, and affecting pollen tube growth.³⁴ However, in recent years, more and more evidences reveal the RALF-mediated signaling is assigned participated in regulating plant-microbe interaction. F oxysporum biosynthesize and utilize a functional homologue of RALF to increase its virulence by activating host FER-mediated alkalinization pathway.³⁵ In Arabidopsis, FER enhances the flg22-induced FLS2-BAK1 complex stability resulting in promoted plant PTI responses, whereas the endogenous RALF23 negatively modulates plant PTI via FER/BAK1 receptor complex.³⁶ Moreover, other RALF receptors ANXUR1 (ANX1), the closest homolog of FER, link with PRRs and nucleotide binding domain leucine-rich repeat (NLR) type R proteins to suppress both PTI and effector-triggered immunity (ETI).³⁷

EPF (EPIDERMAL PATTERN FACTOR) and EPFL (EPF-LIKE) peptides are consist of 45 to 76 amino acid with six or eight conserved Cys residues, which are preserved from the C-terminal region of preproprotein. It has been identified that the LRR-RLK type receptor ERECTA forms a multiprotein receptorsome together with RLP type receptor TMM and RLK co-receptor SERK family to recognize EPFs.³⁸ The functions of EPF/EPFLs are mainly explored in stomatal development, which controls stomatal density, clustering and light response.^38,39 Although there is no direct evidence to show EPF/EPFLs modulating plant immune response against pathogens, their receptorsome is still demonstrated involved in plant resistance to pathogens. Until now, ERECTA-mediated plant immunity pathway efficient rescues plants from infection of the fungus P. cucumerina, Verticillium longisporum, and Magnaporthe oryzae, the bacterium R. solanacearum, the oomycete Pythium irregular, etc.^40–45

Others

The third group of plant peptides so-called “non-Cys-rich/non-PTM peptides” processed from nonfunctional precursors has not been found and characterized by any posttranslational modification yet, and meanwhile these peptides lack more than two Cys resides in their 8 to 36 amino acid length of peptide sequences. Here, we list two representative plant peptides belonged to this group to elucidate their specific role in plant biotic resistance.

SYSTEMIN peptides contain 18 amino acid with four to six Pro residues and a conserved PPKMQTD motif at C terminal end. This type plant peptides particularly characterized in the Solanoideae subfamily of Solanaceae, like tomato, potato and pepper. Recently, a high affinity and specificity systemin receptor SYR1 was identified instead of original SR160 worked in tomato herbivorous resistance.⁴⁶ Except for the role in anti-herbivore defense responses, systemin also elicits endogenous jasmonic acid signaling pathway for tomato plant against necrotrophic B. cinerea.¹ Moreover, systemin also can rescue tomato plants from Glomus clarum infection.⁴⁷

PEPs (PLANT ELICITOR PEPTIDES) are at the length of 23 to 36 amino acid derived from the C terminus of precursor protein PROPEPs.⁴⁸ PEPs are perceived by LRR-RLK type receptor complex of PEPRs and SERKs.^48,49 PEPs as a typical DAMP elicit plant PTI responses via PEPR receptor complex against various pathogen infection. The downstream responses of PEP-PEPR mediated plant immunity signaling is the best characterized model so far in DAMP induced PTI, which include receptor complex dynamics and phosphorylation events, production of signal molecular (such as cyclic GMP, nitric oxide, and reactive oxygen species, and defense-related hormones), induction of Ca²⁺-influx and signaling, phosphorylation of MAP kinases, changes in defense gene expression, and callose deposition and seedling growth inhibition.⁵⁰ Until now, PEPs are elucidated to improve plant defense in Arabidopsis and maize (Zea mays) to virulent bacteria P. syringae, fungal pathogens, Cochliobolis heterostrophus and Colletotrichum graminicola, and the oomycete P. irregular.^51,52

Future directions

Up to now, major function of plant peptides revealed still is focus on plant growth and development except for the antimicrobial peptides.² Although multiple works mentioned above prove the immune function of different type plant peptides, our understanding of this field is still limited. For example, some evidence of several plant peptide related immune response are established on the receptor level of plant peptides, like IDA/IDL, and EPF/EPFL, which means there is no direct proof to confirm the immune function of plant peptides. In addition, there is still a blank field for further investigating defense response of other plant peptides not mentioned above, such as embryo surrounding factor1, grim reaper peptide, and so on.

Funding Statement

This work was supported by the National Natural Science Foundation of China [31772355]; National Key Research and Development Program of China [2017YFD0200600];

Acknowledgments

We are grateful to Dr. Frantisek Baluska for kindly inviting this manuscript. This work was supported by the National Natural Science Foundation of China (31772355), and the National Key Research and Development Program of China (2017YFD0200600).

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.