Jasmonate suppresses seedling soil emergence in Arabidopsis thaliana

ABSTRACT

In addition to defense response, phytohormone jasmonate participates in various plant growth and developmental processes. Nonetheless, its role in the seedling stage is not well defined. We recently report that jasmonate suppresses hypocotyl elongation and promotes cotyledon unfolding in etiolated Arabidopsis seedlings. The molecular basis underlying this phenotype is that jasmonate treatment reduces the biochemical activity of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) to stabilize several COP1-targetted transcription factors for eliciting a proportion of light responsive transcriptome. We further reveal that jasmonate receptor CORONATINE INSENSITIVE 1 (COI1) and bHLH transcription factor MYC2 are required for the suppression of COP1 activity. Because elongated hypocotyl and closed cotyledons secure the success for seedling soil emergence, here we investigate seedling soil emergence under jasmonate treatment and find that jasmonate reduces the seedling emerging rates in the wild-type plants. Consistent with the largely insensitive to jasmonate in the suppression of skotomorphogenesis, the soil emerging rates in coi1 or myc2 mutants are almost not altered in the presence of jasmonate. Our data addendum describe that jasmonate-triggered inhibition of etiolation growth results in the defects in seedling soil emergence and suggest that defense-stimulated jasmonate biosynthesis might affect seedling germination in soil.

As a defense hormone in plants, jasmonate is required for plant resistance to insects and necrotrophic fungi attacks. Additional to its role in defense, jasmonate modulates a myriad of growth and developmental events. For example, jasmonate inhibits root elongation, stimulates anthocyanin accumulation, delays flowering and triggers senescence.¹ However, whether jasmonate participates in the first developmental stage (seedling development) in plant life history is not clear.

Unlike animals, plant development is much more plastic. Seedlings grown in light exhibit short hypocotyls and opened green cotyledons, while seedlings developed under darkness have long hypocotyls and closed yellow cotyledons. The former process is termed photomorphogenesis and the latter is called skotomorphogenesis.² Light determines the transition from skotomorphogenesis to photomorphogenesis through the repression of photomorphogenesis negative regulators. RING type E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) is one major player during this transition.³ COP1 directly interacts with multiple transcription factors and promotes their protein turnover via the 26S proteasome pathway in darkness. Light triggers COP1 movement out of nucleus and reduces the physical interaction between COP1 and its interaction partner SUPPRESSOR OF PHYTOCHROME A-105 1 (SPA1), thus to inhibit COP1 activity and stabilize these COP1-targetted transcription factors. Then these transcription factors elicit the light-responsive transcriptomic changes and cause photomorphogenesis.⁴

Jasmonate signaling is controlled by a derepression mechanism. Without jasmonate, transcription factors in jasmonate signaling are bound by a group of JASMONATE ZIM-DOMAIN PROTEINs (JAZs) and their transcriptional activities are repressed. After jasmonate perception by the nucleus localized F-box protein CORONATINE INSENSITIVE 1 (COI1),^5-7 jasmonate serves as a molecular glue to stimulate COI1-JAZ interaction and bring JAZ proteins degradation. Therefore, these transcription factors are re-activated to stimulate jasmonate responses.⁸ Basic helix-loop-helix (bHLH) family member MYC2 is the first-identified and most widely-studied transcription factor in jasmonate signaling. MYC2 is involved in jasmonate responsive insect defense, root growth inhibition, and metabolism.⁹

We recently report that jasmonate suppresses hypocotyl elongation and stimulates cotyledon opening in etiolated Arabidopsis seedlings. We further demonstrate that jasmonate not only attenuates the interaction between COP1 and SPA1, but also reduces the COP1 protein accumulation in the nucleus for inhibiting COP1 activity to stabilize the photomorphogenesis-promoting transcription factors. Meanwhile, we notice that COI1-JAZ-MYC2 signaling cascade is required for this process.¹⁰ Our findings suggest that jasmonate is a crucial factor for regulating skotomorphogenesis.

We know that long hypocotyls in germinating seedlings in subterranean darkness secure plants to reach for light, while closed cotyledons protect the apical meristem from mechanical injuries during soil penetration. Seedlings impaired in etiolation growth will have severe soil emergence rate loss. For example, although cop1 mutants grow well in the absence of soil cover, they could not emerge from soil when seeds are covered with a layer of soil.¹¹

Because jasmonate inhibits etiolation growth through the suppression of COP1 activity, we next asked whether jasmonate affects seedling soil emergence. We first compared the emergence rate of the wild-type plants with jasmonate treatment or not. Seeds were covered with different depths of soil and then grown under continuous white light for 7 d. We found that jasmonate reduces the emergence rates under various soil depth conditions (Fig. 1A-B). To further dissect whether the loss of emergence rates is correlated with jasmonate-inhibited skotomorphogenesis, we tested seedling emergence in 2 jasmonate insensitive mutants, coi1 and myc2. Jasmonate did not significantly alter the emergence rates in the coi1–2 mutants, either under thin (1 mm) or medium (2 mm) soil cover (Fig. 1C). Similar to coi1–2, 2 different alleles of myc2 (myc2–2 and myc2–3) germinated well in the presence of jasmonate (Fig. 1D).

Figure 1.

Jasmonate inhibits Arabidopsis seedling soil emergence. (A) Quantitative analysis of wild-type (Col-0) seedlings emerging from different depth of soil covered on MS or jasmonate (JA) medium. Mean ± SD; n = 3. (B) Quantitative analysis of Col-0 seedlings emerging from different depth of soil covered on MS or different concentrations of JA medium. Mean ± SD; n = 6. (C) Quantitative analysis of seedlings emerging from 1 mm or 2 mm soil covered on MS or JA medium. Mean ± SD; n = 5. (D) Quantitative analysis of seedlings emerging from 1 mm soil covered on MS or JA medium. Mean ± SD; n = 5.

Taken together, our data addendum illustrate that exogenous jasmonate treatment hinders seedling soil emergence, most likely due to the impaired etiolation growth triggered by the COI1-MYC2 cascade.

Materials and methods

Materials and growth conditions

The seeds of coi1–2, myc2–2 (SALK_083483) and myc2–3 (SALK_017005) have been described.¹² Seeds were surface sterilized with 10% bleach containing 0.1% Triton X-100 for 5 min, and then washed 5 times with sterile water. Sterilized seeds were placed on MS medium. Methyl jasmonate (Sigma-Aldrich) was used as the bioactive jasmonate treatment in this study. Plates were stratified in darkness for 3 days, and then transferred to growth chamber under continuous white light at 22°C for 7 d.

Soil emergence assay

The sands (50–70 mesh particle size, Sigma-Aldrich) were autoclaved and then evenly spread onto the medium surface to cover the plated seeds as described.¹³ The survival percentages of light-grown seedlings were calculated by dividing the number of green seedlings that successfully emerged from soil by the total number of plated seeds.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

This work is supported by the Natural Science Foundation of Jiangsu Province (BK20140919), the National Natural Science Foundation of China (31470375), the Priority Academic Program Development of Jiangsu Higher Education Institutions and Qing Lan Project.