Abstract
Plant peptide hormones play various roles in plant development, pathogen defense and abiotic stress tolerance. Plant elicitor peptides (Peps) are a type of damage-associated molecular pattern (DAMP) derived from precursor protein PROPEPs. In this study, we identified nine PROPEP genes in the broccoli genome. qRT-PCR analysis indicated that the expression levels of BoPROPEPs were induced by NaCl, ABA, heat, SA and P. syringae DC3000 treatments. In order to study the functions of Peps in salinity stress response, we synthesized BoPep4 peptide, the precursor gene of which, BoPROPEP4, was significantly responsive to NaCl treatment, and carried out a salinity stress assay by exogenous application of BoPep4 in broccoli sprouts. The results showed that the application of 100 nM BoPep4 enhanced tolerance to 200 mM NaCl in broccoli by reducing the Na+/K+ ratio and promoting accumulation of wax and cutin in leaves. Further RNA-seq analysis identified 663 differentially expressed genes (DGEs) under combined treatment with BoPep4 and NaCl compared with NaCl treatment, as well as 1776 genes differentially expressed specifically upon BoPep4 and NaCl treatment. GO and KEGG analyses of these DEGs indicated that most genes were enriched in auxin and ABA signal transduction, as well as wax and cutin biosynthesis. Collectively, this study shows that there was crosstalk between peptide hormone BoPep4 signaling and some well-established signaling pathways under salinity stress in broccoli sprouts, which implies an essential function of BoPep4 in salinity stress defense.
Highlights
As sessile organisms that lack opportunities to avoid environmental stress conditions, plants need to continuously recognize and respond to changing environments for survival
In order to get an idea of the transcriptional regulations of BoPROPEPs under stress, we explored the expression changes of BoPROPEP1 to BoPROPEP4 in 11-dayold broccoli leaves and roots in response to NaCl (100 mM), heat (37 ◦C), ABA (5 μM), salicylic acid (SA) (2 mM) and P. syringae DC3000 (OD600 = 0.02) treatments
The results showed that the transcriptional levels of BoPROPEP1, BoPROPEP2 and BoPROPEP4 were induced when treated with SA (Figure 2G)
Summary
As sessile organisms that lack opportunities to avoid environmental stress conditions, plants need to continuously recognize and respond to changing environments for survival. Plant elicitor peptides (Peps), well-characterized members of the damage-associated molecular patterns (DAMPs) family, were demonstrated to enter into the extracellular space when a cell is attacked by pathogens or destroyed by injury, inducing pattern-triggered immunity (PTI) defense processes [5]. The downstream molecular signaling of salinity tolerance mediated by AtPep3-PEPR1 is not defined because none of the typical abiotic stress-responsive genes or genes related to other phytohormones had been identified in either AtPROPEP3 overexpression or synthetic AtPep peptide-treated plants [9]. Besides studies in model plant Arabidopsis, pretreatment with Peps has been identified to protect maize and rice plants against herbivore attack by activation of mitogen-activated protein kinase signaling and generation of reactive oxygen species (ROS) and defense metabolites [14,15]. It can be speculated that Peps are conserved regulators of immune responses across plant species that contribute to salt stress tolerance; the principles by which Peps enhance plant tolerance to salt still need to be investigated
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