Plant hormones have pivotal roles in the regulation of plant growth, development, and reproduction. Additionally, they emerged as cellular signal molecules with key functions in the regulation of immune responses to microbial pathogens, insect herbivores, and beneficial microbes. Their signaling pathways are interconnected in a complex network, which provides plants with an enormous regulatory potential to rapidly adapt to their biotic environment and to utilize their limited resources for growth and survival in a cost-efficient manner. Plants activate their immune system to counteract attack by pathogens or herbivorous insects. Intriguingly, successful plant enemies evolved ingenious mechanisms to rewire the plant’s hormone signaling circuitry to suppress or evade host immunity. Evidence is emerging that beneficial root-inhabiting microbes also hijack the hormone-regulated immune signaling network to establish a prolonged mutualistic association, highlighting the central role of plant hormones in the regulation of plant growth and survival.

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Hormonal Modulation of Plant Immunity

Plant hormones play important roles in regulating developmental processes and signaling networks involved in plant responses to a wide range of biotic and abiotic stresses. Significant progress has been made in identifying the key components and understanding the role of salicylic acid (SA), jasmonates (JA) and ethylene (ET) in plant responses to biotic stresses. Recent studies indicate that other hormones such as abscisic acid (ABA), auxin, gibberellic acid (GA), cytokinin (CK), brassinosteroids (BR) and peptide hormones are also implicated in plant defence signaling pathways but their role in plant defence is less well studied. Here, we review recent advances made in understanding the role of these hormones in modulating plant defence responses against various diseases and pests.

Role of plant hormones in plant defence responses.

Herbivorous insects use diverse feeding strategies to obtain nutrients from their host plants. Rather than acting as passive victims in these interactions, plants respond to herbivory with the production of toxins and defensive proteins that target physiological processes in the insect. Herbivore-challenged plants also emit volatiles that attract insect predators and bolster resistance to future threats. This highly dynamic form of immunity is initiated by the recognition of insect oral secretions and signals from injured plant cells. These initial cues are transmitted within the plant by signal transduction pathways that include calcium ion fluxes, phosphorylation cascades, and, in particular, the jasmonate pathway, which plays a central and conserved role in promoting resistance to a broad spectrum of insects. A detailed understanding of plant immunity to arthropod herbivores will provide new insights into basic mechanisms of chemical communication and plant-animal coevolution and may also facilitate new approaches to crop protection and improvement.

Plant Immunity to Insect Herbivores

Jasmonates are essential phytohormones for plant development and survival. However, the molecular details of their signalling pathway remain largely unknown. The identification more than a decade ago of COI1 as an F-box protein suggested the existence of a repressor of jasmonate responses that is targeted by the SCF(COI1) complex for proteasome degradation in response to jasmonate. Here we report the identification of JASMONATE-INSENSITIVE 3 (JAI3) and a family of related proteins named JAZ (jasmonate ZIM-domain), in Arabidopsis thaliana. Our results demonstrate that JAI3 and other JAZs are direct targets of the SCF(COI1) E3 ubiquitin ligase and jasmonate treatment induces their proteasome degradation. Moreover, JAI3 negatively regulates the key transcriptional activator of jasmonate responses, MYC2. The JAZ family therefore represents the molecular link between the two previously known steps in the jasmonate pathway. Furthermore, we demonstrate the existence of a regulatory feed-back loop involving MYC2 and JAZ proteins, which provides a mechanistic explanation for the pulsed response to jasmonate and the subsequent desensitization of the cell.

The JAZ family of repressors is the missing link in jasmonate signalling

Jasmonates: biosynthesis, perception, signal transduction and action in plant stress response, growth and development. An update to the 2007 review in Annals of Botany

The importance of long noncoding RNAs (lncRNAs) in plant development has been established, but a systematic analysis of the lncRNAs expressed during plant allelopathy has not been carried out. We performed RNA-seq experiments on S. lycopersicum subjected to different levels of para-hydroxybenzoic acid (PHBA) stress during plant allelopathy and identified 61,729 putative lncRNAs. Of these, 7765 lncRNAs cis-regulated 5314 protein-coding genes (PGs). Among these genes, 1116 lncRNAs and 2239 PGs were involved in a complex web of transcriptome regulation, and we divided these genes into 12 modules. Within these modules, 458 lncRNAs and 975 target genes were found to be highly correlated. Additionally, 989 lncRNAs trans-regulated 1765 PGs, and we classified them into 11 modules, within which 335 lncRNAs were highly correlated with their 633 corresponding target genes. Only 98 lncRNAs in S. lycopersicum had homologs in the lncRNA database of Arabidopsis thaliana, all of which were affected by the PHBA treatments. MiRNAs that interacted with both mRNAs and lncRNAs were selected on the basis of weighted correlation network analysis (WGCNA) results to make lncRNA-miRNA-mRNA triplets. Our study presents a systematic identification of lncRNAs involved in plant allelopathy in S. lycopersicum and provides research references for future studies.

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Yajie Niu

Papers

Systematic Identification of Long Non-Coding RNAs under Allelopathic Interference of Para-Hydroxybenzoic Acid in S. lycopersicum

PeMPK17 interacts with PeMKK7 and participates in para-hydroxybenzoic acid stress resistance by removing reactive oxygen species.