Early signal transduction linking the synthesis of jasmonic acid in plant

Abstract

Jasmonate signaling plays a critical role in protecting plants from pathogens or insect attacks and in limiting damage from abiotic stress. Many events contribute to the regulation of jasmonic acid (JA) synthesis during abiotic or biotic stress, but the details of the underlying mechanism remain unclear. In this Mini-Review paper, we discuss the possible roles of reactive oxygen species (ROS), nitric oxide (NO), calcium influx and mitogen-activated protein kinase (MAPK) cascade during JA synthesis or JA signal transduction.

Jasmonic acid (JA) is a member of the jasmonate group of plant hormones; it is biosynthesized from linolenic acid by the octadecanoid pathway.¹ The main functions of this hormone are growth related, including growth inhibition, senescence and leaf abscission. It also plays an important role in plant response to wounding and in systemic resistance. JA has a structure similar to that of mammal prostaglandins and is synthesized from alpha-linolenic acid, which is a C-18 poly-unsaturated fatty acid. Lipoxygenase, allene oxide synthase and allene oxide cyclase are the putative key enzymes for JA synthesis; these enzymes have chloroplast transit peptides that direct their import into chloroplasts. JA can be conjugated with amino acids, namely, leucine, valine, isoleucine and the sugar, B-glucoside using UDP-glucose. (-)-JA and (-)-methyl jasmonate are major JAs in plants. Methyl jasmonate (MeJA) in particular is a strong candidate for airborne signals that mediate interplant communication for defense responses. JA and its derivates induce the production of vegetative storage proteins, osmotin, thionin (antifungal) and defensin. It also induces enzymes related to phytoalexin, chalcone synthase, phenylalanine ammonia lyase (PAL), and hydroxymethylglutaryl-COA reductase; it also induces protease inhibitors to suppress the insect growth. JA and ethylene induce PR-3, PR-4 and PDF 1.2 chitinases (CHI-B) and hevein-like protein. In plants, ROS, Calcium ion influx, MAP kinase cascade, and NO, a novel signaling molecule are involved in the JA octadecanoid signal pathway.^1–4

ROS and Nitric Oxide

H₂O₂ and NO are widely accepted as important signaling molecules in plant response to biotic or abiotic stress. H₂O₂ is a reactive form of ROS (reactive oxygen species)⁵ and is generated via superoxidation during electron transport processes such as photosynthesis or mitochondrial respiration. The plasma membrane NADPH oxidase complex, which is similar to mammalian NADPH oxidase is also responsible for H₂O₂ synthesis during environmental stress conditions such as excess excitation (light) energy, drought and cold.⁶ In tomato leaves, wounding or JA treatment caused significant increases in the activity of plasma membrane NADPH oxidase and accumulation of ROS; however, this accumulation can be blocked by pretreatment with plasma membrane NADPH oxidase inhibitor or H₂O₂ scavengers. Systemin, OGA, or chitosan can nduce the accumulation of H₂O₂ in the tomato leaves, however, they cannot activate H₂O₂ release for a transgenic tomato line compromised in the octadecanoid pathway. These results indicated that it is essential for the generation of H₂O₂ that the entire octadecanoid signal pathway is intact. Evidence mostly indicated that the activation of the octadecanoid pathway and JA synthesis occurs before H₂O₂ accumulation; however, we proposed the existence of a mechanism linking the synthesis of JA and activation of NADPH oxidase. This hypothesis is based on the finding that in a suspension of ginseng cells, additional DPI can block elicitor-induced JA synthesis, indicating that the activation of NADPH is indispensable to the activation of JA synthesis.⁹ We think that there may be a “double-click mechanism” for the activation of the octadecanoid pathway. The first click represents the exogenous elicitor-induced triggering of the activities of NADPH oxidase and H₂O₂ upstream of JA, but this process might be too early and too weak to be easily measured at the whole plant level; however, it is slightly easier to evaluate this process in cell suspensions. The secondary click constitutes the synthesized JA-induced stimulation of NADPH oxidase activity to cause the release of large amounts of H₂O₂ which acts as the chief secondary signal that induces the expression of the genes encoding the components of the octadecanoid pathway that are downstream to H₂O₂.

NO is a putative plant signal molecule involved in multiple plant physiological processes.¹⁰ Accumulation of wound-induced protein inhibitor I can be inhibited by NO; this inhibition can be reversed by NO scavengers. NO also blocks hydrogen peroxide (H₂O₂) production and synthesis of proteinase inhibitor that was induced by systemin, oligouronides and jasmonic acid (JA). NO appears to be interacting directly with the signaling pathway downstream from JA synthesis and upstream of H₂O₂ synthesis.¹² Pretreatment of the Hypericum perforatum suspension cells with NO scavenger, NO synthase inhibitor, or JA synthesis inhibitor blocks fungal elicitor-induced NO accumulation or secondary metabolite synthesis, while JA synthesis inhibitor does not show inhibitory effect on NO accumulation. This suggested that JA acts downstream of NO generation and that the octadecanoid pathway is regulated by NO.¹³ Both JA and NO caused closure of the stomatal cells of Vicia faba L., and JA enhanced NO synthesis in these cells. NO scavengers and NOS synthase inhibitors repressed JA-induced NO generation and JA-induced stomatal closure.¹⁴ It appears that JA also takes part in NO accumulation in plants and is similar to the mammalian NOS enzyme.

Calcium Influx

Ca²⁺ appears to be an essential signaling molecule ubiquitously expressed in all plant tissues and plays key regulatory roles in response to environmental or developmental stimuli.¹³ Elevated concentrations of magnesium (Mg²⁺) and Ca²⁺ in the appoplast have been shown to significantly enhance the biological activity of systemin and Ca²⁺ ionophores were found to induce the accumulation of proteinase inhibitors.¹⁵ Treatment with JA also induced an increase in cytosolic free-Ca²⁺ concentration ([Ca²⁺] cyt) in Arabidopsis thaliana leaves; JA-induced gene expression could be mimicked by higher concentration of extracellular Ca²⁺. It is possible that the influx of extracellular Ca²⁺ through the Nif-sensitive Ca²⁺ channel in the plasma membrane may be responsible for JA-induced elevation of [Ca²⁺]cyt and the expression of the genes encoding the molecules downstream to JA.^16,17 Our study also demonstrated that the influx of calcium is necessary for elicitor-induced JA synthesis and secondary metabolite accumulation in cultured ginseng cells.

MAP Kinase

Mitogen-activated protein kinases (MAPKs) play a key role in plant responses to stress and pathogens.¹⁸ Recently, MAPK cascades have been proposed as major pathways for signal transduction into intracellular response. Silencing the LeMPK1, LeMPK2 and LeMPK3 genes in these transgenic plants by virus-induced gene silencing (VIGS) resulted in reduced levels of defensive proteinase inhibitors; cosilencing of LeMPK1/2 in 35S:: Prosystemin plants also reduced JA synthesis in response to wounding. This demonstrated that tomato MAPK is required for JA-induced gene expression and is upregulated through the octadecanoid pathway. The LeMPK3 ortholog WIPK is also required for JA synthesis in tobacco. It is possible that systemin activated at least 3 MAPKs that function upstream of JA synthesis. We know that JA synthesis is initiated in the chloroplasts, while most MAPKs are cytosolic proteins; therefore, plant MAPKs are likely to relay the extracellular elicitor signal to activate the processes for JA biosynthesis in chloroplasts.¹⁹

Conclusions

Beside ROS, NO, calcium and MAPK kinase that have been discussed above, other signal molecules such as ABA, ethylene and SA are also important components during JA signal transduction and synthesis. In summary, a combination of molecular, biochemical, and genetic analyses have demonstrated that JA is an indispensable signaling molecule during the plant defense response. The identification of other signals will further help us to understand details underlying the molecular mechanism of JA signal transduction.