Methyl jasmonate signaling and signal crosstalk between methyl jasmonate and abscisic acid in guard cells

Plants tightly control stomatal aperture in response to various environmental changes. A drought-inducible phytohormone, abscisic acid (ABA), triggers stomatal closure and ABA signaling pathway in guard cells has been well studied. Similar to ABA, methyl jasmonate (MeJA) induces stomatal closure in various plant species but MeJA signaling pathway is still far from clear. Recently we found that Arabidopsis calcium dependent protein kinase CPK6 functions as a positive regulator in guard cell MeJA signaling and provided new insights into cytosolic Ca²⁺-dependent MeJA signaling. Here we discuss the MeJA signaling and also signal crosstalk between MeJA and ABA pathways in guard cells.Key words: methyl jasmonate, abscisic acid, guard cell, reactive oxygen species, nitric oxide, calciumStomata, which are formed by pairs of specialized cells called guard cells, control gas exchanges and transpirational water loss. Guard cells can shrink and swell in response to various physiological stimuli, resulting in stomatal closing and opening.^1,2 To optimize growth under various environmental conditions, plants have developed fine-tuned signal pathway in guard cells. Abscisic acid (ABA) is synthesized under drought stress and induces stomatal closure to reduce transpirational water loss.² ABA signal transduction in guard cells has been widely studied. ABA induces increases of various second messengers such as cytosolic Ca²⁺, reactive oxygen species (ROS) and nitric oxide (NO) in guard cells. These early signal components finally evoke ion efflux through plasma membrane ion channels, resulting in reduction of guard cell turgor pressure.Jasmonates are plant hormones synthesized via the octadecanoid pathway and regulate various physiological processes in plants such as pollen maturation, tendril coiling, senescence and responses to wounding and pathogen attacks.³ Similar to ABA, jasmonates also trigger stomatal closure and the response is conserved among various plant species including Arabidopsis thaliana,⁴ Hordeum vulgare,⁵ Commelina benghalensis,⁶ Vicia faba,⁷ Nicotiana glauca,⁸ Paphiopedilum Supersuk⁹ and Paphiopedilum tonsum.⁹ A volatile methyl ester of jasmonic acid (JA), methy jasmonate (MeJA), has been widely used for studying jasmonate signaling pathway. To date, pharmacological and reverse genetic approaches have revealed many important signal components involved in MeJA-induced stomatal closure and suggest a signal crosstalk between MeJA and ABA in guard cells. In this review, we mainly focus on the three important second messengers, ROS, NO and cytosolic Ca²⁺ and discuss recent advance about MeJA signaling and signal interaction between MeJA and ABA in guard cells.