Ethylene Signaling System in Plant Innate Immunity

Abstract

Ethylene signaling is involved in the modulation of plant innate immunity. On perception of pathogen-associated molecular pattern (PAMP) signals, ethylene biosynthesis system is activated. It results in enhanced expression of ethylene biosynthesis genes. G-proteins act as molecular switches switching on the ethylene biosynthesis pathway. PAMP-induced signals induce calcium signaling, which is modulated by specific calcium signatures. Calcium sensors involved in ethylene signaling include Ca2+-dependent protein kinases, which are involved in decoding calcium signals. The calcium waves activate ACC synthase, the key enzyme involved in the biosynthesis of ethylene. PAMP triggers ROS production within seconds or minutes, and an increase in H2O2 triggers transcription of genes involved in the biosynthesis of ethylene. Nitric oxide is involved in the generation of the biphasic pattern of ethylene production. MAP kinase cascades have been shown to play an important role in the induction of ethylene biosynthesis by PAMP signals. Ubiquitin- and proteasome-mediated degradation of enzymes involved in ethylene biosynthesis plays an important role in ethylene biosynthesis pathway. Ethylene signal transduction is initiated by the binding of ethylene by a family of five membrane-bound receptors. In all the receptors, the transmembrane domain at the N-terminus comprises the ethylene-binding domain, and the receptor N-terminus has three to four transmembrane domains that bind ethylene. The receptors interact with each other, and cooperativity for ethylene signaling exists among the receptors. Each member of the ethylene receptor family may have unique roles in ethylene signaling and may act cooperatively, rather than independently. CTR1 functions as a key mediator of ethylene signal transduction, acting just downstream of the receptors. EIN2 acts downstream of CTR1, and CTR1 directly phosphorylates the cytosolic C-terminal domain of EIN2. Upon ethylene perception by the receptors, CTR1 is inhibited and the inhibition of CTR1 upon ethylene perception is a signal for cleavage and nuclear localization of the EIN2 C-terminus, allowing the ethylene signal to reach the downstream transcription factors in the nucleus. In the nucleus, EIN2 activates the transcription factors EIN3 and EIL1. In the absence of ethylene, EIN3 is quickly degraded through a ubiquitin–proteasome pathway mediated by two F-box proteins, EBF1 and EBF2. Ethylene signaling suppresses the ubiquitin–proteasome-mediated proteolysis of EIN3. EIN3 becomes stabilized after perception of ethylene and induces ethylene-responsive gene expression. EIN3 binds to the promoter sequence of the ethylene-inducible transcription factor ERF1. ERFs bind specifically to the GCC-box that is found in several promoters of the pathogenesis-related (PR) genes. Ethylene triggers increased expression of plant pattern recognition receptors (PRRs) and activates the plant immune responses. Ethylene and jasmonate signaling systems act concurrently in triggering plant immune responses.