Auxin Signaling System in Plant Innate Immunity

Abstract

Plant hormones play important role in plant immune signaling system. While salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are integral components of signal transduction systems involved in activation of plant innate immunity, auxin is involved in the modulation of various plant defense signaling pathways. Indole-3-acetic acid (IAA) is the naturally occurring auxin in plants. IAA is synthesized from tryptophan via multiple pathways including indole-3-pyruvic acid (IPA), indole-3-acetamide (IAM), tryptamine (TAM), and indole-3-acetonitrile (IAN) pathways. TIR1/AFB proteins, which are F-box subunits of the SCF ubiquitin complex, have been identified as the receptors for auxins. AUX/IAA proteins are nuclear proteins, and the only function of the AUX/IAA proteins is to repress transcription of auxin-regulated genes. Auxin response factor (ARF) proteins are the transcription factors that bind to the cis-element in auxin-responsive promoters and trigger the expression of auxin-responsive genes. The AUX/IAA protein forms a heterodimer with ARF and represses the transcriptional regulatory function of ARFs. Auxin promotes the degradation of the AUX/IAA repressor through ubiquitin–proteasome pathway and consequently activates auxin-responsive gene expression by removing the inhibition of ARF activity. The auxin receptor TIR1 F-box protein directly links auxin perception to degradation of the AUX/IAA proteins, resulting in depression/activation of auxin response genes. Auxin rapidly alters the expression of hundreds of genes within minutes by removing the inhibitory function of AUX/IAA proteins. Auxin modulates SA, JA, ET, abscisic acid (ABA), and cytokinin signaling systems in the induction or suppression of plant immune responses. Interplay between auxin signaling system and G-protein-, mitogen-activated protein kinase (MAPK)-, reactive oxygen species (ROS), nitric oxide (NO)-, ion channels, and small RNA-mediated signaling systems has been reported. Auxin negatively or positively regulates plant defense responses, resulting in disease susceptibility or disease resistance. Several plant pathogens are known to modulate plant auxin signaling system to cause disease. Auxin, auxin response factors, auxin-responsive genes have been shown to modulate plant defense responses. Every event in auxin signaling and metabolism, including auxin biosynthesis, auxin perception, auxin conjugation, auxin transport, signaling derepression, and inactivation, has been correlated with plant defense or plant disease development.