Abscisic Acid Signaling System in Plant Innate Immunity

Abstract

Abscisic acid is a plant hormone involved in immune signal transduction system, and it positively or negatively regulates the plant immune system. On the perception of pathogen-associated molecular pattern (PAMP) signals, ABA biosynthesis pathway is activated and ABA accumulates in the infected tissues. The ABA signaling complex includes PYR/PYL/RCAR (an ABA receptor), the type 2C protein phosphatase (PP2C, a negative regulator), and SNF1-related protein kinase (SnRK2, a positive regulator). Binding of ABA to the receptor induces a conformational change in the receptor that allows it to sequester the PP2Cs. This excludes the PP2Cs from the negative regulation of the downstream SnRK2-activated kinases which phosphorylate and activate downstream transcription factors. Phosphorylation of the transcription factors initiates the transcription of ABA-responsive promoter elements. ABA regulates the expression of several genes involved in host defense responses. The cis-regulatory elements responsible for the ABA regulation of gene expression share a conserved motif, ACGTGGC, which is known as ABA-responsive element (ABRE). ABRE appears in the promoters of many defense genes. The bZIP-type transcription factors bind ABRE and transactivate downstream gene expression. Transcription factors perceive signals relayed by signaling molecules and translate into a functional response through recruiting or releasing RNA polymerase II (RNAPII). MEDIATOR25 (MED25), a cofactor of RNAPII, acts as a bridge between RNAPII and DNA-binding transcription factors and regulates ABA-triggered gene transcription. ABA has been shown to be a complex modulator of various plant hormone signaling systems in triggering or suppressing host defense responses. Synergistic and antagonistic interaction between ABA and JA, SA, and ET in the modulation of plant immune system has been widely reported. ABA and JA signaling pathways may be interconnected, and they may act cooperatively in the induction of defense genes. ABA may also suppress JA-activated transcription of defense genes in some plant–pathogen interactions. ABA may suppress SA biosynthesis and SA-mediated defense responses. ABA signaling system suppresses SA signaling system and vice versa. In some plant–pathogen interactions, ABA signaling may act synergistically with SA signaling in triggering plant immune responses. ABA activates ethylene biosynthesis, while ethylene activates ABA biosynthesis. Synergistic and antagonistic interaction between ABA and ethylene signaling systems has also been reported. ABA induces resistance against many necrotrophic pathogens, while it negatively regulates defense responses against biotrophic pathogens (with a few exceptions). ABA has been shown to promote susceptibility or resistance against bacterial pathogens, and ABA plays positive roles during early stages and negative roles at later stages of bacterial infection. ABA signaling induces resistance or susceptibility against virus diseases. Different ABA signaling pathway components may decide induction or suppression of defense responses against virus diseases. ABA may regulate host defense responses either positively or negatively through its direct or indirect action on the plant innate immune system. ABA may suppress plant immune responses and induce susceptibility. Pathogens may hijack ABA signaling pathway to cause disease. Pathogens may secrete toxins/effectors and suppress ABA-dependent defense responses.