Cytokinin-induced pathogen resistance and BABA-induced priming in plant immunity: Elucidation of the underlying mechanisms and their distinct effect on the plant growth
- Cytokinin-induced pathogen resistance and BABA-induced priming in plant immunity: Elucidation of the underlying mechanisms and their distinct effect on the plant growth
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- Microbial diseases and pests are major threats to food production. As many biotrophic pathogens secrete plant growth hormones such as cytokinins to augment the sink activity of infected regions, plant defense responses suppress growth hormone signaling to enhance plant immunity. Accordingly, an antagonistic relationship between plant immunity and growth has been suggested, and referred to as allocation costs. In this study, it was found that plant-derived cytokinins, recognized by AHK2 and AHK3, promote resistance of Arabidopsis to Pseudomonas syringae pv. tomato DC3000 (Pst). The cytokinin-activated transcription factor ARR2 contributes specifically to Pst resistance, and the SA response factor TGA3 binds and recruits ARR2 to the PR gene promoters. Other than PR genes, ARR2 induced the transcription of ER and cytosolic protein quality control components such as BiP2, CRT3, ERdj3B, SGT1, and HSP90, which play indispensible roles in microbe-associated molecular pattern (MAMP) triggered immunity (MTI) and R-gene-mediated defense. Hence, ARR2 reinforced plant immunity at both the transcriptional level and the post-translational level. Decreased DNA methylation 1 (DDM1) is a chromatin remodeling factor affecting DNA and histone methylation. It was discovered that the ddm1 knockout hyper-activated priming, a sensitization of the plant defense response, triggered by beta-aminobutyric acid (BABA). In contrast to cytokinin-induced immunity, ddm1 and BABA may reduce CG methylation of pericentromeric regions, ICS1, PAD4 and R-gene loci to activate transcription. However, because DNA methylation of heterochromatin is required for cell cycle progression, it is possible that demethylation induced by ddm1 and BABA-dependent priming suppresses cell cycle progression. Taken together, these results show that although both cytokinins and BABA/ddm1 enhance defense gene transcription, cytokinins and ARR2 cooperatively modulate defense signaling at both the transcriptional level and the post-translational level to efficiently reinforce immunity, whereas BABA/ddm1 suppresses DNA methylation for transcriptional induction, which may disrupt cell cycle progression, resulting in retarded growth.
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