Innate immunity plays a vital role in protecting plants and animals from pathogen infections. Immunity varies with age in both animals and plants. However, little is known about the ontogeny of plant innate immunity during seedling development. We report here that the Arabidopsis (Arabidopsis thaliana) microRNA miR172b regulates the transcription of the immune receptor gene FLAGELLIN-SENSING2 (FLS2) through TARGET OF EAT1 (TOE1) and TOE2, which directly bind to the FLS2 promoter and inhibit its activity. The level of miR172b is very low in the early stage of seedling development but increases over time, which results in decreased TOE1/2 protein accumulation and, consequently, increased FLS2 transcription and the ontogeny of FLS2-mediated immunity during seedling development. Our study reveals a role for the miR172b-TOE1/2 module in regulating plant innate immunity and elucidates a regulatory mechanism underlying the ontogeny of plant innate immunity.

Immune responses are triggered when pattern recognition receptors (PRRs) recognize microbial molecular patterns. The Arabidopsis (Arabidopsis thaliana) receptor-like cytoplasmic kinase BOTRYTIS-INDUCED KINASE1 (BIK1) acts as a signaling hub of plant immunity. BIK1 homeostasis is maintained by a regulatory module in which CALCIUM-DEPENDENT PROTEIN KINASE28 (CPK28) regulates BIK1 turnover via the activities of two E3 ligases. Immune-induced alternative splicing of CPK28 attenuates CPK28 function. However, it remained unknown whether CPK28 is under proteasomal control. Here, we demonstrate that CPK28 undergoes ubiquitination and 26S proteasome-mediated degradation, which is enhanced by flagellin treatment. Two closely related ubiquitin ligases, ARABIDOPSIS TÓXICOS EN LEVADURA31 (ATL31) and ATL6, specifically interact with CPK28 at the plasma membrane; this association is enhanced by flagellin elicitation. ATL31/6 directly ubiquitinate CPK28, resulting in its proteasomal degradation. Furthermore, ATL31/6 promote the stability of BIK1 by mediating CPK28 degradation. Consequently, ATL31/6 positively regulate BIK1-mediated immunity. Our findings reveal another mechanism for attenuating CPK28 function to maintain BIK1 homeostasis and enhance immune responses.