Downstream of a Kinase Cascade: A Trihelix Transcription Factor Represses Immune Genes

Immunity in plants relies in large part on signaling mediated by protein kinase cascades (reviewed in Tena et al., 2011). Cascades that lead to pattern-triggered immunity are initiated by the recognition of microbe-associated molecular patterns (MAMPs), such as flg22, by receptors such as FLAGELLIN-SENSING2 (FLS2). Whereas much is known about the phosphorylation and signaling events induced upon the perception of MAMPs, it is less clear how the downstream transcriptional reprogramming is executed. New work from Li et al. (2015) reveals that a trihelix transcription factor called ARABIDOPSIS SH4-RELATED3 (ASR3) is involved in regulating this reprogramming.

ASR3 was identified in a screen for Arabidopsis thaliana genes regulating the expression of genes related to immunity (so-called immune genes) and encodes a member of the SH4 clade of the trihelix transcription factor family (see figure; Kaplan-Levy et al., 2012). Li et al. found that the asr3 null mutant had increased expression of MAMP-responsive genes upon treatment with flg22 and displayed increased resistance to infection with virulent bacterial pathogens. Accordingly, ASR3 appears to be a negative regulator of immune responses.

Functional domains of ASR3. Li et al. used biochemical and genetic analyses to determine the biochemical activities (blue boxes) of the ASR3 trihelix transcription factor. (Adapted from Li et al. [2015], Figure 5.)

Intriguingly, ASR3 was phosphorylated upon treatment with flg22, and inhibitor treatments indicated that a mitogen-activated protein kinase (MAPK) cascade was responsible for this phosphorylation. Li et al. screened all 20 MAPKs (MPKs) from Arabidopsis for activity on ASR3, finding that MPK4 had the highest activity and that ASR3 was not phosphorylated in the mpk4 mutant. In Arabidopsis, a MAPK cascade involving MPK4 suppresses pattern-triggered immunity, allowing plants to fine-tune immune responses. The finding that MPK4 mediates the MAMP-triggered phosphorylation of ASR3 suggests that ASR3 acts downstream of this MAPK cascade to repress pattern-triggered immune responses.

Consistent with a function as a transcription factor, ASR3 localizes to the nucleus. Other trihelix transcription factors bind GT-like elements in the promoters of their downstream genes (Kaplan-Levy et al., 2012). ASR3 was shown to bind to similar elements in the promoter of the early immune response gene FLG22-INDUCED RECEPTOR-LIKE KINASE1, and this binding was increased by flg22 treatment. Furthermore, RNA-seq experiments using asr3 mutant plants and plants overexpressing ASR3 revealed that ASR3 negatively regulates a large portion of the genes that respond to flg22.

This beautifully detailed genetic and biochemical characterization (see figure) of ASR3 establishes it as an important mediator of the negative regulation of pattern-triggered immunity. It also helps elucidate how immune responses are both positively and negatively regulated by MAPK cascades. Furthermore, this work advances our understanding of trihelix transcription factors, which have been relatively poorly studied but are known to function in development and responses to the environment (Kaplan-Levy et al., 2012).