Genetic and Biochemical Analysis of Innate Immunity in Arabidopsis thaliana

Perception of evolutionarily conserved pathogen-associated molecular patterns (PAMPs) elicits rapid and profound transcriptional reprogramming in hosts and activates defense to pathogen attack. The molecular signaling networks underlying this plant pattern-triggered immunity (PTI) remain fragmented. We identified a series of mutants with altered pFRK1::LUC activity were identified and named as Arabidopsis genes governing immune gene expression (aggie) through forward genetic screening. Map-based cloning identified Aggie1 as encoding Arabidopsis C-terminal domain (CTD) phosphatase-like 3 (CPL3), a homolog of yeast FCP1 phosphatase that dephosphorylates the CTD of RNA polymerase II (RNAPII) during the transcription cycle. MAMP perception induced a rapid and transient CTD phosphorylation in Arabidopsis, underlying the modulation of CTD phosphorylation dynamics controlling plant immune responsive gene expression. Aggie1/CPL3 specifically dephosphorylated Ser2 of the CTD in vivo and in vitro and preferentially interacted with phosphorylated CTD. Transcriptional analysis indicates that cpl3 showed overall enhanced flg22-mediated transcription responses. Thus, Aggie1 negatively regulates immune responsive gene expression essential for suppression of pathogen growth by modulating the phosphorylation status of RNAPII CTD. Cyclin-dependent kinases C (CDKC) functions as RNAPII kinases. Interestingly, we also found the silencing of cdkc1 and cdkc2 in wild type reduced flg22-mediated transcription responses and the plants were more susceptible to Pseudomonas syringae DC3000, suggesting their positive role in PAMP-triggered immunity.

Temperature fluctuation is a key determinant for microbial invasion into the host and for host evasion of the microbe. In contrast to mammals that maintain constant body temperature, plant internal temperature oscillates on a daily basis. It remains elusive how plants operate inducible defenses in response to temperature fluctuation. We report that ambient temperature changes lead to pronounced shifts of two distinct plant immune responses: pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). Plants preferentially activate ETI signaling at relatively lower temperatures (10~23?C), whereas they switch to PTI signaling at moderately elevated temperatures (23~32?C). The Arabidopsis arp6 and hta9hta11 mutants, phenocopying plants grown at the elevated temperatures, exhibit enhanced PTI and yet reduced ETI responses. As the secretion of bacterial effectors favors low temperatures whereas bacteria multiply vigorously at elevated temperatures accompanied with increased MAMP production, our findings suggest that temperature oscillation might have driven dynamic co-evolution of distinct plant immune signaling responding to pathogen physiological changes.