AUTHOR=Xu Yong-Ju , Lei Yang , Li Ran , Zhang Ling-Li , Zhao Zhi-Xue , Zhao Jing-Hao , Fan Jing , Li Yan , Yang Hui , Shang Jing , Xiao Shunyuan , Wang Wen-Ming 

TITLE=XAP5 CIRCADIAN TIMEKEEPER Positively Regulates RESISTANCE TO POWDERY MILDEW8.1–Mediated Immunity in Arabidopsis

JOURNAL=Frontiers in Plant Science

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.02044

DOI=10.3389/fpls.2017.02044

ISSN=1664-462X

ABSTRACT=<p>Ectopic expression of the Arabidopsis <italic>RESISTANCE TO POWDERY MILDEW8.1</italic> (<italic>RPW8.1</italic>) boosts pattern-triggered immunity leading to enhanced resistance to different pathogens in Arabidopsis and rice. However, the underlying regulatory mechanism remains largely elusive. Here, we report that <italic>XAP5 CIRCADIAN TIMEKEEPER</italic> (<italic>XCT, At2g21150</italic>) positively regulates <italic>RPW8.1</italic>-mediated cell death and disease resistance. Forward genetic screen identified the <italic>b3-17</italic> mutant that exhibited less cell death and susceptibility to powdery mildew and bacterial pathogens. Map-based cloning identified a G-to-A point mutation at the 3′ splice site of the 8th intron, which resulted in splice shift to 8-bp down-stream of the original splice site of <italic>XCT</italic> in <italic>b3-17</italic>, and introduced into a stop codon after two codons leading to a truncated XCT. <italic>XCT</italic> has previously been identified as a circadian clock gene required for small RNA biogenesis and acting down-stream of <italic>ETHYLENE-INSENSITIVE3</italic> (<italic>EIN3</italic>) in the ethylene-signaling pathway. Here we further showed that mutation or down-regulation of <italic>XCT</italic> by artificial microRNA reduced <italic>RPW8.1</italic>-mediated immunity in R1Y4, a transgenic line expressing <italic>RPW8.1-YFP</italic> from the <italic>RPW8.1</italic> native promoter. On the contrary, overexpression of <italic>XCT</italic> in R1Y4 background enhanced <italic>RPW8.1</italic>-mediated cell death, H<sub>2</sub>O<sub>2</sub> production and resistance against powdery mildew. Consistently, the expression of <italic>RPW8.1</italic> was down- and up-regulated in <italic>xct</italic> mutant and <italic>XCT</italic> overexpression lines, respectively. Taken together, these results indicate that <italic>XCT</italic> positively regulates <italic>RPW8.1</italic>-mediated cell death and disease resistance, and provide new insight into the regulatory mechanism of <italic>RPW8.1</italic>-mediated immunity.</p>