1Rice Research Institute and Research Center for Major Crop Diseases, Sichuan Agricultural University, Chengdu, China. However, the underlying regulatory mechanism remains largely elusive.ģCollaborative Innovation Center for Hybrid Rice in Yangtze River Basin, Sichuan Agricultural University, Chengdu, ChinaĮctopic expression of the Arabidopsis RESISTANCE TO POWDERY MILDEW8.1 ( RPW8.1) boosts pattern-triggered immunity leading to enhanced resistance to different pathogens in Arabidopsis and rice.2Institute for Bioscience and Biotechnology Research and Department of Plant Sciences and Landscape Architecture, University of Maryland, College Park, College Park, MD, United States. Here, we report that XAP5 CIRCADIAN TIMEKEEPER ( XCT, At2g21150) positively regulates RPW8.1-mediated cell death and disease resistance. Forward genetic screen identified the b3-17 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 XCT in b3-17, and introduced into a stop codon after two codons leading to a truncated XCT. XCT has previously been identified as a circadian clock gene required for small RNA biogenesis and acting down-stream of ETHYLENE-INSENSITIVE3 ( EIN3) in the ethylene-signaling pathway. Here we further showed that mutation or down-regulation of XCT by artificial microRNA reduced RPW8.1-mediated immunity in R1Y4, a transgenic line expressing RPW8.1-YFP from the RPW8.1 native promoter. On the contrary, overexpression of XCT in R1Y4 background enhanced RPW8.1-mediated cell death, H 2O 2 production and resistance against powdery mildew. Consistently, the expression of RPW8.1 was down- and up-regulated in xct mutant and XCT overexpression lines, respectively. Taken together, these results indicate that XCT positively regulates RPW8.1-mediated cell death and disease resistance, and provide new insight into the regulatory mechanism of RPW8.1-mediated immunity. To prevent the invasion of pathogenic microbes, plants have evolved two major defense systems in addition to pre-formed barriers such as cell walls and leaf hairs ( Jones and Dangl, 2006). The first system is termed pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), which is activated when the receptors on the surface of host cells perceive conserved PAMPs ( Zipfel et al., 2006 Boller and Felix, 2009). PTI is featured by a series of defense responses, including burst of reactive oxygen species (ROS), deposition of callose, induction of defense–related genes ( Ebel and Mithöfer, 1998 Asai et al., 2002 Zipfel et al., 2006). However, adapted pathogens can subvert PTI by using virulent effectors ( Dou and Zhou, 2012). In turn, plants employ the second defense system, called effector-triggered immunity (ETI) that is activated upon recognition of pathogen effectors by plant intracellular nucleotide-binding and leucine-rich repeat receptors (NLRs) known as resistance (R) proteins ( Spoel and Dong, 2012 Dangl et al., 2013). Most identified plant R genes encode proteins possessing an intracellular nucleotide-binding site and leucine-rich repeat (NBS-LRR) domain ( Bonardi et al., 2012) or an extracellular LRR (eLRR) domain ( Dangl and Jones, 2001).ĭefense responses in ETI are stronger than those in PTI and often culminate in hypersensitive response (HR) at the infection site ( Greenberg and Yao, 2004 Dangl et al., 2013). The R proteins can activate race-specific resistance via direct or indirect interaction with their cognate effectors ( Dodds et al., 2006 Krasileva et al., 2010). A few R genes encode atypical R proteins which are structurally different from the typical R proteins (NBS-LRRs and eLRRs), and mediate broad-spectrum and/or durable resistance to single or multiple pathogens. For example, tomato Pto encodes a serine-threonine protein kinase conferring resistance to Pseudomonas syringae pv. Wheat Lr34 encodes a putative ABC transporter protein conferring resistance to wheat rust and powdery mildew ( Krattinger et al., 2009). Barley Rpg1 encodes a receptor kinase-like protein with two tandem kinase domains conferring resistance to barley stem rust fungus ( Brueggeman et al., 2002). The Arabidopsis RESISTANCE TO POWDERY MILDE RPW8.1 ( RPW8.1) and RPW8.2 encode non-NLR R protein with a putative trans-membrane or signal peptide domain and one or two coiled-coil motifs ( Xiao et al., 2001 Wang et al., 2013).
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