SPOTTED LEAF 11 (SPL11) is a rice U-box/Arm protein identified by its loss-of-function phenotype (Zeng et al. 2004). The spl11 mutants display lesions resembling pathogen-induced cell death in the absence of pathogen attack. The spl11 mutants show enhanced resistance to fungal and bacterial pathogens, suggesting that, in contrast to the ACRE proteins, SPL11 is a negative regulator of cell death in the defence against pathogens (Zeng et al. 2004). Importantly, SPL11 can act as an E3 ubiquitin ligase in vitro (Zeng et al. 2004).
The data described show that targeted protein degradation by plant U-box/Arm proteins is key to regulating cell death programmes in different contexts. In particular, U-box/Arm proteins provide a balance between disease resistance and activation of cell death pathways in the plant, and act at multiple levels within plant disease signalling networks. Discovering the target proteins of ACRE/PUB/SPL11 ubiquitin ligases is critical to understanding how they regulate cell death.
Other Arm-repeat and related proteins also function in pathogen signalling: Arabidopsis SNI1, a negative regulator of pathogen responses, is related in structure to Arm proteins (Mosher et al. 2006), while MOS6 is an importin-a required for disease resistance (Palma et al. 2005). In addition, Arabidopsis AtPLA1, an acyl hydrolase with Arm repeats, promotes resistance to a necrotrophic fungus (Yang et al. 2007). Discovering the mechanisms of action of these diverse Arm proteins will enhance our understanding of how plants respond to pathogens and biotic stress.
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