Plants also activate cell death during pathogen infection. Cell death is induced during disease-causing pathogen infection (a "basal" pathogen response), and also during the hypersensitive response (HR), an immune response where localised cell death occurs at the infection site to prevent spread of the pathogen through the plant. Thus, plants are resistant to pathogens that they can mount the HR against. Regulated protein degradation is important for pathogen responses. Studies in several plants have identified U-box/Arm genes rapidly induced during pathogen infection, which are essential for the HR and disease resistance. These genes are the tobacco and tomato ACRE276 (for Avr9/Cf-9 rapidly elicited), which is homologous to Arabidopsis AtPUB17; and also tobacco/tomato ACRE74 (Arabidopsis counterparts AtPUB20 and AtPUB21, parsley counterpart CMPG1) (Kirsch et al. 2001; Heise et al. 2002; Gonzalez-Lamothe et al. 2006; Yang et al. 2006). Reduction of ACRE276 or ACRE74 function abolishes tobacco and tomato HR, generating plants no longer resistant to leaf mould fungus. Correspondingly, Arabidopsis atpub17 mutants have compromised resistance to bacterial infection. Thus, like ARC1, these Arm-repeat proteins are positive regulators of cell death (Gonzalez-Lamothe et al. 2006; Yang et al. 2006).
Both ACRE276/AtPUB17 and ACRE74 function as ubiquitin ligases in vitro (Gonzalez-Lamothe et al. 2006; Yang et al. 2006). In addition, U-box mutant forms of AtPUB17 cannot promote HR showing that E3 ligase activity is required in vivo (Yang et al. 2006). Interestingly, overexpression of ACRE74 increases the HR, and ACRE74 may be a limiting factor for HR-induced cell death. However, overexpression of U-box mutant ACRE74 acts in a dominant negative fashion, reducing the HR (Gonzalez-Lamothe et al. 2006). ACRE74 is required for resistance to Pseudomonas and Phytophthora elicitor proteins in tobacco and is thus likely to be involved in a widespread range of immunity responses (Gonzalez-Lamothe et al. 2006).
Interestingly, AtPUB17 is the closest Arabidopsis homologue of ARC1 (Azevedo et al. 2001), suggesting that the same U-box/Arm protein has been co-opted to function in more than one plant cell death pathway, as may be the case for AtPUB8 (see Sect. 4.1).
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