The Arabidopsis homologs of CCR4-associated factor 1 show mRNA deadenylation activity and play a role in plant defence responses

Messenger RNA (mRNA) turnover in eukaryotic cells begins with shortening of the poly (A) tail at the 3' end, a process called deadenylation. In yeast, the deadenylation reaction is predominantly mediated by CCR4 and CCR4-associated factor 1 (CAF1), two components of the well-characterised protein complex named CCR4-NOT. We report here that AtCAF1a and AtCAF1b, putative Arabidopsis homologs of the yeast CAF1 gene, partially complement the growth defect of the yeast caf1 mutant in the presence of caffeine or at high temperatures. The expression of AtCAF1a and AtCAF1b is induced by multiple stress-related hormones and stimuli. Both AtCAF1a and AtCAF1b show deadenylation activity in vitro and point mutations in the predicted active sites disrupt this activity. T-DNA insertion mutants disrupting the expression of AtCAF1a and/or AtCAF1b are defective in deadenylation of stress-related mRNAs, indicating that the two AtCAF1 proteins are involved in regulated mRNA deadenylation in vivo. Interestingly, the single and double mutants of AtCAF1a and AtCAF1b show reduced expression of pathogenesis-related (PR) genes PR1 and PR2 and are more susceptible to Pseudomonas syringae pv tomato DC3000 (Pst DC3000) infection, whereas transgenic plants over-expressing AtCAF1a show elevated expression of PR1 and PR2 and increased resistance to the same pathogen. Our results suggest roles of the AtCAF1 proteins in regulated mRNA deadenylation and defence responses to pathogen infections.