Studies On Molecular Mechanisms Of Plant Immunity Suppression By The Pseudomonas Syringae Effector AvrRpt2

Plants are immune from most microbes due to perception of MAMPs (Microbe-Associated Molecular Patterns) by cell surface-localized pattern recognition receptors that transduce dangerous signals from outside into plant cells. Adapted pathogens evolve numerous effectors that are secreted into plant cells to suppress MAMP-triggered immunity (MTI), and thus favoring pathogens’ infection. Meanwhile, plants evolve intracellular receptors that directly or indirectly recognize effector proteins and trigger hypersensitive responses to protect plants from infection.Molecular mechanisms under virulence function(s) of the effector AvrRpt2were studied here using Arabidopsis and Pseudomonas syringae as a model pathosystem. It is known that AvrRpt2cleaves the host protein RIN4(Rpm1-interacting protein4) that is monitored by the corresponding resistance protein RPS2. AvrRpt2-mediated RIN4cleavage leads to the activation of RPS2-mediated signaling pathway. However, Pseudomonas syringae carrying avrRpt2suppresses MTI through virulence function(s) of AyrRpt2and is more virulent on rps2plants lacking RPS2. So far, it is not clear about molecular mechanisms of AvrRpt2virulence function(s). It was reported that AvrRpt2promotes Arabidopsis auxin biosynthesis and signaling. Besides, exogenous application of auxin promotes bacterial infection. Based on these experimental data, we proposed that AvrRpt2may be involved in the regulation of key components in auxin signaling. Thus, AvrRpt2effect on the stability of transcription repressors Aux/IAA and auxin receptor TIR1was investigated. The results showed that AvrRpt2promotes the degradation of Aux/IAA proteins but does not affect TIR1stability in Arabidopsis protoplast transfection systems and during bacterial infection. The degradation of Aux/IAA proteins promoted by AvrRpt2is proteasome-dependent. Interestingly, the dominant mutant axr2-1(AXR2P87S) is resistant to AvrRpt2-mediated degradation. Surprisingly, AvrRpt2acts additively with auxin to stimulate Aux/IAA turnover, suggesting distinct, yet proteasome-dependent, mechanisms operated by AvrRpt2and auxin to control Aux/IAA stability. The cysteine protease activity of AvrRpt2is also required for AvrRpt2-stimulated auxin signaling and Aux/IAA degradation. And AvrRpt2-mediated AXR2degradation is associated with AvrRpt2virulence function but not with its avirulence activity.Furthermore, a new mutant screening system using the type Ⅲ secretion system mutant hrcC of Pseudomonas syringae pv. tomato (Pst) and Arabidopsis pFRKl::Luciferase system was built up. Through the screening system, dozens of Arabidopsis Aggie(Arabidopsis genes governing immune gene expressions) mutants that are altered in plant immunity were obtained. In one of these mutants, aggie101, pFRKl::Luciferase is highly induced upon inoculation of Pst DC3000hrcC while aggie101is insensitive to the treatment of flg22(the conserved22-amino-acid peptide from the N terminus of flagellin of bacteria). Unexpectedly, aggie101is more resistant to the infection of virulent P. syringae pv. tomato and P. syringae pv. maculicola.In summary, promoting auxin response via modulating the stability of the key transcription repressors Aux/IAA is an important virulence strategy used by the bacterial type Ⅲ effector AvrR.pt2to render plants more susceptible to bacterial pathogens. Besides, Arabidopsis Aggie mutants provide new materials to identify new components in MTI signaling and to further understand molecular mechanisms of plant immunity.