Dynamics of Defense Responses and Cell Fate Change during Arabidopsis-Pseudomonas syringae Interactions

Abstract

Plant-pathogen interactions involve sophisticated action and counteraction strategies from both parties. Plants can recognize pathogen derived molecules, such as conserved pathogen associated molecular patterns (PAMPs) and effector proteins, and subsequently activate PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI), respectively. However, pathogens can evade such recognitions and suppress host immunity with effectors, causing effector-triggered susceptibility (ETS). The differences among PTI, ETS, and ETI have not been completely understood. Toward a better understanding of PTI, ETS, and ETI, we systematically examined various defense-related phenotypes of Arabidopsis infected with different Pseudomonas syringae pv. maculicola ES4326 strains, using the virulence strain DG3 to induce ETS, the avirulence strain DG34 that expresses avrRpm1 (recognized by the resistance protein RPM1) to induce ETI, and HrcC- that lacks the type three secretion system to activate PTI. We found that plants infected with different strains displayed dynamic differences in the accumulation of the defense signaling molecule salicylic acid, expression of the defense marker gene PR1, cell death formation, and accumulation/localization of the reactive oxygen species, H2O2. The differences between PTI, ETS, and ETI are dependent on the doses of the strains used. These data support the quantitative nature of PTI, ETS, and ETI and they also reveal qualitative differences between PTI, ETS, and ETI. Interestingly, we observed the induction of large cells in the infected leaves, most obviously with HrcC- at later infection stages. The enlarged cells have increased DNA content, suggesting a possible activation of endoreplication. Consistent with strong induction of abnormal cell growth by HrcC-, we found that the PTI elicitor flg22 also activates abnormal cell growth, depending on a functional flg22-receptor FLS2. Thus, our study has revealed a comprehensive picture of dynamic changes of defense phenotypes and cell fate determination during Arabidopsis-P. syringae interactions, contributing to a better understanding of plant defense mechanisms.

Differential Roles of Two Homologous Cyclin-Dependent Kinase Inhibitor Genes in Regulating Cell Cycle and Innate Immunity in Arabidopsis

“…Our data presented here and in earlier studies (Vanacker et al, 2001;Hamdoun et al, 2013) suggest SA-mediated pathway is an integral part of cell-cycle signaling. The lack of SA biosynthesis and signaling in sid2-1 and npr1-1, respectively, had reduced overall ploidy pattern of leaf cells when assayed by flow cytometry (Fig.…”

“…Interestingly, our previous study using nuclear quantification of mesophyll cells from fixed sections near the middle vein of npr1-1 showed increased ploidy, suggesting a possibility that npr1-1 leaves harbor cells with different ploidy. Both sid2-1 and npr1-1 mutants were also compromised in P. syringae-induced large cell growth (Hamdoun et al, 2013). Since activation of SA signaling with exogenous application of an SA analog did not change cell ploidy and the formation of tumor-like growth in the leaf (Supplemental Fig.…”

“…Since activation of SA signaling with exogenous application of an SA analog did not change cell ploidy and the formation of tumor-like growth in the leaf (Supplemental Fig. S5; Hamdoun et al, 2013), it is possible that SA signaling requires the activation of additional pathways to regulate cell-cycle progression.…”

“…Pathogens, such as the bacteria Pseudomonas syringae and Xanthomonas citri, the actinomycete Rhodococcus fascians, the fungal pathogen powdery mildew, and nematodes, can induce endoreplication in some cells at or near the infected loci of host tissue (Swarup et al, 1991;Chandran et al, 2010;Wildermuth, 2010;de Almeida Engler and Gheysen, 2013;Hamdoun et al, 2013). Consistent with pathogeninduced endoreplication in the host, some pathogen effectors are known to interact with host cell-cycle components, either through direct physical proteinprotein interaction and/or through modulating expression of genes critical for cell-cycle progression (Kay et al, 2007;Mukhtar et al, 2011).…”

Endopolyploidization and flowering time are antagonistically regulated by checkpoint component MAD1 and immunity modulator MOS1

“…35). Increased nuclear DNA content was observed in leaf cells during infection by bacterial pathogen, suggesting an involvement of endocycle modulation either in defense or susceptibility responses 36 . It is therefore possible that the downregulation of SNC1 expression in mos1 mutants is due to a perturbed cell cycle progression.…”