Mutation of a Single Core Gene, tssM, of Type VI Secretion System of Xanthomonas perforans Influences Virulence, Epiphytic Survival, and Transmission During Pathogenesis on Tomato

Abstract

Xanthomonas perforans is a seed-borne hemi-biotrophic pathogen that successfully establishes infection in the phyllosphere of tomato. While the majority of the studies investigating mechanistic basis of pathogenesis have focused on successful apoplastic growth, factors important during asymptomatic colonization in the early stages of disease development are not well understood. In this study, we show that tssM gene of the type VI secretion system cluster i3* (T6SS-i3*) plays a significant role during initial asymptomatic epiphytic colonization at different stages during the life cycle of the pathogen. Mutation in a core gene, tssM of T6SS-i3*, imparted higher aggressiveness to the pathogen, as indicated by higher overall disease severity, higher in planta growth as well as shorter latent infection period compared to the wild-type upon dip-inoculation of 4-5-week-old tomato plants. Contribution of tssM towards aggressiveness was evident during vertical transmission from seed-to-seedling with wild-type showing reduced disease severity as well as lower in planta populations on seedlings compared to the mutant. Presence of functional TssM offered higher epiphytic fitness as well as higher dissemination potential to the pathogen when tested in an experimental setup mimicking transplant house high-humidity conditions. We showed higher osmotolerance being one mechanism by which TssM offers higher epiphytic fitness. Taken together, these data reveal that functional TssM plays a larger role in offering ecological advantage to the pathogen. TssM prolongs the association of hemi-biotrophic pathogen with the host, minimizing overall disease severity, yet facilitating successful dissemination.

Comparative Genomic Analysis of the Lettuce Bacterial Leaf Spot Pathogen, Xanthomonas hortorum pv. vitians, to Investigate Race Specificity

“…The type VI secretion system (T6SS) in plant pathogenic bacteria has antimicrobial ability; they are used to directly inject toxins into competitor bacteria cells to reduce competition for resources ( Bernal et al, 2018 ). The contribution of T6SS in plant host interactions, however, has not yet been investigated ( Liyanapathiranage et al, 2021 ). A cluster of thirteen type six secretion genes, tssA-tssM , encode the system and were formerly referred to as imp ( imp aired in nitrogen fixation) genes.…”

HpaP divergently regulates the expression of hrp genes in Xanthomonas oryzae pathovars oryzae and oryzicola

“…The type VI secretion system (T6SS) is defined as a cell envelope‐spanning machine that bacteria use to translocate toxic effector proteins into eukaryotic and prokaryotic cells (Costa et al, 2015 ). Recent works have demonstrated that the T6SS is involved in virulence or bacterial competition in several Xanthomonas pathogens (Choi et al, 2020 ; Liyanapathiranage et al, 2022 ; Montenegro Benavides et al, 2021 ; Zhu et al, 2020 ). Here, the transcription profile showed that the HpaP in Xoo and Xoc affects the T6SS.…”

Phylogenetic Distribution and Evolution of Type VI Secretion System in the Genus Xanthomonas

“…Evolutionary history of X. axonopodis complex has been recently well studied, where it has been proposed that the first step of generalist diversification into the X. axonopodis subgroups was followed by the second step of ecology-driven specialization that favored the emergence of novel pathovars ( Mhedbi-Hajri et al, 2013 ). Given our prior work that suggested the potential role of core gene tssM of cluster T6 i3*, in overall epiphytic fitness in X. perforans and during initial stages of disease development ( Liyanapathiranage et al, 2021 ), we hypothesized that during diversification into X. axonopodis subgroups, a combination of specific T6SS clusters in individual species provided significant advantage during the process of niche adaptation onto specific hosts. We closely examined the patterns of the evolutionary history of the i3* and i3*** T6 clusters in the X. axonopodis complex subspecies groups and the lifestyle of these pathogens.…”

“…The role of i3* has been suggested not only in competition against resident flora, more specifically, resistance to predation by the bacterivorous amoeba, Dictyostelium discoideum , but also in adaptation to the phyllosphere non-vascular environment ( Bayer-Santos et al, 2018 ; Liyanapathiranage et al, 2021 ). Loss of i3* in lineages containing X. oryzae and X. vasicola , both vascular pathogens, might be in response to the vascular lifestyle.…”