Fortunella margarita Transcriptional Reprogramming Triggered by Xanthomonas citri subsp. citri

Abstract

BackgroundCitrus canker disease caused by the bacterial pathogen Xanthomonas citri subsp. citri (Xcc) has become endemic in areas where high temperature, rain, humidity, and windy conditions provide a favourable environment for the dissemination of the bacterium. Xcc is pathogenic on many commercial citrus varieties but appears to elicit an incompatible reaction on the citrus relative Fortunella margarita Swing (kumquat), in the form of a very distinct delayed necrotic response. We have developed subtractive libraries enriched in sequences expressed in kumquat leaves during both early and late stages of the disease. The isolated differentially expressed transcripts were subsequently sequenced. Our results demonstrate how the use of microarray expression profiling can help assign roles to previously uncharacterized genes and elucidate plant pathogenesis-response related mechanisms. This can be considered to be a case study in a citrus relative where high throughput technologies were utilized to understand defence mechanisms in Fortunella and citrus at the molecular level.ResultscDNAs from sequenced kumquat libraries (ESTs) made from subtracted RNA populations, healthy vs. infected, were used to make this microarray. Of 2054 selected genes on a customized array, 317 were differentially expressed (P < 0.05) in Xcc challenged kumquat plants compared to mock-inoculated ones. This study identified components of the incompatible interaction such as reactive oxygen species (ROS) and programmed cell death (PCD). Common defence mechanisms and a number of resistance genes were also identified. In addition, there were a considerable number of differentially regulated genes that had no homologues in the databases. This could be an indication of either a specialized set of genes employed by kumquat in response to canker disease or new defence mechanisms in citrus.ConclusionFunctional categorization of kumquat Xcc-responsive genes revealed an enhanced defence-related metabolism as well as a number of resistant response-specific genes in the kumquat transcriptome in response to Xcc inoculation. Gene expression profile(s) were analyzed to assemble a comprehensive and inclusive image of the molecular interaction in the kumquat/Xcc system. This was done in order to elucidate molecular mechanisms associated with the development of the hypersensitive response phenotype in kumquat leaves. These data will be used to perform comparisons among citrus species to evaluate means to enhance the host immune responses against bacterial diseases.

Surface Barriers of Mandarin ‘Okitsu’ Leaves Make a Major Contribution to Canker Disease Resistance

“…The replacement of citrus cultivars in current cultivation with those having full or partial resistance represents a sustainable strategy to manage the disease (39). However, characterization of citrus resistance to X. citri has only recently begun and is not completely understood (10,12,15,23,24,26,41). In this work, we have revealed some of the factors that contribute to the enhanced resistance to citrus canker disease of 'Okitsu' mandarin under both field and controlled-growth conditions.…”

“…Pathogenic and histo-anatomic assays suggest that plant preformed defenses, involving physical barriers such as cell wall and membrane ultrastructure, waxy cuticles, and the number and anatomy of the stomata, could be involved in resistance to bacterial invasion (10,22,41). However, artificial inoculation through infiltration or pin prick (which bypasses the preformed defenses) on resistant 'Chinese' citron (C. medica) and 'Nagami' kumquat (Fortunella margarita) suggests that the specific recognition of a X. citri virulence effector could also occur in the plant (12,23,24).…”

Comparative RNA-Seq Analysis of Early-Infected Peach Leaves by the Invasive Phytopathogen Xanthomonas arboricola pv. pruni

“…in the A. thaliana – P. syringae pathosystem [59], but also in kumquat leaves challenged by X. citri subsp. citri [60]. It was hypothesized that the down-regulation of genes involved in photosynthesis could induce a hypersensitive response following the infection [60].…”

“…citri [60]. It was hypothesized that the down-regulation of genes involved in photosynthesis could induce a hypersensitive response following the infection [60]. This could also be due to a strategy of the plant to limit the availability of sugars for the pathogen, to fitness costs for the plant which has to reallocate for defense [61], or to protect the photosynthetic apparatus against oxidative damage [62].…”

PthA4AT, a 7.5‐repeats transcription activator‐like (TAL) effector fromXanthomonas citrissp.citri, triggers citrus canker resistance

“…Several reports suggest specific recognition of effectors by host gene products that render plants resistant to X. citri , i.e. interactions in which the effector acts as an avirulence factor (Chen et al ., ; Deng et al ., ; Khalaf et al ., ; Lee et al ., ). Recently, we have isolated a X. citri strain ( X. citri A T ) that triggers a host‐specific defence response in Citrus limon that is associated with the interference of biofilm development and arrest of bacterial growth (Chiesa et al ., ; Roeschlin et al ., ).…”