Transcriptional analysis of the sweet orange interaction with the citrus canker pathogensXanthomonas axonopodispv.citriandXanthomonas axonopodispv.aurantifolii

Abstract

Xanthomonas axonopodis pv. citri (Xac) and Xanthomonas axonopodis pv. aurantifolii pathotype C (Xaa) are responsible for citrus canker disease; however, while Xac causes canker on all citrus varieties, Xaa is restricted to Mexican lime, and in sweet oranges it triggers a defence response. To gain insights into the differential pathogenicity exhibited by Xac and Xaa and to survey the early molecular events leading to canker development, a detailed transcriptional analysis of sweet orange plants infected with the pathogens was performed. Using differential display, suppressed subtractive hybridization and microarrays, we identified changes in transcript levels in approximately 2.0% of the approximately 32,000 citrus genes examined. Genes with altered expression in response to Xac/Xaa surveyed at 6 and 48 h post-infection (hpi) were associated with cell-wall modifications, cell division and expansion, vesicle trafficking, disease resistance, carbon and nitrogen metabolism, and responses to hormones auxin, gibberellin and ethylene. Most of the genes that were commonly modulated by Xac and Xaa were associated with basal defences triggered by pathogen-associated molecular patterns, including those involved in reactive oxygen species production and lignification. Significantly, we detected clear changes in the transcriptional profiles of defence, cell-wall, vesicle trafficking and cell growth-related genes in Xac-infected leaves between 6 and 48 hpi. This is consistent with the notion that Xac suppresses host defences early during infection and simultaneously changes the physiological status of the host cells, reprogramming them for division and growth. Notably, brefeldin A, an inhibitor of vesicle trafficking, retarded canker development. In contrast, Xaa triggered a mitogen-activated protein kinase signalling pathway involving WRKY and ethylene-responsive transcriptional factors known to activate downstream defence genes.

Closely-related Xanthomonas citri subsp. citri isolates trigger distinct histological and transcriptional responses in Citrus limon

“…In the interaction with the virulent isolate, up-regulated genes involved in cell cycle, growth and differentiation (and related with hyperplasia and hypertrophy) were identified, as has been previously described (Cernadas et al, 2008).…”

“…One of the primary roles of SNARE proteins is to mediate fusion between cellular transport vesicles and the cellular plasma membrane for exocytosis. A study using inhibitors of vesicle trafficking has shown that this inhibition delays pustule development induced by Xcc in sweet orange Pera leaves (Cernadas et al, 2008), so the suppression of this gene would be consistent with the lack of symptoms in Xcc A T inoculated leaves. In conclusion, unlike the virulent type, the avirulent isolate Xcc A T did not prevent the development of physical barriers, such as starch accumulation or cell wall thickening, both events associated with plant defense mechanisms.…”

“…aurantifolii (Xaa) pathotype C, which does not produce canker in this genotype. Xaa induces ethylene related genes like ACC oxidases, as well as ethylene receptors, ethylene-induced esterase and ethylene response factors 48 hpi, and these responses are associated to disease resistance (Cernadas et al, 2008).…”

1H HRMAS NMR spectroscopy and chemometrics for evaluation of metabolic changes in citrus sinensis Caused by Xanthomonas axonopodis pv. citri

“…According to the literature, 28,29 Xac induces gene transcription related to resistance, defense and response to pathological stress induced by alkaloids, terpenes and volatile compound production. However, during lesion development, there is a suppression of the defensive responses and the mobilization of nutrients toward infection sites.…”

“…It is known that Xac decreases biosynthetic rates 30 and protein catabolism, 28 whereas carbohydrates are considered signals of metabolism regulation, development and stress response. 31,32 Moreover, the production of these compounds was practically stopped with infection, which may be associated with the supply of energy required for metabolism reactions important for defense mechanisms.…”