TALE-induced bHLH transcription factors that activate a pectate lyase contribute to water soaking in bacterial spot of tomato
Wednesday, 2017/02/01 | 14:39:13
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Allison R. Schwartz, Robert Morbitzer, Thomas Lahaye, and Brian J. Staskawicz SignificanceAvrHah1 [avirulence (avr) gene homologous to avrBs3 and hax2, no. 1] is a transcription activator-like (TAL) effector (TALE) in Xanthomonas gardneri that enhances water soaking in its known hosts tomato, pepper, and Nicotiana benthamiana. We observe that the water soaking conferred by AvrHah1 is due to the movement of water into the infected apoplast from a wet environment. RNA sequencing identified two basic helix–loop–helix (bHLH) transcription factors that we confirmed as targets of AvrHah1. We discovered that a pectate lyase was upregulated by both of the bHLH transcription factors. Designer TALEs (dTALEs) for both bHLH transcription factors and the pectate lyase complemented the water-soaking phenotype of X. gardneriΔavrHah1. This report demonstrates virulence activity from an indirect TALE target.
AbstractAvrHah1 [avirulence (avr) gene homologous to avrBs3 and hax2, no. 1] is a transcription activator-like (TAL) effector (TALE) in Xanthomonas gardneri that induces water-soaked disease lesions on fruits and leaves during bacterial spot of tomato. We observe that water from outside the leaf is drawn into the apoplast in X. gardneri-infected, but not X. gardneriΔavrHah1 (XgΔavrHah1)-infected, plants, conferring a dark, water-soaked appearance. The pull of water can facilitate entry of additional bacterial cells into the apoplast. Comparing the transcriptomes of tomato infected with X. gardneri vs. XgΔavrHah1 revealed the differential up-regulation of two basic helix–loop–helix (bHLH) transcription factors with predicted effector binding elements (EBEs) for AvrHah1. We mined our RNA-sequencing data for differentially up-regulated genes that could be direct targets of the bHLH transcription factors and therefore indirect targets of AvrHah1. We show that two pectin modification genes, a pectate lyase and pectinesterase, are targets of both bHLH transcription factors. Designer TALEs (dTALEs) for the bHLH transcription factors and the pectate lyase, but not for the pectinesterase, complement water soaking when delivered by XgΔavrHah1. By perturbing transcriptional networks and/or modifying the plant cell wall, AvrHah1 may promote water uptake to enhance tissue damage and eventual bacterial egression from the apoplast to the leaf surface. Understanding how disease symptoms develop may be a useful tool for improving the tolerance of crops from damaging disease lesions.
See: http://www.pnas.org/content/114/5/E897.full PNAS January 31 2017; vol.114; no.5: E897–E903
Fig. S1. XgΔavrHah1 has an in-frame deletion of the DBD. (A) Schematic of the in-frame deletion that removes the DBD from avrHah1 to create avrHah1ΔDBD. NLS, nuclear localization signal. (B) Southern blot analysis of BamHI-digested genomic DNA from X. gardneri strains listed probed for the DBD (Top) or the T3SS (Bottom). The size of BamHI-digested avrHah1 is 2,964 bp, and the size of BamHI-digested DBD deletion of avrHah1 is 1,491 bp. (C) Cell death (or HR) occurs in response to activation of the Bs3 resistance gene in pepper 30R and appears here as a dark, opaque area. XgΔavrHah1 does not activate Bs3 and is pathogenic on pepper 30R. Complementation of XgΔavrHah1 with avrHah1 or avrBs3 restores activation of Bs3 resistance and HR in pepper 30R. |
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