Xanthomonas oryzae TAL Effectors as Tools to Identify Disease Resistance in Rice

Genomes of the rice xylem and mesophyll pathogens Xanthomonas oryzae pv. oryzae (Xoo) and pv. oryzicola (Xoc) encode numerous secreted transcription factors called TAL effectors. Based on studies in a few rice varieties, some TAL effectors contribute to virulence by activating corresponding host susceptibility genes, and some activate disease resistance genes. The roles of X. oryzae TAL effectors in diverse rice backgrounds, however, are poorly understood.  We expressed a set of Xoo TAL effectors that promote infection by activating SWEET sucrose transporter genes in the TAL effector-deficient X. oryzae strain X11-5A, and assessed the induced phenotypes in 21 rice varieties. Xoo TAL effectors enhanced X11-5A virulence on most varieties, but to varying extents depending on the effector and variety. SWEET genes were activated in all tested varieties, but increased virulence did not correlate with activation level. When expressed in an Xoc strain, SWEET activators also enhanced Xoc virulence on Nipponbare, however, Xoc TAL effectors did not alter X. oryzae X11-5A virulence. Our results suggest that SWEET-targeting TAL effectors contribute broadly and without tissue specificity to virulence in rice and their function is affected by host differences besides target sequences. Furthermore, we establish the utility of the TAL-deficient strain of X. oryzae X11-5A for characterizing individual TAL effectors in rice.