Abstract
Bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the major diseases that impact rice production in Asia. The bacteria use transcription activator-like effectors (TALEs) to hijack the host transcription machinery and activate key susceptibility (S) genes, specifically members of the SWEET sucrose uniporters through the recognition of effector-binding element (EBEs) in the promoter regions. However, natural variations in the EBEs that alter the binding affinity of TALEs usually prevent sufficient induction of SWEET genes, leading to resistance phenotypes. In this study, we identified candidate resistance alleles by mining a rice diversity panel for mutations in the promoter of OsSWEET13 and OsSWEET14, which are direct targets of three major TALEs PthXo2, PthXo3 and AvrXa7. We found natural variations at the EBE of both genes, which appeared to have emerged independently in at least three rice subspecies. For OsSWEET13, a 2-bp deletion at the 5th and 6th positions of the EBE, and a substitution at the 17th position appear to be sufficient to prevent activation by PthXo2. Similarly, a single nucleotide substitution at position 10 compromised the induction of OsSWEET14 by AvrXa7. These findings might increase our opportunities to reduce pathogen virulence by preventing the induction of SWEET transporters. Pyramiding variants along with other resistance genes may provide durable and broad-spectrum resistance to the disease.
Highlights
Rice provides more than 20% of the total caloric intake for half of the world’s population [1]
transcription activator-like effectors (TALEs)-mediated activation usually occurs by sequence-specific recognition of effector binding elements (EBEs) in the promoter sequence of host target genes [10]
Mining for variations in the promoter sequence of OsSWEET13 and OsSWEET14 in the 3k SNP-seek database resulted in 631 accessions with SNPs/InDels within the region 1.5kb upstream of both genes
Summary
Rice provides more than 20% of the total caloric intake for half of the world’s population [1]. Development of resistant rice varieties, by pyramiding multiple resistance genes, is a cost-effective disease management strategy as it reduces the net cost for crop production and substantially preserves the yield potential [3]. Oryzae (Xoo), which enters the rice host through wounds or hydathodes and establishes itself in the vascular tissues [4]. For further proliferation in the xylem, Xoo relies mainly on type III secretion system-mediated translocation of effector proteins into the host cell. The second group includes transcription activator-like effectors (TALEs), which induce the expression of host susceptibility (S) genes to promote a favorable environment for the bacteria [6]. TALE-mediated activation usually occurs by sequence-specific recognition of effector binding elements (EBEs) in the promoter sequence of host target genes [10]
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