Abstract
Xanthomonas oryzae (Xo) are globally important rice pathogens. Virulent lineages from Africa and Asia and less virulent strains from the United States have been well characterized. Xanthomonas campestris pv. leersiae (Xcl), first described in 1957, causes bacterial streak on the perennial grass, Leersia hexandra, and is a close relative of Xo. L. hexandra, a member of the Poaceae, is highly similar to rice phylogenetically, is globally ubiquitous around rice paddies, and is a reservoir of pathogenic Xo. We used long read, single molecule real time (SMRT) genome sequences of five strains of Xcl from Burkina Faso, China, Mali, and Uganda to determine the genetic relatedness of this organism with Xo. Novel transcription activator-like effectors (TALEs) were discovered in all five strains of Xcl. Predicted TALE target sequences were identified in the Leersia perrieri genome and compared to rice susceptibility gene homologs. Pathogenicity screening on L. hexandra and diverse rice cultivars confirmed that Xcl are able to colonize rice and produce weak but not progressive symptoms. Overall, based on average nucleotide identity (ANI), type III (T3) effector repertoires, and disease phenotype, we propose to rename Xcl to X. oryzae pv. leersiae (Xol) and use this parallel system to improve understanding of the evolution of bacterial pathogenicity in rice agroecosystems.
Highlights
Rice is a staple crop for more than half the world
We found in our whole genome sequences that X. oryzae pv. leersiae (Xol) strains contained 12 or 13 transcription activator-like effectors (TALEs) each, which is more than the nine TALEs per genome found in African X. o. pv. oryzae (Xoo), and less than what is commonly found in Xanthomonas oryzae pv. oryzicola (Xoc) (22–29 TALEs) and Asian Xoo (13–20 TALEs)
We propose Xol, which was isolated from rice and southern cutgrass (L. hexandra), a weedy grass closely related to rice, as a new member of the X. oryzae species
Summary
Rice is a staple crop for more than half the world. Severe rice diseases, such as bacterial leaf streak (BLS) caused by Xanthomonas oryzae pv. Oryzae (Xoo), are increasing in prevalence in parts of Asia and sub-Saharan Africa and cause significant yield losses. Southern cutgrass (Leersia hexandra Swartz) is a common grass found in the southern United States, South America, Africa, and Asia. It is a member of the Poaceae family and is closely related to rice, but diverged from Oryza approximately 14 mya (Guo and Ge, 2005). The high amount of newly discovered repeats (35%) indicates that the L. perrieri genome is evolving rapidly relative to the Oryza genus (Copetti, 2013)
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