A Pathovar of Xanthomonas oryzae Infecting Wild Grasses Provides Insight Into the Evolution of Pathogenicity in Rice Agroecosystems.

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.

Rice yellow mottle virus Is Transmitted by Cows, Donkeys, and Grass Rats in Irrigated Rice Crops.

Rice yellow mottle virus (RYMV), endemic in Africa, is believed to be spread by chrysomelid beetles, although the infections in a field often cannot be explained by the prevailing number of beetles. We show that the grass rat Arvicanthis niloticus, domestic cows (Bos spp.), and donkeys (Asinus spp.) are potent and efficient transmitters of RYMV. Spread of RYMV by rats was demonstrated in cage experiments wherein individual rats transmitted the virus from healthy to infected rice plants and confirmed in a field experiment. Experiments with cows and donkeys showed that they could transmit the virus in plots with healthy and infected plants and to plots with healthy plants. A high percentage of seedlings became infected when a cow grazed in a seedbed after being fed with infected rice plants. Transmission also was observed when cows were allowed to graze on the stubble of infected fields. The disease incidence increased at least fourfold over time to approximately 36% of the plants infected in the experimental plots of two stubble fields. The results obtained in these stubble fields suggest that cattle-mediated spread will enhance the size of the virus load in the contraseason and the infection potential to infect the next crop.