Abstract
BackgroundHerbaspirillum rubrisubalbicans was first identified as a bacterial plant pathogen, causing the mottled stripe disease in sugarcane. H. rubrisubalbicans can also associate with various plants of economic interest in a non pathogenic manner.ResultsA 21 kb DNA region of the H. rubrisubalbicans genome contains a cluster of 26 hrp/hrc genes encoding for the type three secretion system (T3SS) proteins. To investigate the contribution of T3SS to the plant-bacterial interaction process we generated mutant strains of H. rubrisubalbicans M1 carrying a Tn5 insertion in both the hrcN and hrpE genes. H. rubrisulbalbicans hrpE and hrcN mutant strains of the T3SS system failed to cause the mottled stripe disease in the sugarcane susceptible variety B-4362. These mutant strains also did not produce lesions on Vigna unguiculata leaves. Oryza sativa and Zea mays colonization experiments showed that mutations in hrpE and hrcN genes reduced the capacity of H. rubrisulbalbicans to colonize these plants, suggesting that hrpE and hrcN genes are involved in the endophytic colonization.ConclusionsOur results indicate that the T3SS of H. rubrisubalbicans is necessary for the development of the mottled stripe disease and endophytic colonization of rice.
Highlights
Herbaspirillum rubrisubalbicans was first identified as a bacterial plant pathogen, causing the mottled stripe disease in sugarcane
Organization of the hrp/hrc gene cluster in H. rubrisubalbicans M1 The hrp/hrc genes cluster of H. rubrisubalbicans M1 contains 26 genes distributed in a 21 kb region, composed of seven hrp, eight hrc, and eleven genes encoding for hypothetical proteins (GenBank accession JN256203) (Figure 1)
Comparison of the DNA sequence of the hrp/hrc cluster of H. seropedicae SmR1 with H. rubrisubalbicans showed that the genes are almost identically arranged (Figure 1)
Summary
Herbaspirillum rubrisubalbicans was first identified as a bacterial plant pathogen, causing the mottled stripe disease in sugarcane. The mottled stripe disease was first described in Louisiana (USA) in 1932 and is characterized by the development of red streaks with white spots on the leaves of sugarcane It is a disease of relatively small economic importance and affects sugarcane varieties B-4362 and Taiwang [3,6,7]. The point of injection becomes red and necrotic and, after seven days, red stripes are formed along the vessels near the inoculation site, accompanied by different degrees of chlorosis. At this stage the bacteria infest the protoxylem and the metaxylem of the leaves. Host plant responds with the production of phenolic compounds, gum, and localized cell death [1]
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