Xylella fastidiosa is an important phytopathogenic bacterium that causes many serious plant diseases, including Pierce's disease of grapevines. Disease manifestation by X. fastidiosa is associated with the expression of several factors, including the type IV pili that are required for twitching motility. We provide evidence that an operon, named Pil-Chp, with genes homologous to those found in chemotaxis systems, regulates twitching motility. Transposon insertion into the pilL gene of the operon resulted in loss of twitching motility (pilL is homologous to cheA genes encoding kinases). The X. fastidiosa mutant maintained the type IV pili, indicating that the disrupted pilL or downstream operon genes are involved in pili function, and not biogenesis. The mutated X. fastidiosa produced less biofilm than wild-type cells, indicating that the operon contributes to biofilm formation. Finally, in planta the mutant produced delayed and less severe disease, indicating that the Pil-Chp operon contributes to the virulence of X. fastidiosa, presumably through its role in twitching motility.
Three races of Xanthomonas axonopodis pv. glycines were identified on pustule disease resistant and susceptible soybean cultivars based on virulence phenotype. For race 3, an avrBs3 homolog, avrXg1 was identified that conferred resistance expressed as a hypersensitive response on resistant cultivar Williams 82. Mutations in two predicted functional domains of avrXg1 resulted in gained virulence on Williams 82 and an increase in bacterial population number on susceptible cultivars. Expression of avrXg1 in race 1, that is predicted to confer a nonspecific HR, led to virulence on susceptible cultivars Spencer and PI 520733. Expression of avrXg1 in race 2, that is predicted of carrying avrBs3-like genes, resulted in gained virulence and fitness of pathogen on both resistant and susceptible cultivars. The results demonstrate multifunctions for avrXg1 dependent on pathogen and plant genetic backgrounds.
Targeted mutations in flgK, and pilD genes in strain KU‐P‐SW005 of Xanthomonas axonopodis pv. glycines, the cause of pustule disease on soybean, led to altered motility phenotypes. The flgK mutants lacked a monopolar flagellum and lost swimming motility, whereas the pilD mutant lacked type IV pili and was unable to move via twitching, a form of surface motility not previously reported for this pathogen. The flgK and pilD mutants were also altered in biofilm production. The flgK and pilD mutants caused reduced disease in susceptible soybean cultivars Spencer when compared to KU‐P‐SW005. Cell counts of the flgK and pilD mutants on plants remained equivalent to KU‐P‐SW005 10 days after inoculation. Complementation of flgK and pilD mutants restored all phenotypes to wild‐type levels. Therefore, flgK and pilD genes that are required for swimming and twitching motility also affected biofilm formation and virulence on soybean.
Xylella fastidiosa is an important phytopathogenic bacterium that causes many serious plant diseases including Pierce’s disease of grapevines. X. fastidiosa is thought to induce disease by colonizing and clogging xylem vessels through the formation of cell aggregates and bacterial biofilms. Here we examine the role in X. fastidiosa virulence of an uncharacterized gene, PD1671, annotated as a two-component response regulator with potential GGDEF and EAL domains. GGDEF domains are found in c-di-GMP diguanylate cyclases while EAL domains are found in phosphodiesterases, and these domains are for c-di-GMP production and turnover, respectively. Functional analysis of the PD1671 gene revealed that it affected multiple X. fastidiosa virulence-related phenotypes. A Tn5 PD1671 mutant had a hypervirulent phenotype in grapevines presumably due to enhanced expression of gum genes leading to increased exopolysaccharide levels that resulted in elevated biofilm formation. Interestingly, the PD1671 mutant also had decreased motility in vitro but did not show a reduced distribution in grapevines following inoculation. Given these responses, the putative PD1671 protein may be a negative regulator of X. fastidiosa virulence.
Twenty (20) of Verticillium dahliae were isolated from wilted potato specimens collected from six districts in Guizhou, China. All the isolates were evaluated for pathogenicity on two potato cultivars, Favorita (susceptible) and Hui-2 (resistant) using the root dip inoculation (RDI) and microsclerotia inoculation (MI). All of the V. dahliae isolates appeared to be pathogenic on both cultivars but VGZ-HZ-4 isolate gave the highest wilt incidence comparing to the others, seconded by VGZ-SC-1 and VGZ-XW-1. Combined analysis of wilt incidence resulting from using two inoculation methods for VGZ-HZ-4 and VGZ-XW-1 isolates on the two potato cultivars showed that the MI gave a higher wilt incidence than that of the RDI and cultivar Favorita had a higher wilt incidence than that of Hui-2. These two V. dahliae isolates were further used as representative isolates for mycelial inhibition (MyI) test with 33 Trichoderma isolates under a dual culture condition on potato dextrose agar plate. The 33 Trichoderma isolates consisting of 21 isolates isolated from potato soils from seven districts of Guizhou, 11 isolates from single spore isolates of the TGZ-150 isolate preserved at Guizhou Institute of Plant Protection (GZIPP) and one isolate TGZ-OLD-81 also preserved at the GZIPP. Most of the single spore isolates and TGZ-SC-4 were found to have higher MyI efficiency than that of the rest. The results indicate that the Trichoderma isolates in this study have initial modes of action of biological control to protect potato crop against V. dahlia.
Bacillus sp. strain CaSUT007, a plant growth promoting rhizobacterium isolated from cassava, was investigated for the secretion of compounds that might be involved in plant growth promotion. Extracts containing phytohormone and extracellular proteins were made from the cell-free fluid of CaSUT007 broth cultures. These extracts, along with a whole culture of CaSUT007 and the raw fluid and cellular fractions from a CaSUT007 culture, were applied separately to cassava stakes. The stakes were planted into pots of soil maintained in a greenhouse condition. Under this condition, all of the extracts including phytohormones and extracellular proteins increased root and shoot lengths and cassava biomass as compared to negative control. Our results indicate that the culture extracts, when applied to cassava stakes, increased root and shoot lengths by more than 30%, and increased fresh and dry weights by more than 25% compared to the distilled water control. Thus, photohormone and extracellular proteins secreted by CaSUT007 can influence plant growth and development. Analysis of the photohormone and extracellular proteins extracts revealed indole-3-acetic acid and peptides to be the primary compounds.
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