To gain insights into the virulence suppressive mechanism of a nonpathogenic strain of Rhizobium vitis ARK-1, we co-inoculated ARK-1 with a tumorigenic (Ti) strain of R. vitis to examine the expression of two essential virulence genes (virA and virG) and one non-essential gene (virD3) of the Ti strain at the wound site of grapevine. Co-inoculation of ARK-1 with a Ti strain VAT03-9 at a 1:1 cell ratio into grapevine shoots resulted in significantly lower expression of the virulence genes virA, virD3, and virG of VAT03-9 at 1 day after inoculation compared with those when shoots were inoculated only with VAT03-9. ARK-1 was not able to catabolize acetosyringone, which is the plant-derived metabolites inducing the entire vir regulon in Ti strains, suggesting the direct effect of ARK-1 on the induction of broad range of vir genes of R. vitis Ti strains.

Summary: Colletotrichum higginsianum is a fungal pathogen that infects a wide variety of cruciferous plants, causing important crop losses. We have used map‐based cloning and natural variation analysis of 19 Arabidopsis ecotypes to identify a dominant resistance locus against C. higginsianum. This locus named RCH2 (for recognition of C. higginsianum) maps in an extensive cluster of disease‐resistance loci known as MRC‐J in the Arabidopsis ecotype Ws‐0. By analyzing natural variations within the MRC‐J region, we found that alleles of RRS1 (resistance to Ralstonia solanacearum 1) from susceptible ecotypes contain single nucleotide polymorphisms that may affect the encoded protein. Consistent with this finding, two susceptible mutants, rrs1‐1 and rrs1‐2, were identified by screening a T‐DNA‐tagged mutant library for the loss of resistance to C. higginsianum. The screening identified an additional susceptible mutant (rps4‐21) that has a 5‐bp deletion in the neighboring gene, RPS4‐Ws, which is a well‐characterized R gene that provides resistance to Pseudomonas syringae pv. tomato strain DC3000 expressing avrRps4 (Pst‐avrRps4). The rps4‐21/rrs1‐1 double mutant exhibited similar levels of susceptibility to C. higginsianum as the single mutants. We also found that both RRS1 and RPS4 are required for resistance to R. solanacearum and Pst‐avrRps4. Thus, RPS4‐Ws and RRS1‐Ws function as a dual resistance gene system that prevents infection by three distinct pathogens.

Plant activators are compounds, such as analogs of the defense hormone salicylic acid (SA), that protect plants from pathogens by activating the plant immune system. Although some plant activators have been widely used in agriculture, the molecular mechanisms of immune induction are largely unknown. Using a newly established high-throughput screening procedure that screens for compounds that specifically potentiate pathogen-activated cell death in Arabidopsis thaliana cultured suspension cells, we identified five compounds that prime the immune response. These compounds enhanced disease resistance against pathogenic Pseudomonas bacteria in Arabidopsis plants. Pretreatments increased the accumulation of endogenous SA, but reduced its metabolite, SA-O-β-d-glucoside. Inducing compounds inhibited two SA glucosyltransferases (SAGTs) in vitro. Double knockout plants that lack both SAGTs consistently exhibited enhanced disease resistance. Our results demonstrate that manipulation of the active free SA pool via SA-inactivating enzymes can be a useful strategy for fortifying plant disease resistance and may identify useful crop protectants.

Summary: In this study we characterized the sensitive to low humidity 1 (slh1) mutant of Arabidopsis ecotype No‐0 which exhibits normal growth on agar plate medium but which on transfer to soil shows growth arrest and development of necrotic lesions. cDNA microarray hybridization and RNA gel blot analysis revealed that genes associated with activation of disease resistance were upregulated in the slh1 mutants in response to conditions of low humidity. Furthermore, the slh1 mutants accumulate callose, autofluorescent compounds and salicylic acid (SA). We demonstrate that SA is required for the slh1 phenotype but not PAD4 or NPR1. SLH1 was isolated by map‐based cloning and it encodes a resistance (R)‐like protein consisting of a domain with Toll and interleukin‐1 receptor homology (TIR), a nucleotide‐binding domain (NB), leucine‐rich repeats (LRR) and a carboxy‐terminal WRKY domain. SLH1 is identical to the R gene RRS1‐R of the Arabidopsis ecotype Nd‐1, a gene which confers resistance to the bacterial pathogen Ralstonia solanacearum GMI1000 and also functions as an R gene to this pathogen in No‐0. We identified a 3 bp insertion mutation in slh1 that results in the addition of a single amino acid in the WRKY domain; thereby impairing its DNA‐binding activity. Our data suggest that SLH1 disease resistance signaling may be negatively regulated by its WRKY domain in the R protein and that the constitutive defense activation conferred by the slh1 mutation is inhibited by conditions of high humidity.

Summary: Rhizoctonia solani is a soil‐borne fungus causing sheath blight. In consistent with its necrotrophic life style, no rice cultivars fully resistant to R. solani are known, and agrochemical plant defense activators used for rice blast, which upregulate a phytohormonal salicylic acid (SA)‐dependent pathway, are ineffective towards this pathogen. As a result of the unavailability of genetics, the infection process of R. solani remains unclear. We used the model monocotyledonous plants Brachypodium distachyon and rice, and evaluated the effects of phytohormone‐induced resistance to R. solani by pharmacological, genetic and microscopic approaches to understand fungal pathogenicity. Pretreatment with SA, but not with plant defense activators used in agriculture, can unexpectedly induce sheath blight resistance in plants. SA treatment inhibits the advancement of R. solani to the point in the infection process in which fungal biomass shows remarkable expansion and specific infection machinery is developed. The involvement of SA in R. solani resistance is demonstrated by SA‐deficient NahG transgenic rice and the sheath blight‐resistant B. distachyon accessions, Bd3‐1 and Gaz‐4, which activate SA‐dependent signaling on inoculation. Our findings suggest a hemi‐biotrophic nature of R. solani, which can be targeted by SA‐dependent plant immunity. Furthermore, B. distachyon provides a genetic resource that can confer disease resistance against R. solani to plants.