Chemotaxis of Rhizobium leguminosarum biovar phaseoli RP8002 towards a range of carbohydrates, phenolic compounds and flavonoids was assayed. Xylose (peak response M) were strong chemoattractants amongst the carbohydrates, whilst glucose, fructose, galactose and maltose produced little or no detectable response. Of the monocyclic phenolic compounds, vanillyl alcohol, p-hydroxybenzoic acid (both peak responses M) and 3,4-dihydroxybenzoic acid (peak response 1 0-4 M) all evoked strong chemotactic responses. Amongst the nod-inducing flavonoids, apigenin and luteolin were both strong chemoattractants (peaks at M) while naringenin produced a very low response. Competition experiments suggest that apigenin and luteolin are recognized by a common receptor, but that there exists a separate receptor for luteolin alone. The inhibitors of nod-induction, umbelliferone and acetosyringone, both produced strong chemotactic responses, with peaks at loW3 M and M respectively. This evidence is indicative of a role for chemotaxis towards nod-inducing flavonoids in the initiation of root nodule formation by rhizobia, and also suggests that chemotaxis may influence the host range of the interaction.

Twelve phenolic compounds with related structures were analyzed for their ability to act as chemoattractants for Agrobacterium tumefaciens C58C1 and as inducers of the Ti plasmid virulence operons. The results divided the phenolic compounds into three groups: compounds that act as strong vir inducers and are chemoattractants for A, tumefaciens C58C0 harboring the nopaline Ti plasmid pDUB1003A31, but not the isogenic cured strain; compounds that are at best weak vir inducers and are weak chemoattractants for Ti plasmid-harboring and cured A. tumefaciens C58C'; and compounds that are vir noninducers and are also nonattractants. A strong correlation between vir-inducing ability and Ti plasmid requirement for chemotaxis is thus established. In addition, chemical structure rules for vir induction and chemotaxis are outlined. Positive chemotaxis toward root and shoot homogenates from monocotyledonous and dicotyledonous plants was observed. At low extract concentrations, chemotaxis was enhanced by the presence of Ti plasmid. The chemoattractants do not derive from intact cell walls. Lack of attraction is not responsible for the apparent block to monocot transformation by A. tumefaciens.Plant wounding is a prerequisite for crown gall tumor induction by Ti-plasmid-harboring Agrobacterium tumefaciens. The Ti plasmid virulence operons, induced by wound exudates such as acetosyringone (24,32,33), mediate T-DNA transfer to the plant cell, where neoplastic overgrowth results from its expression (19,20,23).A. tumefaciens C58C1 possesses a highly sensitive chemotaxis system, which responds to a range of sugars and amino acids (19a). For example, the chemotactic peak for sucrose occurs at 10-6 M. Thus, release of these compounds from plant roots and chemotaxis of A. tumefaciens toward them can account for the prevalence of the bacterium in the rhizosphere (15,16).The majority of chemotactic responses in A. tumefaciens appear to be chromosomally encoded (3, 19a, 25). However, chemotaxis toward acetosyringone, one of the major plant phenolic inducers, requires the presence of a Ti plasmid and occurs with a threshold sensitivity of <108 M (3), some 1,000-fold below the maximal vir-inducing concentration (32). Agrobacterium strains harboring either octopine or nopaline Ti plasmids respond at this concentration (29), indicating that chemotaxis is important in guiding virulent A. tumefaciens toward a susceptible plant (30). The Ti plasmid genes involved in this specific chemotactic response are virA and virG (29). virA and virG are also involved in mediating vir induction in response to acetosyringone (18,21,34,37). This suggests a multifunctional role for virA and virG: at low concentrations of acetosyringone they trigger chemotaxis, whereas at high concentrations vir induction is effected.To confirm and extend our previous observations, we embarked upon a survey of the vir-inducing and chemotactic properties of a variety of related phenolic compounds found in plant exudates. We present evidence indicative of a strong correlation bet...

No abstract

Light leaf spot (Pyrenopeziza brassicae)is an important disease on winter oilseed rape crops (Brassica napus) in northern Europe. In regions where economically damaging epidemics occur, resistance to P. brassicae in commercial cultivars is generally insufficient to control the disease without the use of fungicides. Two major genes for resistance have been identified in seedling experiments, which may operate by decreasing colonisation of B. napus leaf tissues and P. brassicae sporulation. Much of the resistance present in current commercial cultivars is thought to be minor gene-mediated and, in crops, disease escape and tolerance also operate. The subtle strategy of the pathogen means that early colonisation of host tissues is asymptomatic, so a range of techniques and molecular tools is required to investigate mechanisms of resistance. Whilst resistance of new cultivars needs to be assessed in field experiments where they are exposed to populations of P. brassicae under natural conditions, such experiments provide little insight into components of resistance. Genetic components are best assessed in controlled environment experiments with single spore (genetically fixed) P. brassicae isolates. Data for cultivars used in the UK Recommended List trials over several seasons demonstrate how the efficacy of cultivar resistance can be reduced when they are deployed on a widespread scale. There is a need to improve understanding of the components of resistance to P. brassicae to guide the development of breeding and deployment strategies for sustainable management of resistance to P. brassicae in Europe.

Recent evidence has suggested that cutinase is required for cuticular penetration and, therefore, is essential for pathogenicity of Pyrenopeziza brassicae, the causal organism of light leaf spot disease of oilseed rape and other brassicas. In order to acquire molecular evidence for the role of cutinase in pathogenesis, the single-copy P. brassicae cutinase gene Pbc1 was disrupted by a transformation-mediated approach. Southern hybridization analysis revealed that in one mutant, NH10-1224, the disruption was due to a tandem insertion of two copies of the disruption vector into the 5' coding region of Pbc1. In contrast to the wild type, no expression of Pbc1 was detected during in planta growth or in cutin-induced mycelium of NH10-1224 and no cutinase activity was detected in culture supernatants from NH10-1224 using p-nitrophenyl butyrate as substrate. Scanning electron microscopy of Brassica napus cotyledons infected with wild-type P. brassicae confirmed that entry into the host is by direct penetration of the cuticle. In contrast, the cutinase-deficient mutant NH10-1224 failed to penetrate the cuticular layer and was unable to develop disease symptoms. This evidence is consistent with the hypothesis that Pbc1 is required for P. brassicae to penetrate the plant cuticle. Demonstration that complementation of NH10-1224 with the Pbc1 wild-type gene restores both cutinase activity and pathogenicity will be required to definitively establish that cutinase is required for successful pathogenesis of brassicas by P. brassicae.