Pectic Enzymes in Tissue Degradation

One characteristic feature of many phytopathogenic organisms is their ability to produce an array of enzymes capable of degrading the complex polysaccharides of the plant cell wall (Bateman and Millar, 1966; Wood, 1967; Albersheim et al., 1969; Wood, 1973) and membrane constituents (Porter, 1966; Tseng and Bateman, 1968). These enzymes usually are produced inductively. Generally, they are extracellular, highly stable and present in infected host tissues. In most plant diseases caused by microbial agents, cell walls are penetrated, tissues are colonized, and permeability of host cells is altered. A brief summary of our understanding of cell wall and membrane structure, coupled with knowledge of the enzymes capable of degrading the components of these structures, and an analysis of the association of these enzymes with diseased tissue, should enable us to make an appraisal of their involvement in pathogenesis and point the way to an objective consideration of this area of disease physiology.

Degradation of Plant Cell Walls and Membranes by Microbial Enzymes

Some leading characteristics and historical notes on Botrytis spp. are described here. Botrytis spp. infect many host plants in all climate areas of the world, infecting mainly upper plant parts at pre- and post-harvest stages. Bulbs, seeds and other propagation material also suffer infection. Infection can occur in high humidity in the presence or absence of water films. Infection may be quiescent, aggressive, restricted or widely developing. The production of high numbers of conidia poses a long lasting threat to susceptible hosts. Genotypic and phenotypic variation is most important in the broad spectrum pathogen B. cinerea. Moreover, changes in populations in response to selection by exposure to xenobiotics, especially fungicides, are quite common in the genus and fungicide resistance has been recorded in Botrytis populations throughout the history of the modern fungicide era. Detailed studies on the precise conditions that promote infection, disease development and survival of inoculum have provided the essential epidemiological information required for design of control strategies. For example, cultural methods have been developed that increase aeration and drying of the plant canopy to reduce the risk of Botrytis epidemics. The increasing requirement for alternative approaches to reduce farmers' dependency on use of fungicides led to the evaluation and exploitation of potential biocontrol agents capable of substantial disease suppression in a commercial context, and within integrated crop management systems.

Botrytis spp. and diseases they cause in agricultural systems - an introduction

Animal tetraspanins are membrane proteins controlling cell adhesion, morphology and motility. In fungi, the tetraspanin MgPls1 controls an appressorial function required for the penetration of Magnaporthe grisea into host plants. An orthologue of MgPLS1, BcPLS1, was identified in the necrotrophic fungal plant pathogen Botrytis cinerea. We constructed a Bcpls1::bar null mutant by targeted gene replacement. Bcpls1::bar is not pathogenic on intact plant tissues of bean, tomato or rose, but it infects wounded plant tissues. Both wild type and Bcpls1::bar differentiate appressoria on plant and artificial surfaces, a process involving an arrest of polarized growth, apex swelling and its cell wall reinforcement. Although wild-type appressoria allowed the penetration of the fungus into the host plant within 6-12 h, no successful penetration events were observed with Bcpls1::bar, suggesting that its appressoria are not functional. An eGFP transcriptional fusion showed that BcPLS1 was specifically expressed in conidia, germ tubes and appressoria during host penetration. Our results indicate that BcPLS1 is required for the penetration of B. cinerea into intact host plants. The defect in pathogenicity of Bcpls1::bar also demonstrates that functional B. cinerea appressoria are required for a successful penetration process. As Bcpls1::bar and Mgpls1 Delta::hph penetration defects are similar, fungal tetraspanins are likely to be required for an essential appressorial function widespread among fungi.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2958.2003.03866.x

The tetraspanin BcPls1 is required for appressorium-mediated penetration of Botrytis cinerea into host plant leaves.

Quantitative measurements, using thin layer chromatography plus ultraviolet spectophotometry and gas-liquid chromatography, have shown that phaseollin accumulation was limited to infected tissue which was visibly brown. Phaseollin accumulated much earlier following infection by an incompatible race, which caused hypersensitivity, than by a compatible race which eventually caused lesions to form. Basal phaseollin metabolism was not affected by the extensive intracellular growth of compatible races of Colletotrichum lindemuthianum which continued for several days before host cells collapsed and became brown. Concentrations of 71 to 147 μg phaseollin/g fresh weight excised hypersensitive tissue were detected. However, calculations suggest that phaseollin concentrations in hypersensitive cells might be many-fold higher than these values. Germination of all races was equally sensitive to low concentrations of phaseollin (less than 10 μg/ml). Although germ-tube growth was also inhibited by small amounts of phaseollin, mycelial growth was less sensitive and experiments indicated that phaseollin was metabolized by hyphae in liquid culture. The production, cellular distribution and concentration, and fungitoxicity of phaseollin are discussed in relation to the importance of phaseollin in restricting hyphal growth during hypersensitive reactions.

Formation and activity of phaseollin in the interaction between bean hypocotyls (Phaseolus vulgaris) and physiological races of Colletotrichum lindemuthianum

R. K. S. Wood

Papers

Studies in the Physiology of Parasitism XVIII. Pectic Enzymes secreted by Bacterium aroideae