The suppressor action of water-soluble glucans (WSG) isolated from Phytophthora capsici, P. infestans and P. nicotianae var. nicotianae on hypersensitive cell death caused by elicitors was investigated using suspension-cultured of tomato (Lycopersicon esculentum Mill., L, peruvianum), sweet pepper (Capsicum frutescence L.) and tobacco (Nicotiana tabacum L.). The hypersensitive cell death was caused by elicitor of hyphal wall components (HWC) both from pathogenic and nonpathogenic Phytophthora spp. and depended on the concentration of the elicitor and duration of treatment. On the other hand, the WSG from respective species of Phytophthora suppressed the elicitor-induced cell death in the manner of host selectivity: the suppressor activity was specific only to its typical host (i.e., P. capsici for sweet pepper, P. infestans for tomato and P. nicotianae var. nicotianae for tobacco) and the common host (tomato). These results suggest that WSG from the above Phytophthora spp. may be involved in the determination of host-selectivity for the establishment of compatible interaction with their respective host plants.
Water‐soluble glucans (WSG) from a virulent isolate of Phytophthora capsici (PCAP‐3) which were released during germination of cystospore markedly suppressed the elicitor‐induced death of suspension‐cultured cells of susceptible sweet pepper and tomato but not that of resistant pepper and tobacco. PCAP‐3, its polygalacturonase (PGase)‐deficient mutant (PCAP3‐M16), and galacturonic acid non‐utilizable mutant carrying the PGase (PCAP‐1) activity could penetrate in epidermal cells of host leaves, but similarly caused a hypersensitive response (HR) on non‐injured leaves of resistant host (sweet pepper). In the case of inoculation on press‐injured leaves, however, both of the resistant and nonhost plant leaves became quite susceptible to PCAP‐3similar to susceptible hosts, but not to PCAP3‐M16 and PCAP‐1. The results suggested that host‐selectivity of P. capsici may be determined in the leaf epidermal cells where the suppressor glucans released during infection effectively suppressed the occurrence of hypersensitive reaction. Furthermore, during growth of the fungus in intercellular spaces of leaf tissues, PGase may contribute not only to the virulence experession but also the supply of initial nutrition for fungal growth in the intercellular space of host tissues.
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