CYTOLOGY AND PHYSIOLOGY OF SILVERLEAF WHITEFLY-INDUCED SQUASH SILVERLEAF

摘要

Squash silverleaf is a disorder of certain cultivars of Cucurbita which results from feeding by the silverleaf whitefly Bemisia argentifolii (formerly, Bemisia tabaci, biotype B), but not by the sweetpotato whitefly, Bemisia tabaci Biotype A (B. tabaci-A). Because squash silverleaf aetiology is unknown, we compared the symptomology, cytology and physiology of Cucurbita pepo plants Fed upon by B. argentifolii and B. tabaci-A to find the mechanism of squash silverleaf induction. B. argentifolii induced rapid-onset, 100% (of upper leaf surface) expression of squash silverleaf regardless of the season, whereas B. tabaci-a induced some intervenal chlorosis but did not induce full expression of squash silverleaf. Cytology of silvered tissue revealed large intercellular air spaces between deformed mesophyll palisade cells and the adaxial epidermis. The chloroplast in the palisade cells and the plasmalemma around some of the vascular cells showed minor ultrastructural damage. Cellular autolysis, similar to other homopteran-induced phytotoxemias, was observed in tissue where the whitefly nymphs had fed and this damage was not due to direct stylet penetration. Ultrastructural examination of squash silverleaf-affected plants did not reveal any viral particles or inclusion bodies. Both whiteflies caused reductions in chlorophyll content, but the loss was greater with B. argentifolii. Two new intercellular fluid proteins were induced in silvered leaf tissue and the constitutive expression of another protein was suppressed. Enzyme assays performed on the intercellular fluid proteins indicated it exhibited reduced activities of chitinase and peroxidase. Similarities between this insect-induced host response and pathogen-induced responses are discussed.