HISTOPATHOLOGICAL CHANGES ASSOCIATED WITH SILVERLEAF SYNDROME IN SQUASH

Silverleaf syndrome on squash (Cucurbita pepo) is characterized by vei n clearing, and leaf whitening, and has been associated with the prese nce of nymphs of Bemisia tabaci (sweetpotato whitefly), and the presen ce of two double-stranded RNAs (4.2 and 4.6 kb) in symptomatic leaves. However, the aetiology of the disorder remains unknown. Anatomical st udies using transverse and paradermal sections from symptomatic leaves of C. pepo cv. Dixie showed turgescent, aligned adaxial epidermal cel l layer, flat cuticle and the presence of sub-epidermal spaces. Histoc hemical studies failed to reveal any breakdown in the cellulose cell w alls; however, the cuticular layer, the pectin layer and cellulose wal l of epidermal cells formed a flat compact structure. Some changes wer e observed in the lipid content, with the presence of lipid bodies nea r cell wall membranes. Sodium dodecyl sulphate-polyacrylamide gel elec trophoresis of preparations of symptomatic squash leaves, partially pu rified by differential centrifugation, revealed a protein with a molec ular mass of about 48 kDa that was not found in comparable preparation s of asymptomatic leaves. This protein was localized by immunological staining in the spongy mesophyll cells and in the layer of bundle shea th associated with sieve elements and companion cells of the leaf phlo em.