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.