Chemical and immunological similarities between the phloem proteins of three genera of the Cucurbitaceae

Phloem exudates from Cucurbita, Cucumis, and Citrullus were gelled by oxidative formation of disulphide bridges between the phloem filaments. Gellation could be inhibited by dithiothreitol or iodoacetamide and did not require the presence of the phloem lectin. Each exudate contained a dimeric lectin of similar relative molecular mass and purified specific activity; these were all specific for oligomers of N-acetyl-glucosamine, and shared antigenic determinants. The similarity of the phloem proteins between Cucurbita, Cucumis, and Citrullus implied that they served the same function in each genus. This is postulated to be the sealing of wounded sieve-tubes, with the lectin on the filaments binding and preventing the entry of micro-organisms. The phloem lectin and the filament-forming protein from Cucurbita shared sequence homologies as judged by amino-acid-composition comparisons, but antibodies raised against each showed no cross-reactivity with the other protein. The exudates from Cucurbita and Cucumis may contain a high concentration of phloem proteins because the large diameter of their sieve-pores does not allow rapid blocking by callose synthesis on wounding, and a chemical mechanism of gellation is required.