A study of phloem proteins with particular reference to plant defense mechanisms
A number of proteins present within cucurbit phloem exudate were studied, and their potential relevance to plant defense mechanisms considered. A group of serine protease inhibitors were partially characterised and compared to other inhibitors described within the literature. A number of these inhibitors differed from those described from cucurbit seeds in either size or enzyme specificity. Experimental data indicated that the phloem inhibitors do not regulate protease native to the plants, and no evidence was found to suggest that they are part of an antipathogenic mechanism, although this remains a theoretical possibility. The inhibitors were found to be capable of inhibiting the gut extracts from a number of herbivorous insects. Some peroxidase isozymes were found to be characteristic of species and cultivar, while others appeared to be induced by infection. Whether these latter isozymes were of host or pathogen origin is not known. No other evidence was found to indicate that the phloem peroxidases are involved in plant defense mechanisms, while some of their properties suggest a possible role in the maintenance of redox conditions within the sieve elements. The PP2 lectin of cucurbit exudate was found to agglutinate fungal spores, and some data suggests that the phloem exudate is capable of delaying spore germination by a few hours. Hydroxyproline was detected in the exudate, but appeared to be present as a free amino acid or as part of a small peptide.