Submitted to: Journal of Florida State Horticulture Society
Publication Type: Proceedings
Publication Acceptance Date: July 14, 1995
Publication Date: N/A
Interpretive Summary: We are interested in the roles of chitinase and glucanase, two enzymes known to be involved in plants' resistance to fungal and insect pests, in Citrus species. This report presents results of experiments that were designed to determine levels of these enzymes in various citrus tissues, the effects of challenges by fungal and insect pests on levels of these enzymes in citrus, and on the effects of potential chemical elicitors on levels of the enzymes. We found that levels of the two enzymes vary with tissue and tissue age and that the levels of the two enzymes are regulated independently. Infection by fungi or feeding by insects affected levels of the two enzymes indicating that they were indeed part of a defense response. Three chemicals (gibberellic acid, salicylic acid, or KeyPlex 350) all resulted in increased levels of the two enzymes. Our results suggest that chitinase and glucanase are part of citrus plants' defense system and that levels of the enzymes may be regulated either genetically, chemically, or by a combination of both. It is expected that such manipulation may lead to enhanced resistance of citrus to fungal and insect pests.
Among the various defense mechanisms that plants exhibit in response to attack by pests is the expression of a number of proteins collectively referred to as 'pathogenesis-related proteins'. We are interested in these proteins in citrus with the long term goal of enhancing resistance to fungal and insect pests by developing improved germplasm and the use of elicitors to induce these proteins on demand. Two classes of enzymes, chitinases and b-1,3-glucanases, which are known to have antifungal characteristics are active in all citrus tissues studied (roots, leaves, blossoms, and fruit). The amount of activity varies with tissue, tissue age, and in some cases cultivar. Chitinase and b-1,3-glucanase activities increase with age in leaves whereas chitinase activity decreases and b-1,3-glucanase activity increases with age in flavedo. Infection of fruit by Penicillium digitatum induced increased activities of these enzymes. Feeding by sugar cane rootstock borer weevil (Diaprepes abbreviatus) larvae induced increased chitinase activity substantailly in roots of 2 of 8 cultivars that we examined and in leaves of sour orange; in contrast to chitinase, b-1,3-glucanase activity is reduced by weevil feeding. Treatment of grapefruit trees with gibberellic acid, salicylic acid, or Keyplex 350 resulted in significant, although transient, increases in both enzymes.