Cell Wall Degrading Enzymes of Fusarium moniliforme var. subglutinans, Incitant of Mango Malformation

摘要

Fusarium moniliforme var subglutinans, incitant of malformation disease could not enter the host without injuries or natural cracks and even after entry remained confined at the intercellular space near the epidermal region inside the host. Therefore, we investigated on the ability of the fungus to produce cell wall degrading enzymes (CWDE) and also on the possibility of any inhibition of CWDE secretion or their inactivation by the host in its defense. The fungus in the presence of suitable substrates in vitro showed a very high activity of protease. However, activity of cellulase (C-x) was moderate and that of polygalacturonase (PG) was very low. There was no pectin methyl esterase (PME) activity. In malformed mango buds activity of protease was the highest followed by PME. PG activity was moderate but that of C-x was very low. However, activity of the host enzyme, beta-1,3 glucanase was at higher level than that of other enzymes. In healthy mango buds, activity of PME and C-x was not noticed; activity of protease and beta-1, 3 glucanase in comparison with malformed cells was less. However, its PG activity was higher than that in the malformed buds. Incorporation of mangiferin (1,3,6,7-tetrahydroxyxanthone-C-2-beta-D glucoside), a defense metabolite of the host, in the culture broth considerably reduced the activity of all the enzymes. Apparently mangiferin that occurred in malformed buds in huge amounts reduced the activity of PG and C-x in malformed cells. A higher protease activity in malformed buds was presumably due to the increased enzyme secretion by the pathogen. The PME was recorded only in the malformed cells which apparently was either derived from or secreted by the host. The mycelial dry weight of the Fusarium in carboxymethyl cellulose, pectin and sodium polypectate as compared with sucrose used for carbon sources, was poor indicating inadequate secretion of the corresponding enzymes. Similarly, casein as source of nitrogen was less consumed than potassium nitrate. Leakage of electrolytes due to damage of cell wall permeability from malformed leaflets was significantly higher than that from healthy leaves. Healthy leaves when treated with protease rich culture filtrate of the fungus showed max. electrolyte leakage. However, mangiferin treatment reduced the leakage; thus further confirming its CWDE inhibiting role. The histopathological tests with healthy and malformed buds showed erosion of pectic substances, cellulose and protein from cell wall of the latter. The treatment of healthy buds with a mixture of extracellular enzymes of F. moniliforme var. subglutinans created gaps in between two adjacent cells through which the hyphae of the fungus moved inside the tissues. No maceration or collapsing of cells was noticed.