Hydrolysis of bordered pits during colonization of conifers by brown-rot fungi

IRG/WP 95-10103

F Green III, J L Tschernitz, T A Kuster, T L Highley

Brown-rot decay results in rapid reduction in degree of polymerization (DP) of holocellulose with concomitant strength loss (MOR) without removing lignin. Development of new methods of wood protection will require focusing on early events in the sequence of depolymerization. Bordered pit membranes (sapwood) represent a readily available source of non-lignified carbohydrate, ie. pectin and cellulose. Commercial pectinases (Pectinol) and Trichoderma sp. have been shown to degrade pit membranes and increase penetration of preservatives. Brown-rot fungi have previouely been shown to produce oxalic acid (OA) during the decay process. Plant pathogens have been shown to degrade pectin by the synergistic action of OA and polygalacturonase (PG). The OA solubilizes the pectin by chelating the Ca++ and the PG hydrolyses the a-1,4 linkages. We have demonstrated the ability of Postia placenta, Gloeophyllum trabeum and Serpula incrassata to use pectin as a sole carbon sourse and to produce OA and PG on both liquid media and wood. Aspergillus niger and Trichoderma sp. also produce PG on wood but no OA or weight loss. The optimal pH of brown-rot polygalacturonase activity is circa 4.0. As the pH of the wood drope below pH 4, due to acid production during decay, there is a progressive decrease in PG activity and the possibility of acid catalyzed hydrolysis of pit membranes is suggested by increased permeability. We hypothesize that pectin utilization is an essential step during incipient brown-rot decay which helps to initiate fungal metabolism and promote the spread of fungal hyphae between tracheids.


Keywords: BROWN ROT; DECAY; MECHANISMS OF DECAY; PECTIN; POLYGALACTURONASE; BORDERED PITS; OXALIC ACID

Conference: 95-05-11/16 Helsingør; Denmark


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