Developmental expression and biochemical properties of a β‐1,4‐endoglucanase family in the soybean cyst nematode, Heterodera glycines

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Abstract
The soybean cyst nematode, Heterodera glycines, produces β‐1,4‐endoglucanases (cellulases) that are secreted during infection of soybean. The gene structures of three, hg‐eng‐4, hg‐eng‐5 and hg‐eng‐6, of the six β‐1,4‐endoglucanase genes, all family 5 glycosyl hydrolases previously identified from H. glycines, are presented here. Furthermore, we present the detailed expression analyses of β‐1,4‐endoglucanase genes as well as the biochemical properties of four H. glycines endoglucanase enzymes. Two of the endoglucanases, HG‐ENG‐5 and HG‐ENG‐6, differed significantly in their amino acid sequence of the catalytic domains and their gene structure from that of the other four β‐1,4‐endoglucanases. Quantitative real‐time RT‐PCR revealed distinct developmental expression differences among the hg‐eng family members during the early stages of parasitism and relatively low expression levels in late parasitic stages, with the exception of the adult male stage for some eng genes. Recombinant HG‐ENGs degraded carboxymethylcellulose and optimum enzyme activity ranged from pH 5.5 for HG‐ENG‐5 to pH 8 for HG‐ENG‐6. EDTA, Ca2+, Co2+, Mg2+ and Fe2+ did not affect enzyme activity of any ENG protein, whereas Zn2+, Cu2+ and Mn2+ inhibited enzyme activity from 23% to 73% in some cases. In tests with 12 different polysaccharide substrates, enzyme activity was restricted to β‐1,4 linkages with all ENG proteins tested. Only HG‐ENG‐5 and HG‐ENG‐6 had relatively high activity on xylan and slightly degraded microcrystalline cellulose. Together, these data reveal distinct differences in expression and biochemistry of cyst nematode parasitism genes and proteins, respectively, and cast light on the intricate interactions between a parasitic animal and its plant host.