Correlation of viral RNA biosynthesis with glucose-6-phosphate dehydrogenase activity and host resistance.

Changes in glucose-6-phosphate dehydrogenase (G6P DH; EC 1.1.1.49) activity caused by infection of tobacco ( Nicotiana tabacum L.) leaves with potato virus Y (PVY), cucumber mosaic virus, potato virus X, tobacco rattle virus and turnip mosaic virus, the subcellular localisation of G6P DH isozymes in mesophyll protoplasts derived from healthy and PVY-infected tobacco leaves, as well as G6P DH control and the relationship of its isozymes with the degree of tobacco resistance to PVY multiplication, were studied. The activities of G6P DH were markedly increased in locally and systemically infected leaves and the time courses of the activity linearly correlated with those of virus multiplication. In leaves infected with PVY, the activity time courses of the crude and the partially purified G6P DH were coincident. This probably indicates the involvement of coarse regulation of the enzyme. PVY content linearly correlated with enhanced G6P DH activity in leaf discs derived from susceptible, tolerant and resistant cultivars of tobacco. The increased activity of the enzyme in infected protoplasts and plant tissues was predominantly caused by the increased activity of chloroplastic isozymes. This was confirmed by the specific staining of isozymes after electrophoretic separation of chloroplastic proteins of tobacco leaves. These findings enable the degree of resistance to virus multiplication to be quantified for the use of gene manipulation and breeding.