The mechanism of
nitrate reductase (NR) regulation under long-term
anoxia in roots of whole plants and the putative role of
nitrate in
anoxia tolerance have been addressed. NR activity in tomato roots increased significantly after 24 h of anaerobiosis and increased further by 48 h, with a concomitant release of
nitrite into the culture medium.
Anoxia promoted NR activation through dissociation of the
14-3-3 protein inhibitor and NR dephosphorylation. After 24 h of
anoxia, the total amount of NR increased slightly up to 48 h. However, NR-
mRNA levels remained constant between 0 h and 24 h of root
anoxia and decreased after 48 h. This is probably due to the inhibition of NR degradation and the accumulation of its native form. NR was slightly dephosphorylated in the absence of
oxygen and
nitrate. Under
anoxia, NR dephosphorylation was modulated by
nitrate-controlled NR activity. In addition, the presence of
nitrate prevents anoxic symptoms on leaves and delays wilting by 48 h during root
anoxia. In the absence of
nitrate, plants withered within 24 h, as they did with
tungstate treatment, an inhibitor of NR activity. Thus,
anoxia tolerance of tomato roots could be enhanced by
nitrate reduction.