Different response of an elite <Emphasis Type="Italic">Bt</Emphasis> restorer line of hybrid rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) in adaptation to nitrogen deficiency

Transgenic Bacillus thuringiensis (Bt) rice have been reported to acquire effective resistance against the target pests; however, the insertion and expression of alien Bt genes may have some unintended effects on the growth characteristics of rice. A screen-house experiment was conducted and repeated twice to investigate the growth characteristics and Bt protein expressions in two Bt rice lines MH63 (Cry2A*) and MH63 (Cry1Ab/Ac)], which had different Bt protein expression levels in leaves, under zero nitrogen (N0) and recommended nitrogen (NR) fertilizer applications. Compared to the counterpart MH63, MH63 (Cry2A*) under N0 experienced accelerated leaf senescence and a lower internal N use efficiency (IE_N), resulting in a 23.2% decrease in grain yield and a lower accumulated biomass. These variations were revealed to be correlated to the higher ratio of the Bt protein content to the soluble protein content (BTC/SPC) with a maximum value of 4.3‰ in MH63 (Cry2A*) leaves in the late growth stage. Under NR, no differences in growth characteristics between MH63 (Cry2A*) and MH63 were found. The growth characteristics of MH63 (Cry1Ab/Ac), with a lower BTC/SPC in the late growth stage compared to MH63 (Cry2A*), were identical to those of MH63 under the two N applications. Results show that the transgenic Bt rice MH63 (Cry2A*), with a relatively higher Bt protein expression in the late growth stage, had an inferior adaptation to nitrogen deficiency compared to its non-Bt counterpart. And this inferior adaptation was found to be correlated with the higher BTC/SPC in MH63 (Cry2A*) leaves in the late growth stage.