Improved water-use strategies in oilseed rape transformed with Rhizobium rhizogenes under soil drought

Root-inducing (Ri) lines are plants that have undergone transformation with Rhizobium rhizogenes in which the transfer (T)-DNA of the bacterium has been incorporated into their genomes. Two Ri lines of Brassica napus (A11 and B3), developed in our lab, demonstrated improved tolerance to osmotic stress compared to the wild-type (WT) plants grown in hydroponics. However, these responses have not been confirmed for plants grown under soil drought. In the current study, A11 and B3 exhibited better maintenance of hydraulic integrity under conditions of severe soil drought, as exemplified by higher leaf water potential and delayed decline of daily plant water consumption in response to drought. In respect to the higher water maintenance, both Ri lines exhibited lower stomatal conductance (g_s) and transpiration rate (T_r) (0.4 times lower than WT) under drought, which could be attributed to their significantly higher concentrations of leaf abscisic acid (ABA). Therefore, the improved water-use strategies in the Ri lines under severe soil drought could be attributed to ABA-mediated fine-tuned control of g_s and thus T_r compared to WT. We furthermore analyzed the T-DNA insertion events in the two Ri lines and found that a high copy number of T_L-DNA genes (rolA, rolB, rolC, rolD, and ORF13) and complete T-DNA insertion (both T_L-DNA and T_R-DNA) seemed to be correlated with a more severe Ri phenotype (e.g., dwarfing). Finally, the potential association of morphological modifications after T-DNA insertion with drought resilience in Ri oilseed rape was discussed. These findings provide evidence that R. rhizogenes-mediated transformation can be beneficial for developing pre-breeding oilseed rape lines coping better with drought stress.