Drought adaptability of Agrobacterium rhizogenes ‐induced roots in oilseed rape ( Brassica napus var. oleifera )

. Soil grown oilseed rape ( Brassica napus L. var. oleifera M., cv. Darmor) seedlings at the cotyledon stage (one week old), were inoculated in vivo at the base of the hypocotyl with Agrobacterium rhizogenes harbouring the pRi 15834 plasmid. Resulting adventitious root formation was observable about 2 or 3 weeks after infection. Differential Ri‐induced root emergence and subsequent development occurred depending on water conditions and closeness of the wounding site to the soil surface: either thin, hairy roots growing rapidly and plagiotropically at the soil level under humid atmosphere, or hairless and fleshy, slowly growing aerial roots developed. The hairy roots were highly drought susceptible, whereas aerial roots revealed some potential for drought tolerance. Unlike normal roots, none of these Ri‐induced roots appeared able to give rise to drought rhizogenesis in plants subjected to progressive drought stress. However, under hardening, achieved through successive and moderate drought stress‐rehydration cycles, both types of Ri‐induced roots improved drought tolerance and could express the morphogenetic differentiation programme leading to the formation of short, tuberized, drought‐adapted, roots. These results, discussed in terms of hormonal imbalance and drought tolerance regulation, suggest that the Ri T‐DNA gene expression, responsible for adventitious root induction and growth behaviour, is further regulated through the host plant.