THE RESPONSE TO AUXIN OF RAPESEED (BRASSICA-NAPUS L) ROOTS DISPLAYINGREDUCED GRAVITROPISM DUE TO TRANSFORMATION BY AGROBACTERIUM-RHIZOGENES

It has recently been documented that, compared to untransformed contro ls, the roots of oilseed rape (Brassica napus L. CV CrGC5) seedlings t ransformed by Agrobacterium rhizogenes A4 show a reduced gravitropic r eaction (Legue et al. 1994, Physiol Plant 91: 559-566). After stimulat ion at 90 degrees C or 135 degrees, the transformed root tips curve, b ut never reach a vertical orientation. In the present study, we invest igated the causes of reduced gravitropic bending observed in stimulate d transformed root tips. First, we localized the gravitropic curvature in normal and in transformed roots after 1.5 h of stimulation. The ce lls involved in root curvature (target cells) corresponded at the cell ular level to the apical part of the zone of increasing cell length. I n transformed roots grown in the vertical position, these cells showed a reduction in cell length compared to controls. Because auxin is con sidered to be the gravitropic mediator, the response of normal and tra nsformed roots to exogenous auxin was studied. Indole-3-acetic acid (I AA) was applied along the first 3 mm using resin beads loaded with the hormone. In comparison to normal roots, transformed roots showed redu ced bending toward the bead at all points of bead application. Moreove r, the cells which responded to IAA corresponded to the target cells i nvolved in the gravitropic reaction. The level of endogenous IAA was l ower in transformed roots. Thus, it was concluded that the modified be havior of transformed roots during gravitropic stimulation could be du e to differences either in IAA levels or in reactivity of the target c ells to the message from the cap.