PATTERNS OF AUXIN AND ABSCISIC-ACID MOVEMENT IN THE TIPS OF GRAVISTIMULATED PRIMARY ROOTS OF MAIZE

Because both abscisic acid (ABA) and auxin (IAA) have been suggested a s possible chemical mediators of differential growth during root gravi tropism, we compared with redistribution of label from applied H-3-IAA and H-3-ABA during maize root gravitropism and examined the relative basipetal movement of H-3-IAA and H-3-ABA applied to the caps of verti cal roots. Lateral movement of H-3-ABA across the tips of vertical roo ts was non-polar and about 2-fold greater than lateral movement of H-3 -IAA (also non-polar). The greater movement of ABA was not due to enha nced uptake since the uptake of H-3-IAA was greater than that of H-3-A BA. Basipetal movement of label from H-3-IAA or H-3-ABA applied to the root cap was determined by measuring radioactivity in successive 1 mm sections behind the tip 90 minutes after application. ABA remained la rgely in the first mm (point of application) whereas IAA was concentra ted in the region 2-4 mm from the tip with substantial levels found 7- 8 mm from the tip. Pretreatment with inhibitors of polar auxin transpo rt decreased both gravicurvature and the basipetal movement of IAA. Wh en roots were placed horizontally, the movement of H-3-IAA from top to bottom across the cap was enhanced relative to movement from bottom t o top whereas the pattern of movement of label from H-3-ABA was unaffe cted. These results are consistent with the hypothesis that IAA plays a role in root gravitropism but contrary to the idea that gravi-induce d asymmetric distribution of ABA contributes to the response.