Regulation of auxin transport in pea (Pisum sativum L.) by phenylacetic acid: Inhibition of polar auxin transport in intact plants and stem segments

The transport of [14C]phenylacetic acid (PAA) in intact plants and stem segments of light-grown pea (Pisum sativum L. cv. Alderman) plants was investigated and compared with the transport of [14C]indol-3yl-acetic acid (IAA). Although PAA was readily taken up by apical tissues, unlike IAA it did not undergo long-distance transport in the stem. The absence of PAA export from the apex was shown not to be the consequence of its failure to be taken up or of its metabolism. Only a weak diffusive movement of PAA was observed in isolated stem segments which readily transported IAA. When [1-14C]PAA was applied to a mature foliage leaf in light, only 5.4% of the 14C recovered in ethanol extracts (89.6% of applied 14C) had been exported from the leaf after 6.0 h. When applied to the corresponding leaf, [14C]sucrose was readily exported (46.4% of the total recovered ethanol-soluble 14C after 6.0 h). [1-14C]phenylacetic acid applied to the root system was readily taken up but, after 5.0 h, 99.3% of the recovered 14C was still in the root system. When applied to the stem of intact plants (either in lanolin at 10 mg·g-1, or as a 10-4 M solution), unlabelled PAA blocked the transport through the stem of [1-14C]IAA applied to the apical bud, and caused IAA to accumulate in the PAA-treated region of the stem. Applications of PAA to the stem also inhibited the basipetal polar transport of [1-14C]IAA in isolated stem segments. These results are consistent with recent observations (C.F. Johnson and D.A. Morris, 1987, Planta 172, 400—407) that no carriers for PAA occur in the plasma membrane of the light-grown pea stem, but that PAA can inhibit the carrier-mediated efflux of IAA from cells. The possible functions of endogenous PAA are discussed and it is suggested that an important role of the compound may be to modulate the polar transport and — or accumulation by cells of IAA.