[Auxin transport and phototropism]

Abstract

3-M, an oxidation product of IAA, inhibits the transport of auxin through coleoptiles (HAGER and SCHMIDT, 1968); the mode of action of this inhibition has been investigated.During incubation of 1 mm long sections of corn coleoptiles with IAA-2-(14)C, the presence of 3-M increases the concentrations of labeled auxin in the tissue.Since the influx of IAA-2-(14)C into the sections is not at all changed during the first 30 minutes, the accumulation of auxin has to be due to some other mechanism besides uptake.The active exit step, that is the secretion of IAA-2-(14)C out of 1 mm long coleoptile sections into the surrounding solution, is strongly impaired by 3-M. It is this phenomenon which has to be regarded as the cause of the reduced polar transport of auxin.The polar transport of NAA-1-(14)C through coleoptile sections, just like that of IAA, is inhibited by addition of 3-M, or TIBA or by illumination of the coleoptiles.From the results mentioned above and from the fact that 3-M does not influence important metabolic processes like respiration but on the other hand is able to react with compounds containing SH-groups (like cystein), it is concluded that the transport of auxin is directly influenced by 3-M.It is suggested that systems secreting auxin ("permeases") in the outer border layers of the plasma, mainly in the basal parts of the cell (polar transport!) could be partly blocked by 3-M (possibly by the formation of an adduct with SH-groups of the "permeases").The asymmetric photooxidative formation of 3-M in each single cell illuminated laterally and the subsequent lateral secretion of auxin may be the reason for the transversal shift of auxin during phototropism.