Auxin transport and phototropism

Abstract

Short illumination of excised coleoptiles (with or without apex) inhibits the subsequent transport of IAA-2-¹⁴C in these sections during darkness. To a certain extent the inhibition is dependent both on the light intensity and on the duration of illumination. Only the blue region of the visible spectrum is effective. The light induced inhibition is due to a decrease of the quantity of IAA transported; on the other hand, the velocity of transport remains unchanged. The inhibition of auxin transport can be observed only if coleoptiles contain endogenous or fed auxin during the preceding illumination period. Besides illumination inhibition of auxin transport can also be brought about by incubation of coleoptile sections with a previously illuminated IAA/FMN solution. Auxin transformed by peroxidase operates in the same way. The different oxidation products of IAA in the solutions used were identified: The only product which inhibits elongation growth and auxin transport was 3-M. The conversion of IAA to 3-M is accomplished by crude cell-free extracts from corn coleoptiles. An increased formation of labeled 3-M from IAA-2-¹⁴C during illumination of coleoptiles could be demonstrated. Since 3-M is not actively transported in coleoptiles, it must be assumed that 3-M functions as an inhibitor of auxin transport only at its site of formation. It is concluded that the phototropic curvature of coleoptiles and stems is triggered by the photooxidative formation of 3-M from IAA in the side exposed to light. The flow of growth substances will be partly blocked by 3-M in this side and can be directed to the shaded side. On the strength of these findings some phenomena of phototropism (transmission of stimulus, “mneme”, quantum yield) can easily be explained.