The effect of light on IAA metabolism in different parts of maize seedlings in correlation with their growth

The inhibitory effect of light on the growth of plants correlates with a decrease of free IAA content in their tissues and might be mediated through changes of IAA metabolism. In different parts of Zea mays L. seedlings (roots, mesocotyls and coleoptiles) that respond to light with a different growth rate, the effect of light on the formation of IAA metabolites was examined in feeding experiments with (1)4C-IAA. In all tissues, IAA was taken up and metabolised mainly into six compounds, four of them were tentatively identified as IAA-1-O-glucose (IAGlc), IAA-myo-inositol, indole acetamide and IAA-aspartate (IAAsp). IAA was metabolised most slowly in the roots. In coleoptiles and mesocotyls, IAGlc was the most abundant metabolite, except for mesocotyls in the light. In roots, a relatively large amount of IAA was also metabolised into IAAsp. Light stimulated the rate of IAA metabolism in all tissues, but its effect on the conversion of IAA was exceptionally high in mesocotyls. In mesocotyl tissue the conversion into IAAsp was greatly stimulated by light. Conversely, the content of IAGlc in mesocotyls was decreased by light. Since light inhibited mesocotyl growth significantly and specifically, it is possible that the high capacity of mesocotyls to synthesise IAAsp in the light may have caused a depletion of free IAA, which then led to an inhibition of growth. In mesocotyls from the light-grown plants IAA conjugated into IAGlc was probably used for IAAsp biosynthesis.