Direct and Indirect Effects of Auxin

It has been observed that promotion of cell enlargement by auxin is accompanied by an increase in synthesis of cell wall material (2, 3, 5, 6, 18, 25, 26, 27). Because auxin does not cause an increase in wall synthesis in oat coleoptile segments whose elongation is inhibited with mannitol, the increase in synthesis caused by auxin in uninhibited segments has been regarded as induced by elongation rather than directly by auxin (2, 23). This kind of effect, in which a promotion of synthesis results from a promotion of elongation, we term an indirect effect of auxin. We reported previously (20) that when elongation is inhibited by Ca+ +, a substantial promotion by auxin of incorporation of labelled glucose into the cell wall can still be detected. This shows that auxin actually exerts a direct promotive effect on wall synthesis, direct in the sense that it is not a consequence of the promotion of elongation. Of course this effect is not necessarily a primary action of auxin. This paper presents a more detailed comparison of the effect of IAA on wall synthesis in oat coleoptile segments in the absence and presence of Ca+ +, which makes it possible to distinguish the occurrence of both kinds of effects on wall synthesis during auxinpromoted elongation, namely direct (due to the auxin) and in :irect (due to the elongation).