Developmental regulation of indole-3-acetic acid turnover in Scots pine seedlings

Indole-3-acetic acid (IAA) homeostasis was investigated during seed germina tion and early seedling growth in Scots pine (Pinus sylvestris). IAA-ester conjugates were initially hydrolyzed in the seed to yield apeak of free IAA prior to initiation of root elongation. Developmental regulation of IAA sy nthesis was observed, with tryptophan-dependent synthesis being initiated a round 4 d and tryptophan-independent synthesis occurring around 7 d after i mbibition. Induction of catabolism to yield 2-oxindole-3-acetic acid and ir reversible conjugation to indole-3-acetyl-N-aspartic acid was noticed at th e same time as de novo synthesis was first detected. As a part of the homeo static regulation IAA was further metabolized to two new conjugates glucopy ranosyl-1-N-indole-3-acetyl-N-aspartic acid and glucopyranosyl-1-N-indole-3 -acetic acid. The initial supply of IAA thus originates from stored pools o f VLA-ester conjugates, mainly localized in the embryo itself rather than i n the general nutrient storage tissue, the megagametophyte. We have found t hat de novo synthesis is first induced when the stored pool of conjugated I AA is used up and additional hormone is needed for elongation growth. it is interesting that when de novo synthesis is induced, a distinct induction o f catabolic events occurs, indicating that the seedling needs mechanisms to balance synthesis rates for the homeostatic regulation of the IAA pool.