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.