Origin and dynamics of indoleacetic acid under polar transport in Pinus sylvestris

Polarly-transported IAA is regarded as a long distance correlative signal important in many aspects of plant physiology, including, for example, apical dominance and growth and development of vascular tissues. In this study, we investigated the importance of apical sources in supplying stem tissues with IAA. The current-year-shoots of 4-year-old Pinus sylvestris L. saplings were replaced with a source of [C-13(6)]IAA. Subsequent mass spectrometric analysis showed that most of the IAA present at two positions in the subjacent 1-year-old internode consisted of [C-13(6)]IAA, while [C-12]IAA of endogenous origin formed a minor pool. However, the pool of [C-13(6)]IAA decreased from 90 to 80% of the total free IAA pool ([C-13(6)]IAA + [C-12]IAA) while being transported down the shoot. This dilution with [C-12]IAA indicates that de novo biosynthesis of IAA occurred. An additional defoliation experiment showed that the synthesis took place in stem tissues rather than in the mature leaves. The results confirm the role of apical shoots as the major source of polarly-transported IAA, but also indicate that synthesis of IAA takes place in stem tissues. This is important when considering IAA balance at the whole plant level.