Involvement of indole-3-acetic acid in the circadian growth of the first internode of Arabidopsis

The extension rate of the first inflorescence node of Arabidopsis was measu red during light/dark or continuous light exposure and was found to exhibit oscillations which showed a circadian rhythmicity. Decapitation induced a strong inhibition of stem extension. Subsequent application of IAA restored growth and the associated extension-rate oscillations. In addition, IAA tr eatments, after decapitation, re-established the circadian rhythmicity visi ble in the intact plants during free run. This indicates that the upper zon e of the inflorescence has a major influence on the extension rate of flora l stems and implies a role for auxin. Application of N-(1-naphthyl)phthalam ic acid, an IAA transport inhibitor, to an intact floral stem inhibited gro wth and the rhythmicity in the extension rate oscillations, indicating that IAA polar transport may play a role in the dynamics of stem elongation. Fu rthermore, IAA-aspartate application, after decapitation, did not restore g rowth and rhythmicity. Nevertheless, biochemical analysis of IAA and IAA-as partate demonstrated circadian fluctuations of the endogenous levels of bot h compounds. These observations suggest that IAA metabolism is an essential factor in the regulation of the circadian growth rhythm of Arabidopsis flo ral stems.