EXPRESSION STUDIES OF THE ZEAXANTHIN EPOXIDASE GENE IN NICOTIANA-PLUMBAGINIFOLIA

Abscisic acid (ABA) is a plant hormone involved in the control of a wi de range of physiological processes, including adaptation to environme ntal stress and seed development. In higher plants ABA is a breakdown product of xanthophyll carotenoids (C-40) via the C-15 intermediate xa nthoxin. The ABA2 gene of Nicotiana plombaginifolia encodes zeaxanthin epoxidase, which catalyzes the conversion of zeaxanthin to violaxanth in. In this study we analyzed steady-state levels of ABA2 mRNA in N. p lumbaginifolia. The ABA2 mRNA accumulated in all plant organs, but tra nscript levels were found to be higher in aerial parts (stems and leav es) than in roots and seeds. In leaves ABA2 mRNA accumulation displaye d a day/night cycle; however, the ABA2 protein level remained constant . In roots no diurnal fluctuation in mRNA levels was observed. In seed s the ABA2 mRNA level peaked around the middle of development, when AB A content has been shown to increase in many species. In conditions of drought stress, ABA levels increased in both leaves and roots. A conc omitant accumulation of ABA2 mRNA was observed in roots but not in lea ves. These results are discussed in relation to the role of zeaxanthin epoxidase both in the xanthophyll cycle and in the synthesis of ABA p recursors.