Abscisic acid biosynthesis in water-stressed leaves

Although abscisic acid (ABA) was discovered 30 years ago, very little is known about its biosynthetic pathway in higher plants. Two hypotheses have been proposed: (i) a direct pathway involving only C-15 intermediates like farnesyl pyrophosphate, (ii) an indirect pathway involving C-40 intermediates like the xanthophylls. When 14CO2 was fed into greened bean plants, the 14C specific activity of ABA was always lower than those in xanthophylls, such as violaxanthin and lutein, regardless of 12CO2 chase periods. The ABA accumulation in green leaves was not affected by fluridone when plants were stressed once, but the 14C incorporation into ABA was inhibited to the same extent as those of xanthophylls. The incorporation of 18O into the ABA ring when violaxanthin was labeled by 18O in vivo via the violaxanthin cycle indicates that at least a portion of ABA was derived from 18O-labeled violaxanthin during water stress