Abscisic acid-dependent algal morphogenesis in the unicellular green alga Haematococcus pluvialis

To study the physiological role of abscisic acid (ABA) in the unicellular green alga Haematococcus pluvialis, we investigated the effect of ABA on both algal morphogenesis and carotenogenesis in liquid and plate cultures. When ABA was added to vegetative cells of H. pluvialis, red mature cyst cells with enhanced carotenogenesis rapidly appeared on agar plates in Petri dishes. We considered these conditions as drought stress. In plate culture, the morphological change from vegetative to cyst cells was prevented by the inhibitor of chloroplastic protein synthesis, chloramphenicol (CP), resulting in algal death. Exogenous ABA caused recovery of algal encystment even in the presence of CP. The relationship between ABA concentration and morphogenesis in H. pluvialis showed that a decrease in ABA coincided with cyst formation. In contrast, immature cyst cells underwent maturation accompanied by enhanced carotenogenesis in either the presence of CP or the absence of ABA. Therefore, ABA might regulate algal morphogenesis from vegetative to cyst cells, but not carotenogenesis in cyst cells of H. pluvialis. Furthermore, endogenous active oxygen species generated under drought stress were involved in all algal events, including ABA biosynthesis, encystment, and enhanced carotenogenesis. These results indicate that ABA, induced by oxidative stress, could function as a stress hormone in algal morphogenesis in H. pluvialis under drought stress.