Abstract
Progesterone is an animal steroid hormone that functions as a corpus luteum hormone, which maintains pregnancy and implantation of fertilized egg. We identified progesterone by using GC-MS in various plants such as Arabidopsis, rice, tomato, pea and apple. Among these tissues, pea shoots contained the highest level of progesterone, about 10 mg/kg fw and rice shoots contained the next highest level of progesterone, about 1.5μg/kg. Thus it was found that progesterone is a ubiquitous constituent and that the level of progesterone is similar to that of other plant hormones. Furthermore, we isolated three rice genes (OsPB1, OsPB2, OsPB3) analogous to human progesterone-binding protein genes. These genes encoded proteins that have a putative steroid-binding domain. OsPB2 and OsPB3 are expressed highly throughout the rice plant ( leaf blade, leaf sheath and spike), suggesting that OsPBs, as well as progesterone, may have physiological functions.
