Thidiazuron increases fruit number in the biofuel plant Jatropha curcas by promoting pistil development

Jatropha curcas L. is a potential biofuel plant because the composition of its seed oil is suitable for biodiesel and bio-jet fuel production and it is able to grow in unproductive subtropical or subdesert soils. Many studies have been performed to improve the seed yield of J. curcas to meet the needs of the biodiesel industry. As female flower number is an important factor affecting seed yield, an increase in the number of female flowers through the modification of sex expression is critical to the improvement of J. curcas for use as a biofuel. In this study, thidiazuron (TDZ), a synthetic compound with cytokinin (CK) activity, was exogenously applied to inflorescence meristems in four developmental stages to study its effect on sex expression in J. curcas. The results revealed that TDZ treatments of 75μM and 225μM promoted pistil development, which significantly increased the number of female flowers along with the development of inflorescence meristems. Number of female flowers reached a peak (40.0 female flowers per inflorescence) at 225μM TDZ on stages III and IV inflorescence meristems. TDZ also reversed stamen abortion in stages II, III, and IV female flowers and induced bisexual flowers, which largely depends on the development stage of the inflorescence meristems. Furthermore, TDZ treatment increased the branch orders of the dichasia on the inflorescence, as observed by scanning electron microscopy. However, the total number of flowers was significantly decreased, as a result of the abortion of flower buds caused by TDZ. The number of mature fruits, which determines seed yield, was significantly increased by TDZ treatment, although this treatment resulted in a greater number of premature fruits. This study found that treatment with TDZ improved the fruit number of J. curcas by promoting pistil development. TDZ may play dual roles in the determination of flower sex, i.e., promoting pistil development and reversing stamen abortion in female flowers, which could shed light on the mechanism of sex determination in J. curcas and/or other non-model plants.