Low transcription of CmsIAA9 in the basal pistil is related to parthenocarpic fruiting of fingered citron (Foshou)

Seedlessness is a commercial trait that provides a good model for uncovering fruiting mechanisms. Fingered citron (Citrus medica L. var. sarcodactylis Swingle, Foshou), a variant of Citrus medica L. (Xiangyuan), is a traditional ornamental tree in China with a finger-like fruit that is seedless. Its fruit-setting mechanism has not been studied before. The emasculation of Foshou showed that fruits set and were in a good development condition consistent with the non-emasculated control, indicating that Foshou produces parthenocarpic fruit. Foshou pollen can normally germinate and elongate in vitro. The pollen tubes of Xiangyuan entered the ovary 3 days after pollination, while Foshou pollen only germinated in the finger portion of the pistil and did not stretch to the ovary after a week. Moreover, paraffin sections of Foshou pistils revealed that Foshou ovules were aborted or abnormal during flower development, presumably having lost the attraction of female gametes for fertilization. These observations indicate that Foshou shows obligate parthenocarpy. Real-time quantitative PCR showed the mRNA expression levels of auxin-related genes, IAA9, AUCSIA, and PIN4, were down-regulated after anthesis. The auxin response gene CmsIAA9 was significantly down-regulated at 3 days post-anthesis in the basal pistil compared with CmIAA9 in Xiangyuan. Furthermore, the fruit set rate was reduced if the apical pistil was treated with NPA (N-1-naphthylphthalamic acid; an auxin transport inhibitor). In summary, the low transcription of CmsIAA9 in the basal pistil is related to parthenocarpic fruiting in Foshou. This result highlights that auxin could be used in field fruit breeding to obtain a greater number of Foshou fruits.