Histological characteristics and related gene expression analysis of ovule abortion in Camellia oleifera

  Objective  The ovule abortion of Camellia oleifera is serious. Only a few ovules could develop into mature seeds, but its abortion mechanism is not clear. The research investigated abortion period, microstructure changes and abortion reasons of ovules to clarify the abortion process and to provide certain theoretical basis and practical significance for increasing the yield of Camellia oleifera.

  Method  The fruits of C. oleifera cv. ‘Huashuo’ were chosen as the experimental material. The ovule morphology in Camellia oleifera fruit was observed under stereomicroscope. We counted the proportion of abortive ovules. The stage of losing activity of abortive ovules was observed by FDA staining technique and stereomicroscope. The histological changes of fertile and abortive ovules were clarified by paraffin technique and microscopic observation, and the starch grain distribution in fertile and abortive ovules was marked by K-KI₂ staining and PAS reaction. The CFDA fluorescence tracing experiment and laser confocal microscopy were used to reveal assimilate transport pathways in fertile and abortive ovules. The qRT-PCR was used to analyze the expression of genes related to sugar and energy metabolism, reactive oxygen species metabolism, and apoptosis processes in fertile and abortive ovules.

  Result  (1) The stereomicroscope observation showed that the size of fertile and abortive ovules differed after 26 WAA (weeks after anthesis). The results of FDA staining indicated that the abortive ovules gradually lost their activity during fruit development. (2) At 37 WAA, the proportion of abortive ovules reached 64.08%. (3) Microscopic observation showed that the embryo and endosperm of fertile ovules developed normally, and the inner and outer integuments were tightly united; the abortive ovules had no endosperm cells, and the space between the inner and outer integument was wild. Starch grains were observed in both the suspensor and endosperm of fertile ovules, and only a few starch grains were observed on the shrunken inner integuments of abortive ovules. (4) CFDA fluorescence tracing results revealed differences in assimilate transport modes between fertile and abortive ovules. There was no callose deposition on the inner integument of fertile ovules, while callose deposition can be seen on the inner integument of abortive ovules. (5) Genes related to sugar and energy metabolism, reactive oxygen species metabolism, and apoptosis processes were differentially expressed in fertile and abortive ovules.

  Conclusion  The abortive ovules have abnormal structure and lack starch accumulation. There are some callose depositions on inner integument. There are differences in the transport modes of assimilates between abortive ovules and fertile ovules. Different expressions of genes about material and energy metabolism, antioxidant action and apoptosis processed in the ovules might be related to the abortion of Camellia oleifera ovules.