The Floral Development And Sex Determination Mechanism Of Vernicia Fordii

Vernicia fordii (tung tree) is a perennial deciduous woody oil plant in subtropical and tropical areas with tremendous potential as raw material to produce biodiesel. The seeds of the tung tree contain 50-60% oil with approximately 75% a-eleostearic acid(9 cis, 11 trans, and 13 trans octadecatrienoic acid), which determines the quality of tung oil. In recent years, the tung tree has been highlighted as an important woody bioenergy plant in China for resolving the increasing energy crisis. Tung trees are monoecious species with male and female flowers on the same inflorescence, the flowers of which are developed in panicled cymose inflorescences that are terminal and solitary on new branches. In an inflorescence, female flowers are located at the center of the main or second inflorescence axis, which are surrounded by male flowers. The number of staminate flowers greatly exceeds that of the pistillate flowers and the average ratio of female to male flowers is 1:27, which results in low yield.Therefore, it is important to investigate the flower development and sex determination processes in the tung tree for further alteration of the sex ratio and to increase in the yield. However, few studies are available on floral development in the tung tree. The main conclusions of this paper are as follows:1. Flower development of V. fordii1) We observed that the differentiation of inflorescences occurred between July and August in the current growing season in Hefei. The development of inflorescences started from July and lasted until April of the following year. Based on the phonological study, we divided the inflorescences differentiation into 6 stages: bud development, shoot elongation, initiation of inflorescence differentiation,inflorescence dormancy, bud burst, inflorescence axes elongation.2) Flower development was divided into 12 stages by key morphological events and developmental order of flower organs with the results of SEM and histological section analyses. The development of male and female flowers was identical at the first six stages. The developmental divergence between the male and female flowers was observed at stage 7, which was defined as the initiation of sex determination. The male flowers are always unisexual, but the female flowers achieved full transformation from bisexuality to unisexuality with stamens abortion.3) The staminode development was blocked during the development of the anther in stage 10 in the female flowers. Meanwhile, the anther differentiated abnormally and microspore mother cells could not undergo meiosis to form microspores because of the disappearance of the tetrads in later development stages the female flowers. Thus, the stage of stamens abortion is the pre-meiosis.4) The staminode abortion process was determined by cell death with a loss of nuclear integrity and DNA fragmentation in stage 11 during female flower development.2. Comparative transcriptome analysis of male and female flowers in V. fordiiRNA sequencing was performed by Illumina Hiseq 2500. A total of 56,065 unigenes was assembled with an average length of 1,044 bp and N50 of 1,463 bp. To annotate the unigenes, the BlastX program was used to search against the Arabidopsis thaliana protein sequence dataset TAIR 10 and 24,567 were matched. Each of V.fordii unigene has unique AGI. According to DEG analysis, 812 DEGs existed between males and females, 608 of which homologously corresponded to unique AGIs. A total of 310 and 298 DEGs showed significant expression in male and female flowers, respectively. The transcriptome data showed that the sexual dimorphism of female flowers was affected by jasmonic acid (LOX2 and LOX3), transcription factors(GL3, MYB3, MYB73. WRKY33 and WRKY53), and some genes related to the floral meristem activity (SPLAYED and AGO10).3. Proteomic analysis of male and female flowers in V. fordiiThe proteins that are differentially expressed between male and female flowers were isolated using a combination of two-dimensional gel electrophoresis (2-DE) and matrix-assisted laser desorption/ionisation time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF MS). A proteomic analysis led to the identification of 14 proteins, of which two were expressed specifically in male flowers and 10 in female flowers. Among the 10 female-specific proteins, aspartic proteinase, ascorbate peroxidase, and cyclophilin were thought to be involved in stamen abortion and gynoecium formation during the development of female flowers.