Transcription Profile Analysis Of Maize Embryo Sac And Preliminary Identification Of Genes Involved In Embryo Sac-Pollen Tube Interaction

Embryo sac, the female gametophyte of plants, plays important roles in the pollination and fertilization process. Compatible embryo sac-pollen tube interaction and sperm-egg cell interaction are required for successful fertilization. During the process, genes expressed in embryo sac are under highly regulation. Maize (Zea mays) has special floral organs and completed genome information, which make it is convenient to study embryo sac-polle tube interaction in maize. However, until now, a study of the female gametophyte transcriptome has not been performed at the whole-genome level in maize and little is known about the function of embryo sac factors in the interaction between maize embryo sac and pollen tube. In this study, we identified the genes specifically and preferentially expressed in maize embryo sac and genes differentially expressed before and after pollination in maize embryo sac via RNA-seq. The main results are listed as follows:(1) By comparing the transcriptome of maize mature embryo sac with nucellus, we identified3463genes expressed preferentially in maize embryo sac. By comparing the3463genes with nucellus, pollen, silk, seedling and leaf expressed genes, we identified376genes expressed specifically in maize embryo sac. Most of them were involved in RNA transcription, protein targeting, protein degradation, transport, signal transduction and stress. Comparative analysis of embryo sac-specific and preferential expressed genes between maize and Arabidopsis (Arabidopsis thaliana) revealed both common and distinct characteristics between the two species. Although the characteristics of maize and Arabidopsis embryo sac-specifical and preferential expressed genes are similar, the homology between them is low suggesting that the mechanism of embryo sac-pollen tube interaction between them might be different.(2) Through comparison of maize embryo sac before and after pollination, we obtained3382differentially expressed genes, including1609up-regulated genes and1773down-regulated genes. Most of them were involved in RNA transcription, protein targeting, protein degradation and signaling transduction. Analysis of the differentially expressed genes induced by pollination suggested that RNA transcription, protein metabolism and signaling transduction are essential to embryo sac-pollen tube interaction in maize. (3) The genes participated in ubiquitin-mediated protein degradation were enriched in embryo sac and changed significantly before and after pollination, we suggest that the intracellular or extracellular ubiquitin proteasome system may play important roles in maize embryo sac-pollen tube interaction via mediating different substrates.(4) Actin-myosin system mediates directional movement of organelles in the cell, former predicted that sperm motion may be in a similar mechanism. The genes associated with myosin were enriched in the differentially expressed gene sets of maize embryo sac before and after pollination. The special expression patterns of myosin-associated genes indicate that they may play important roles in maize embryo sac-mediated sperm transport.(5) Signal molecules are critical components in each step of embryo sac-pollen tube interaction. We focus on the most enrichment signaling molecules, such as receptor-like kinase and small secretory cysteine-rich proteins. Through our analysis, LRR receptor-like kinases and DUF26receptor-like kinases were enriched in maize embryo sac and changed significantly before and after pollination. In addition, we predicted109cysteine-rich proteins in maize embryo sac,56of them are maize specific. These discoveries might provide information for investigation of their functions in maize embryo sac-pollen tube interaction.In conclusion, a large number of genes involved in embryo sac-pollen tube interaction were identified through transcriptome analyses of three different experimental tissues in maize, more than30%of which are unknown functions and more than80%of which are first reported in maize embryo sac. Our results provided important data for further analysis of embryo sac-pollen tube interaction in maize.