miRNA and mRNA studies reveal pollination activates hormonal signaling and increases fruit set in the apomictic tree Zanthoxylum bungeanum Maxim

Abstract

Background Apomixis is a form of reproduction that does not involve fertilization of female by male gametes but instead produces offspring from the female parent directly. The progeny of apomixis is genotypically identical to the female parent and so maintains any elite traits of the female parent. Apomixis has considerable potential in genetic plant breeding. However, the mechanism of apomictic reproduction remains unclear. Zanthoxylum bungeanum is an apomictic plant. Studies on miRNAs, mRNAs, hormone changes and fertilization process of the pollinated and non-pollinated materials of Zanthoxylum bungeanum allows screening of the important regulatory factors of the apomictic process at both physiological and molecular levels. This information should be of considerable help in understanding the mechanism of apomixis in this species.Results Our results show that Zanthoxylum bungeanum pollen can germinate on the stigma, and that the pollen tube can extend to the ovary wall after two days but that it then degenerates about the fifth day and before reaching the egg cell. So fertilization does not occur. Nevertheless, pollination did increase fruit set from 74.12% (unpollinated) to 89.31% (pollinated). The reproductive mode of the Zanthoxylum bungeanum cultivar ‘Hancheng Dahongpao’ was identified as obligate apomixis. Enrichment analysis of differential genes indicates that pollination activates genes involved in the synthesis of ABA, IAA, GA3 and JA, and that genes related to asexual development, genes involved in flower development, and transcription factors are also activated. RT-qPCR verification shows that the relative expression levels of mRNAs and miRNAs related to hormone signaling pathways and flower development were negatively correlated. Also, that miRNA is an important factor influencing hormone regulation after pollination of this apomictic species. The contents of ABA, IAA, GA3 and JA were all significantly increased following pollination.Conclusions In general, although pollination does not result in fertilization in this species, it does activate mRNAs and miRNAs which serve as signals to activate the anabolic pathways for ABA, IAA, GA3 and JA. Increased levels of these hormones result in increased fruit set.