Role of ethylene in the regulatory mechanism underlying the abortion of ovules after fertilization in Xanthoceras sorbifolium.

KEY MESSAGE: Genes related to the MAPK cascade, ethylene signaling pathway, Pi starvation response, and NAC TFs were differentially expressed between normal and abortive ovules. Receptor-mediated ethylene signal perception and transmission play an important role in regulating fruit and ovule development. Xanthoceras sorbifolium, a small to medium-sized tree endemic to northern China, is an emerging dedicated oilseed crop designed for applications in advanced biofuel, engine oil, and functional food, as well as for pharmaceutical and cosmetic applications. Despite the importance of Xanthoceras seed oil, low seed productivity has constricted commercial exploitation of the species. The abortion of developing seeds (ovules after fertilization) is a major factor limiting fruit and seed production in the plant. To increase fruit and seed yields, a better understanding of the mechanisms underlying the abortion of fertilized ovules is critical. This study revealed differences in nucellus degeneration, endosperm development, and starch grain content between normally and abnormally developing ovules after fertilization. We constructed 6 RNA-sequencing (RNA-seq) libraries from normally and abnormally developing ovules at the onset of their abortion process. Comparative transcriptome analysis between the normal and abnormal ovules identified 818 differentially expressed genes (DEGs). Among DEGs, many genes involved in mitogen-activated protein kinase (MAPK) cascades, ethylene signaling pathway, and NAC transcription factor genes showed up-regulated expression in abnormal ovules. The RNA-seq data were validated using quantitative reverse-transcription PCR. Using virus-induced gene silencing (VIGS) methods, evaluation of an ethylene receptor gene (XsERS) function indicated that the gene was closely related to early development of fruits and seeds. Based on the data presented here, we propose a model for a MAPK-ethylene signaling-NAC2 gene regulatory cascade that plays an important role in the regulation of the ovule abortion process in X. sorbifolium. The present study is imperative for understanding the mechanisms of ovule abortion after fertilization and identifying the critical genes and gene networks involved in determining the fate of ovule development.