Transcriptomics Reveal an Integrated Gene Regulation Network of Early Flowering Development in an Oil Sunflower Mutant Induced by Heavy Ion Beam

The oil sunflower is an important oil crop and ornamental plant. Flowering time affects the environmental adaptability and final yield of oil sunflowers. Floral induction is one of the important events that determines subsequent reproductive growth and seed setting, but there has been no systematic study on the regulation of gene expression during the transition from vegetative growth to reproductive growth in oil sunflowers. In this study, an oil sunflower mutant displaying early flowering ( ef ) was obtained by heavy ion beam irradiation. This mutant had a stable genetic trait, and its flowering time was 15 days earlier than the wild type (WT) in the field. The histology result showed that the ef mutant induced floral meristem at 6-leaf stage earlier than WT. The shoot apical meristems (SAMs) of the ef mutant and WT at 4-leaf, 6-leaf, 8-leaf, 10-leaf and budding periods were collected for RNA sequencing. The results showed that the transition from the leaf meristem to the floral meristem resulted in significant changes in the transcriptional landscape. Overall, 632, 1825, 4549, 5407 and 2164 differentially expressed genes (DEGs) were identified at 4-leaf, 6-leaf, 8-leaf, 10-leaf and budding periods, respectively. These DEGs were mainly enriched in biological pathways, including plant hormone signal transduction, carbon metabolism, protein processing in endoplasmic reticulum, secondary metabolism, and photosynthesis. We also found significant differences in the expression levels of starch and sucrose metabolism-related genes in the ef mutant and WT, indicating that sugar signaling plays an important role in the early flowering of oil sunflowers, especially SUC9 and sugar synthesis and degradation enzyme genes. In addition to hormone and sugar signals, flowering integration genes SOC1 , AP1 , FUL and LFY were upregulated in the ef mutant, and genes in photoperiod, aging, autonomous and temperature pathways were also involved in the regulation of floral transition. The results showed that plant hormones, sucrose metabolism, and flowering genes synergistically cause the early flowering of oil sunflowers. Our study provided important information for understanding flowering and is helpful for the genetic improvement of sunflowers.