SPLs-mediated flowering regulation and hormone biosynthesis and signaling accompany juvenile-adult phase transition in Pyrus

Abstract

A long juvenile period limits the breeding process of fruit trees, such as pears. To understand the molecular mechanisms involved in the juvenile-adult phase change, two cDNA libraries of juvenile and adult phases from seedlings of hybrid offspring (Pyrus. pyrifolia Nakai cv.Whangkeumbae × P. bretschneideri Rehd. cv. Zaosu) were subjected to high-throughput sequencing. A total of 23,995 genes were obtained, of which 608 and 290 genes were down- and up-regulated, respectively, in the adult phase compared with the juvenile phase. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that some differentially expressed genes were involved in hormone biosynthesis and signaling pathways. Determination of hormone contents revealed that the levels of gibberellin A4 (GA4), ethylene, and jasmonic acid (JA) were lower in the adult than in the juvenile phase, while the contents of indole-3-acetic acid (IAA), gibberellin A1 (GA1), and abscisic acid (ABA) were significantly higher in the adult than in the juvenile phase. Combining the quantitative PCR (qPCR) analysis, we suggested that JA and ethylene might play negative roles, while ABA and IAA might play positive roles in the juvenile-adult phase transition in Pyrus. Meanwhile, a series of flowering-related genes were differentially expressed between juvenile and adult phases, including SPL (Squamosa promoter binding protein-like) genes, the flowering-time integrator gene SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1), and some floral meristem identity genes. Through qPCR analysis of SPL family members, 27 SPLs were found to participate in the phase transition in Pyrus. However, the regulatory mechanisms of hormone and SPLs require further study.