Transcriptional and phytohormone regulatory network involved in LITTLELEAF-mediated organ size development in cucumber (Cucumis sativus)

Organ size, especially fruit size, is an economically important trait in many horticulture crops like cucumber (Cucumis sativus L.) which affects both fruit yield and quality. Organ size determination is the outcome of complex interplay of many factors such as transcription regulators, phytohormone and the environments. The cucumber littleleaf (ll) mutation exhibits small organ size, which encodes a defective WD40 repeat domaincontaining protein, but the regulatory mechanisms of LL-dependent organ size control are largely unknown. To test the functions of the ll gene in different genetic backgrounds and explore its regulatory network, we developed near isogenic lines (NIL) of ll gene by introgressing the recessive allele into a Chinese Long inbred line 9930 with marker assisted selection (MAS). Both foreground and background selections were practiced which allowed development of a NIL (HAULL) at BC2F2 which were homologous at the ll locus and captured >96% genetic background of the recurrent parent. Comparative morphological analysis between the NILs indicated that the mutant allele for reduced organ size was independent of the genetic background. Transcriptome profiling using RNA-Seq in the unpollinated ovaries of the two NILs identified differentially expressed genes which were highly enriched with genes known to regulate organ size, as well as transcription factors, and genes in various phytohormone biosynthesis/signaling pathways. RNA-Seq and phytohormone assays found that auxin and cytokinin played more important roles in LL-regulated fruit growth. These results will be helpful for further explore the molecular mechanism of organ size in cucumber.

Automated summary

Organ size, especially fruit size, is an economically important trait in cucumber. The littleleaf mutation affects organ size and encodes a defective protein, but the regulatory mechanisms are largely unknown. Near isogenic lines (NIL) were developed to explore the function of the mutated gene and its regulatory network. Transcriptome profiling and phytohormone assays indicated that auxin and cytokinin play important roles in regulating fruit growth.