Genetic Basis And Molecular Regulatory Mechanism Of Cucumber Fruit Elongation

Cucumber（Cucumis sativus L.）is one of the most important vegetables in China.Fruit size and shape are important agronomic traits influencing yield and market value.The fruit length of cucumber has increased significantly during domestication and improvement process,but the genetic and molecular basis of fruit domestication has remained unclear.Plant organ growth and development are determined by a subtle balance between growth stimulation and inhibition,but the underlying mechanisms regulating fruit growth balance to achieve final size and shape is well investigated.Gene editing is a powerful tool for functional gene research and crop improvement,but it is difficult for Cucurbitaceae crops to carry out genetic transformation,which greatly limits the application of gene editing technology.Based on the established genetic transformation cucumber,the genetic basis and molecular regulatory mechanism of cucumber fruit length were studied using genetics,developmental biology,physiological and biochemical methods.The main results were as follows:1.We propose an“optimal infiltration intensity”strategy by introducing sonication,micro-brush and vacuum treatment.We successfully established a genetic transformation system for melon and squash using this strategy and improved the transformation efficiency of cucumber.2.We identified a cucumber mutant that bears short fruits owing to repressed cell division.SF1（Short Fruit 1）encodes a cucurbit-specific RING-type E3 ligase,and the mutation resulted in its enhanced self-ubiquitination and degradation,but accumulation of ACS2（1-aminocyclopropane-1-carboxylate synthase 2）,a rate-limiting enzyme for ethylene biosynthesis.3.SF1 ubiquitinates and degrades both itself and ACS2 to control ethylene synthesis for dose-dependent effect on cell division and fruit elongation.Our findings reveal the mechanism by which ethylene dosage is regulated for the control of cell division in developing fruit.4.We identified two closely linked,epistatically interacting genes:YTH1,encoding an RNA m⁶A reader protein,and an ACC synthase（ACS2）that contributed to longer cucumber（Cucumis sativus）fruit during domestication.The causative single nucleotide polymorphism（SNP）,1287C>T synonymous substitution in ACS2,affects protein translation by altering m RNA m⁶A levels on nearby adenosine residues.5.In wild cucumber,the YTH1^wild recognizes the m⁶A modification on ACS2^1287C transcripts and promotes ACS2 translation,resulting in shorter fruit.In cultivated cucumber,ACS2^1287T is recessive epistasis to YTH1^cul through impairment of m⁶A methylation,leading to attenuated protein translation and fruit elongation.This study provides genetic evidence of synonymous variation shaping a biological trait,by epistatically interacting with another mutation through epitranscriptomic regulation.