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[Objectives]This study aimed to analyze the effects of mutation of cytokinin receptor gene SlHK4 on flowering time of tomato, and to lay a foundation for elucidating molecular mechanisms of the regulation of cytokinin pathways on plant flowering. [Methods]Two homozygous mutant lines slhk4-4 and slhk4-5 of cytokinin receptor gene SlHK4 and wild-type(WT)tomato plants were used as materials. The flowering time of each line was analyzed. The shoot apexes of plants at 26 days after germination were analyzed by microscopic observation of paraffin sections, detection of cytokinin-responsive fluorescence signals and transcriptome sequencing. [Results]slhk4-4 and slhk4-5 mutant plants had the same number of internodes as WT plants when the first flowers opened, but the time from germination to flowering for slhk4-4 and slhk4-5 was 6 and 4 days later than WT plants, respectively. Compared with WT plants, the differentiation of slhk4-5 floral meristem was delayed; the sizes of cells in the slhk4-5 floral meristems decreased significantly while the cell number was slightly smaller; and the cytokinin-responsive fluorescence signals were significantly reduced in the shoot apexes of the F1 hybrids of slhk4-4 and the transgenic line with a GFP gene driven by the cytokinin-sensor promoter TCSn. In the slhk4-5 shoot apexes, expression of Cyclin-B1-2, a positive regulatory gene for cell cycle, was down-regulated; the expression of FUL2 and SOC1, positive regulatory genes for flowering, was up-regulated, while expression of floral homeotic genes AGL12 and AGL19 was down-regulated, in comparison to those in WT shoot apexes. The expression variation trends of the genes related to flowering regulation were not completely consistent with the trait change of slhk4-5 with delayed flowering time. The functional analyses of differentially expressed genes(DEG)showed that 57.6% of the DEG with functional annotation were structural genes enriched in the pathways related to photosynthesis, respiration, protein synthesis and metabolism, etc. Therein, 88.2% of these DEG showed down-regulated expression in slhk4-5 compared to those in WT shoot apexes, indicating that the cell activities in slhk4 shoot apexes were likely weaker than those in WT. [Conclusions]Loss-of-function mutations of cytokinin receptor SlHK4 led to weaker cytokinin signaling in slhk4 shoot apexes, decreased size, reduced activity and delayed differentiation of the cells in slhk4 floral meristems, and thus delayed flowering time of slhk4 plants. SlHK4 was a positive regulator of flowering time in tomato. [ABSTRACT FROM AUTHOR]

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