A Comparative Characterization and Expression Profiling Analysis of Fructokinase and Fructokinase-like Genes: Exploring Their Roles in Cucumber Development and Chlorophyll Biosynthesis

Fructokinase (FRK) and fructokinase-like (FLN), belonging to the phosphofructokinase B type subfamily, share substantial sequence similarity, and are crucial in various plant physiological processes. However, there is limited information regarding what functionally differentiates plant FRKs from FLNs. Here, a total of three CsFRKs and two CsFLNs were identified from the cucumber genome. Their significant difference lay in the structure of their G/AXGD motif, which existed as GAGD in CsFRKs, but as G/ASGD in CsFLNs. Comparative phylogenetic analysis classified CsFRKs and CsFLNs into five sub-branches consistent with their quite different exon/intron organizations. Both transcriptome data and RT-qPCR analyses revealed that CsFRK3 was the most active gene, with the highest expression in the majority of tissues tested. Moreover, the expression levels of two putative plastidic genes, CsFRK1 and CsFLN2, were significantly positively associated with chlorophyll accumulation in the chlorophyll-reduced cucumber mutant. Briefly, both CsFRK and CsFLN genes were involved in the development of sink tissues, especially CsFRK3. CsFRK1 and CsFLN2 were recognized as candidates in the chlorophyll biosynthesis pathway of cucumber. These results would greatly assist in further investigation on functional characterization of FRKs and FLNs, especially in the development and chlorophyll biosynthesis of cucumber.