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Research advances in plant fructokinases

Sugars are essential to the fundamental processes required for plant growth and development. The hexose sugars glucose and fructose are central to most metabolic pathways and serve as the raw material for organic matter in plants. Fructose accounts for half of the hexose generated by sucrose cleavage, and it can be phosphorylated either by fructokinase (FRK) or hexokinase (HXK). FRKs are the main fructose phosphorylating enzymes because their substrate specificity and affinity for fructose are much higher than those of the HXK, and thereby regulate fructose concentration and the distribution and flow of organic carbon in cells. Therefore, FRKs are considered to be of critical importance for all metabolic pathways and the formation of organic matter in plants. To date, all plant FRKs have been assigned to the phosphofructokinase type B subfamily, a large group within the sugar kinase family. Unlike mammalian FRKs, which phosphorylate fructose to form fructose 1-phosphate (F1P), plant FRKs, similar to bacterial FRKs, phosphorylate fructose to form fructose 6-phosphate (F6P). Moreover, there are both cytosolic and plastidic FRKs in plants, indicating that they play roles in different cellular compartments. Phylogenetic analysis shows that most of the FRK genes that are located on similar branches encode proteins with similar subcellular localizations, suggesting that they may perform similar functions. A large number of studies have implicated plant FRKs in vascular tissue development, and in recent years, it has been found that FRK is related to seed, fruit, and pollen development. FRK can also interact with the plant FLOWERING LOCUS T protein to co-regulate flowering. Additionally, a large number of experiments have shown that FRK affects chloroplast development mainly by affecting chloroplast gene expression, thus affecting photosynthesis and the source-sink relationship. Beyond these functions, FRK, as an important carbon flux-regulated kinase in plants, also plays an important role in plant stress response and can be used as an early marker of stress response signaling. Despite the fact that numerous studies have highlighted the functions and importance of FRK, detailed studies on the physiological and metabolic functions of this enzyme and the underlying regulatory mechanisms are lacking. Most research on FRK has been focused on model plants such as Arabidopsis thaliana , and limited progress has been made studying FRK functions in crops. With the aim of systematically summarizing the characteristics of FRKs and their important roles in plant life activities, this paper reviews their roles in regulating plant growth and development, responding to stress, and regulating photosynthetic and metabolic pathways. In addition, several scientific questions about FRKs are discussed, including whether they act as sugar-sensing signals, whether they are involved in plant defense processes, how they regulate plant source-sink metabolism, whether they are involved in regulating the phase transitions from the vegetative to reproductive growth, and whether there is redundancy in function among family members. Recently developed omics and gene editing technologies are expected to be applied toward answering these questions and discovering the related regulatory networks in order to fully understand the role of FRKs in plant sugar metabolism.