The hexokinases HbHXK2 and 4 are key enzymes involved in glucose metabolism and contribute to rubber productivity in Hevea brasiliensis (para rubber tree)

Abstract In higher plants, hexokinase (HXK) is a crucial enzyme that phosphorylates glucose, because there is no evidence for the existence of glucokinase (GLK). In the rubber tree laticifers, the phosphorylation of glucose initiates glycolysis and pentose phosphate pathways, which provide energy and other compounds required for latex regeneration. However, the mechanism of how HXKs participate in the latex regeneration process in laticifers is currently unknown. In this study, four HXK cDNAs (HbHXK1-4) were isolated from the rubber tree (Hevea brasiliensis) and sequenced. Nucleotide-derived amino acid sequences revealed the presence of conserved amino acids that are required for catalytic activity and subcellular targeting. Among these genes, HbHXK2 and 4 were found to be the predominant isoforms in latex (cytoplasm of laticifers) when compared with others. Phylogenetic and subcellular localization analysis revealed that HbHXK2 was presumably targeted to the chloroplast and HbHXK4 to the mitochondria. Both HbHXK2 and 4 were confirmed to have catalytic activities towards glucose and fructose with a significantly higher affinity for glucose. In yield-stimulating experiments, HbHXK4 was strongly induced by tapping and ethephon treatments; whereas, HbHXK2 had the opposite expression pattern. The abundance of HbHXK4 transcripts was consistent with the observed catalytic activity and latex yield. Furthermore, in laticifers, HbHXK4 was more efficient in phosphorylating glucose compared with HbHXK2. Based on these results, it is concluded that HbHXK2 and 4 are the key enzymes for glucose metabolism during latex regeneration and may contribute to the productivity of H. brasiliensis (para rubber tree), especially HbHXK4.