The role of citrate synthase HbCS4 in latex regeneration of Hevea brasiliensis (para rubber tree)

The rubber tree, Hevea brasiliensis, is a tropical rubber-yielding crop with economic importance and provides over 90% of natural rubber (NR) for the global rubber industry. NR biosynthesis belongs to a typical secondary metabolism in laticifers. Definitely, NR production is a huge energy-consuming metabolism, especially during latex regeneration. The tricarboxylic acid (TCA) cycle, as the primary energy source, strongly supports NR synthesis in laticifers. In this study, a mitochondrial citrate synthase (CS) gene named as HbCS4 was isolated from rubber tree and related changes of expression patterns, enzyme activity and metabolites assays in response to tapping and ethephon (ethylene-releasing agent) treatments were compared. These molecular analyses were also conducted in two typical varieties Reyan 7-33-97 and PR107 as well as in HbCS4-overexpressing lines. As a result, the expression characteristics of HbCS4 or its enzyme activity tallied with the energy requirement of rubber synthesis in both treatments and the yielding traits of varieties Reyan 7-33-97 and PR107. In addition, in HbCS4-overexpressing lines, the activities of three key enzymes in TCA cycle, namely CS, NAD-IDH and alpha-KGDH, were all increased simultaneously. To conclude, these results suggested that HbCS4 is the initial enzyme required for energy supply in laticifers, which greatly contributes to rubber production through changing its expression or enzyme activity.