It is rather well established that rubber biosynthesis in rubber trees (Hevea brasiliensis) takes place in laticifers and is dependent on mevalonate (MVA). 3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGR, EC 22.214.171.124) has been shown to catalyze a rate-limiting step in this pathway. However, our studies demonstrated that both 3-hydroxy-3-methylglutaryl coenzyme A synthase (HMGS, EC 126.96.36.199) and HMGR are essential enzymes involved in rubber biosynthesis. In this chapter, we report on the current information as to the regulation of both HMGS and HMGR and their effect on rubber biosynthesis in H. brasiliensis. Hevea HMGS is encoded by a small gene family consisting of hmgs-1 and hmgs-2. The available information concerning the nature of the gene(s) encoding HMGS is also summarized. Enzyme activity and mRNA transcripts were mainly found in tissues with more laticifers, the sites of rubber biosynthesis. In latex of the high-yielding rubber clone, hmgs mRNA levels and enzyme activity were significantly higher than in the latex of the low-yield variety. Furthermore, the mRNA transcripts and enzyme activity in latex were higher at night than daytime, which is reflected by the dry rubber content. Ethephon treatment, which is known to increase the latex yield, had a direct effect on both hmgs mRNA transcripts and enzyme activity. The hmgs mRNA levels and dried rubber content per tapping from intraclone rubber trees were also shown to be highly correlated. HMG-CoA acts as a substrate for HMGR to form mevalonate, which is further converted to isoprenoid compounds as well as natural rubber. Three genes are known to encode HMGR in H. brasiliensis, namely, hmgr-1, hmgr-2, and hmgr-3, and hmgr-1 is likely to be involved in rubber biosynthesis. The hmgr-1 mRNA level was well correlated with dried rubber content, similar to those observed in the case of hmgs gene expression.
These findings clearly indicated that both HMGS and HMGR enzyme activities are involved in early steps of rubber biosynthesis in H. brasiliensis at the level of their gene expression. The two enzymes possibly function in concert in response to the supply of substrate for rubber biosynthesis, similar to the synthesis of cholesterol in animals.
Within this Chapter:
IntroductionProperties of H. brasiliensis HMGS and HMGR EnzymesMolecular Biology of HMGSRole of HMGS and HMGR in Rubber BiosynthesisConclusionReferences
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