Function and Expression of a New 2,3-oxidosqualene Triterpenoid Cyclase from Soybean

Mohamad M. Gendy¹, Walaa A. Mohamed², Chigen Tsukamoto³, Seung Hwan Yang⁴, Gyuhwa Chung^{1, * 1}Department of Biotechnology, Chonnam National University, Yeosu 550-749, Korea, ²Department of Botany and Microbiology, Faculty of Science, Alexandria University, 21-526, Egypt, ³Department of Applied Biological Chemistry, Iwate University, Morioka, Iwate 020-8550, Japan, ⁴Center for Nutraceutical and Pharmaceutical Materials, Myongji University, Cheoin-gu, Yongin, Gyeonggi-Do 449-728, Korea

The first committed step in triterpenoid saponin biosynthesis is the cyclization of epoxysqualene into different triterpenoids. The cyclization process is catalysed by different genes of the Oxidosqualene cyclases (OSC) family. Previously, we have reported the molecular cloning and characterization of BAS-Iso, a new OSC gene in soybean. RT-PCR results of BAS-Iso indicated differential temporal and spatial expression pattern within specific tissues including old and young leaves, hypocotyl, stem, root, and callus. Moreover, RT-PCR and quantitative PCR of soybean seedlings growing on Gamborg B5 media with different conditions showed higher expression levels at high concentration of mannitol, NaCl and abscisic acid, as well as old senescent leaf. The BAS-Iso full length cDNA was cloned into the yeast expression vector pYES2 and electroporated to GIL77 mutant strain of Saccharomyces cerevisiae, which is deficient in OSCs. The transformed cells were collected, refluxed in alcoholic KOH and extracted twice with hexane, then analysed using high performance liquid chromatographic technique. Collectively, the newly isolated BAS-Iso gene resulted in the accumulation of β-amyrin and lupeol as end products. In addition, it confers a higher expression rate at stress conditions and may play a role in leaf senescence, in context with its major role in the defence mechanism against fungal infections; which has been detected upon eliciting the soybean cell culture with yeast extract.

*Corresponding Author: chung@chonnam.ac.kr