Overexpression of Phenylalanine Ammonia-Lyase in Transgenic Roots of Coleus blumei Alters Growth and Rosmarinic Acid Synthesis

Abstract

Most natural phenolic compounds in plants are derived from trans-cinnamic acid, formed by the nonoxidative deamination of L-phenylalanine by phenylalanine ammonia-lyase (EC 4.3.1.24). Although a strict line between primary and secondary metabolism cannot be drawn, phenylalanine ammonia-lyase is considered to play a pivotal role in channelling carbon flux from primary metabolism to phenolic synthesis. The objective of this work is to evaluate the impact of phenylalanine ammonia-lyase activity on phenolic production in Coleus blumei. Transgenic roots of C. blumei, harbouring the Arabidopsis thaliana PAL1 gene, under the control of the CaMV 35S promoter, along with empty vector and wild-type roots, were regenerated. Transgenic root lines had disparate phenylalanine ammonia-lyase activities ranging from 67 to 350 %, compared to wild-type roots. Growth rates significantly differed, with the lowest in transgenic roots exerting augmented phenylalanine ammonia-lyase activity. Transgenic roots with high phenylalanine ammonia-lyase activity had lower growth rates, lower amounts of total phenolics, rosmarinic acid (the major phenolic compound in C. blumei) and chlorogenic acid, but increased amounts of caffeic acid. There was no increase in total phenolics and rosmarinic acid content after feeding transgenic roots with casein enzymatic hydrolysate and L-tyrosine. This shows that augmented phenylalanine ammonia-lyase activity inhibits growth and phenolic metabolism, and the probable regulator of these processes is trans-cinnamic acid.

Keywords

• caffeic and rosmarinic acid
• Coleus blumei
• overexpression
• phenolics
• phenylalanine ammonia-lyase
• transgenic roots