Downregulation of a CYP74 Rubber Particle Protein Increases Natural Rubber Production in Parthenium argentatum

Abstract

We report functional genomics studies of a CYP74 rubber particle protein from <i>Parthenium argentatum</i>, commonly called guayule. Previously identified as an allene oxide synthase (AOS), this CYP74 constitutes the most abundant protein found in guayule rubber particles. Transgenic guayule lines with <i>AOS</i> gene expression down-regulated by RNAi (<i>AOSi</i>) exhibited strong phenotypes that included agricultural traits conducive to enhancing rubber yield. <i>AOSi</i> lines had higher leaf and stem biomass, thicker stembark tissues, increased stem branching and improved net photosynthetic rate. Importantly, the rubber content was significantly increased in <i>AOSi</i> lines compared to the wild-type (WT), vector control and <i>AOS</i> overexpressing (<i>AOSoe</i>) lines, when grown in controlled environments both in tissue-culture media and in greenhouse/growth chambers. Rubber particles from <i>AOSi</i> plants consistently had less AOS particle-associated protein, and lower activity (for conversion of 13-HPOT to allene oxide). Yet plants with downregulated <i>AOS</i> showed higher rubber transferase enzyme activity. The increase in biomass in <i>AOSi</i> lines was associated with not only increases in the rate of photosynthesis and non-photochemical quenching (NPQ), in the cold, but also in the content of the phytohormone SA, along with a decrease in JA, GAs, and ABA. The increase in biosynthetic activity and rubber content could further result from the negative regulation of <i>AOS</i> expression by high levels of salicylic acid in <i>AOSi</i> lines and when introduced exogenously. It is apparent that AOS in guayule plays a pivotal role in rubber production and plant growth.