Downregulation of an allene oxide synthase gene improves photosynthetic rate and alters phytohormone homeostasis in field-grown guayule

Abstract

Guayule (Parthenium argentatum) is under development in the southwestern United States as a source of domestic natural rubber; increased rubber yield was found for greenhouse-grown guayule with downregulated allene oxide synthase (AOS). The objective of this study was to evaluate natural rubber production in guayule plants with varying levels of AOS gene expression grown in a field environment. Four plant genotypes: wildtype plants, vector controls, and transgenic plants with overexpressed and downregulated allene oxide synthase (total of 960 plants) were grown at the Bridgestone Guayule Research Farm in Eloy, Arizona between May 2016 to May 2018. Plant phenotypes were evaluated at 6-, 12-, 18-, and 24-months. Downregulated allene oxide synthase (AOSi) genotypes showed remarkable phenotypes including higher photosynthetic activity (net assimilation rate), total number of branches per plant, and plant stembark thickness compared with wildtype, vector control, and overexpressed AOS (AOSoe) lines. Moreover, the rubber particles were smaller and had higher activity (radiolabeled isoprenyl pyrophosphate (IPP) incorporation per g of rubber) in the AOSi genotypes than control and AOSoe lines. The downregulation of AOS also led to significant changes in phytohormone levels, which may have influenced the plants’ morphology and physiology. However, in contrast to laboratory and greenhouse studies, natural rubber concentration and yield were not higher in AOSi plants.