Enhanced Rice Growth is Conferred by Increased Leaf ADP-Glucose Pyrophosphorylase Activity

Abstract

Modification of leaf starch levels may be employed in attempts to increase cereal yield. Few studies have examined leaf starch as a plant biomass limiting factor. Here we test the hypothesis that rice plant productivity may be increased by increasing leaf starch. Starch biosynthesis is controlled by the heterotetrameric rate-limiting enzyme ADP-glucose pyrophosphorylase (AGPase). Rice variety Nipponbare was transformed with a modified form of the maize endosperm AGPase large subunit gene, Sh2r6hs, as well as with the small subunit gene, Bt2, under control of a rice RuBisCO small subunit promoter. RNA sequencing results indicated that Sh2r6hs and Bt2 transcript levels were each greater than 20 times that of the native genes. Increased total AGPase activity was correlated with higher leaf starch accumulation at the end of the day. Yield trials of T1 derived homozygous plants indicate that increased leaf AGPase leads to a 29% increase in plant biomass under the conditions tested without changing the rate of photosynthesis while significantly reducing leaf transpiration and conductance. Additionally, functional annotation clustering of significantly up and down regulated transcripts reveals areas of protein metabolism, specifically protein biosynthesis, transport, and localization, that were altered in response to increased leaf starch. Together, these findings indicate plant growth is limited by native levels of leaf starch and that it is possible to increase plant yield via the starch biosynthesis pathway.