Enhanced Rice Growth Is Conferred By Increased Leaf ADP-Glucose Pyrophosphorylase Activity.

Allocation of photoassimilates from source to sink tissues is a driving force of plant growth and yield.  Modification of sink or source strength may therefore be employed in attempts to increase cereal yield.  Few studies have examined leaf starch as a source limiting factor.  Here we test the hypothesis that higher rice yield may be achieved by increasing source strength via an increase in leaf starch biosynthetic rates.  The rate limiting step in starch biosynthesis is controlled by the heterotetrameric 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.  Results show that increased expression of leaf AGPase transcripts is correlated with higher leaf AGPase enzyme activity as well as increased leaf starch accumulation at the end of the day.  Yield trials of T₂ derived homozygous plants in controlled environment conditions indicate that increased leaf AGPase leads to a significant increase in plant biomass across replicated yield trials in the form of increased tiller number, without apparent changes the in flag leaf photosynthetic rates.