Adapting Kernel Metabolism to Enhance Cereal Yield Under Adverse Conditions.

The objectives of this work are to identify rate-limiting, heat labile reactions in biosynthetic pathways essential to seed yield and then genetically modify them to enhance the heat stability or the amount of the gene product.  These alterations are then monitored in corn plants grown at different temperatures to determine if they affect seed yield.  Candidate biochemical steps are first identified by recessive mutations in genes that condition a seed phenotype. The vast majority of mutants screened were derived from a population of 8,256 F₃ W22 inbred lines (UniformMu population) containing 41,523 sequenced Mutator (Mu) transposable element insertions.  Mutants having seed phenotypes are being tested to identity those that exhibit a gene dosage effect on seed weight.  So far, five genes identified in the Mu population and two of 11 “classic” maize genes produce less seed weight in two doses of the functional allele compared to three doses.  These genes are being cloned via their Mu tags.  Because of their importance in seed development, mutants in genes involved in starch biosynthesis, the oxidative pentose phosphate pathway, and sugar signaling have also been targeted for isolation from the UniformMu population.   

            Genes exhibiting dosage effects that encode rate-limiting, heat labile steps are the main targets of this proposal.  Accordingly, the starch enzyme, ADP-glucose pyrophosphorylase (AGPase) receives considerable attention.  One variant enzyme conditions a 64% increase in yield, depending on growth conditions. Three different strategies have identified 54 amino acids that play important roles in heat stability, kinetics and allostery. In maize pgd3 encodes the plastidial isozyme, 6-phosphogluconatedehydrogenase (6-PGDH). Loss of this enzyme gives rise to kernels lacking starch. Assays reveal that this enzyme is relatively heat sensitive. The gene is presently being expressed in E. coli to isolate heat stable variants.