Effect of nitrogen form and genetic variations of the Asparagine-cycling pathway on maize ear growth

Abstract

The positive growth response of maize ears to nitrate supplementation declines during late vegetative development, whereas ammonia addition continues to promote ear growth through the reproductive phase. The underlying genetic and physiological mechanisms for this phenomenon are currently unknown. Previous studies with DuPont Pioneer found that in the maize line Gaspé Flint, ear growth no longer responded to additional nitrate 20 days after emergence, but a subsequent addition of ammonia significantly increased ear dry weight as well as the transport amino acids asparagine and glutamine. This nitrogen-induction experiment was duplicated, and root, shoot, and ear tissues were profiled via RNA-Seq to characterize responses to ammonia or nitrate at four or eight hours after nitrogen treatment. Results suggest an upregulation of amino acid assimilation and metabolism genes in ammonia-treated plants. Illinois High Protein (IHP) differs from other genotypes in exhibiting an increased rate of nitrogen uptake, vigorous ear growth under low N conditions, very high levels of asparagine, but a reduced response to ammonia. These observations suggest that the mechanism for ammonia-induced ear growth in some genotypes may already be active in IHP even without ammonia supplementation. Compared to many other maize genotypes, IHP has functional variants of the two asparagine-cycling genes, Asparagine synthetase (AS) and Asparaginase (ASNase). We examined the effect of these two genes on ear growth using a range of populations that contain variants of these genes. Greater AS expression was associated with improved ear growth, whereas very low ASNase activity was associated with decreased developing ear size.