Improving Nitrogen Use Efficiency Through Overexpression of Alanine Aminotransferase in Rice, Wheat, and Barley

Abstract

Nitrogen is an essential nutrient for plants, but crop plants are inefficient in the acquisition and utilization of applied nitrogen. This often results in producers over applying nitrogen fertilizers, which can negatively impact the environment. The development of crop plants with more efficient nitrogen usage is, therefore, an important research goal in achieving greater agricultural sustainability. We utilized genetically modified rice lines over-expressing a barley alanine aminotransferase (HvAlaAT) to help characterize pathways which lead to more efficient use of nitrogen. Under the control of a stress-inducible promoter OsAnt1, OsAnt1:HvAlaAT lines have increased above-ground biomass with little change to both nitrate and ammonium uptake rates. Based on metabolic profiles, carbon metabolites, particularly those involved in glycolysis and the tricarboxylic acid (TCA) cycle, were significantly altered in roots of OsAnt1:HvAlaAT lines, suggesting higher metabolic turnover. Moreover, transcriptomic data revealed that genes involved in glycolysis and TCA cycle were upregulated. These observations suggest that higher activity of these two processes could result in higher energy production, driving higher nitrogen assimilation, consequently increasing biomass production. Other potential mechanisms contributing to a nitrogen-use efficient phenotype include involvements of phytohormonal responses and an alteration in secondary metabolism. We also conducted basic growth studies to evaluate the effect of the OsAnt1:HvAlaAT transgene in barley and wheat, which the transgenic crop plants increased seed production under controlled environmental conditions. This study provides comprehensive profiling of genetic and metabolic responses to the over-expression of AlaAT and unravels several components and pathways which contribute to its nitrogen-use efficient phenotype.

Molecular characterization and n use efficiency of LeAlaAT ‘Mekongga’ transgenic rice lines

“…Garnett et al (2013) found no biomass differences among wildtype and maize AlaAT transgenic plants in a hydroponic system under different N rates (0.5 and 2.5 mM N). Tiong et al (2021) found similar results for rice plants expressing OsAnt1:HvAlaAT grown in a hydroponics system. This similarity suggested that hydroponic systems provide steady N supplies even at low concentrations.…”

“…Selvaraj et al (2016) discovered a limited or no increase in the biomass of the 43-day old plants of OsAnt1:HvAlaAT transgenic lines with a NERICA4 background under low N conditions. These results implied that during vegetative growth, the biomass of the transgenic lines may moderately and nonsignificantly increase and is then accumulated throughout the growth stages, resulting in a significant increase in biomass and seed yield (Tiong et al, 2021).…”

“…Root-to-shoot ratio did not significantly differ between the MWT and all transgenic lines under each N rate (Table 2). Similarly, Tiong et al (2021) found low significant differences in the root dry weight of rice overexpressing the HvAlaAT gene. By contrast, Shrawat et al (2008) and Beatty et al (2013) found that the shoot and root biomasses of OsAnt1:HvAlaAT rice lines were higher than those of the control.…”

“…NUE technology can considerably reduce the need for N fertilizer while simultaneously increasing food security and reducing greenhouse gas emissions from the rice ecosystem (Sapkota et al, 2021). A recent study by Tiong et al (2021) on rice genetic transformation by using HvAlaAT under a stress-inducible promoter OsAnt1 reported that OsAnt1:HvAlaAT lines exhibited increased above-ground biomass with a negligible change in nitrate and ammonium uptake rates. The mechanism involved was related to the drastic alteration of OsAnt1:HvAlaAT lines to increase metabolic turnover.…”

Genetic Engineering and Genome Editing for Improving Nitrogen Use Efficiency in Plants

“…The HvAlaAT expression increased the biomass in sugarcane [ 124 ] and the vegetative biomass and grain yield in wheat, yet it did not change the phenotype of sorghum [ 126 ]. As shown by a recent study [ 128 ], a transfer of a gene combination of OsAnt1::HvAlaAT to rice, wheat and barley increased biomass and yield. Researchers found significant changes in carbohydrate metabolism, which includes glycolysis and the TCA cycle, and suggested that the increased production of energy resulted in increased N assimilation.…”