Triose phosphate utilization determines the Yield-Grain protein trade-off in contrasting rice genotypes under varying light intensities

Attempts to increase grain protein content (GPC) most often result in a reduction in yield in cereals. This trade-off between GPC and yield could arise mainly because of the shared source of reductants for carbon and nitrogen assimilation. The primary aim of this study was to investigate whether the negative relationship between yield and GPC is primarily influenced by the availability of reductants for carbon and nitrogen assimilation.

Based on the initial screening of ten high-yielding rice genotypes, we identified two genotypes contrasting in GPC with comparable yield and photosynthesis. This was the most appropriate set to assess the interrelationship between carbon and nitrogen assimilation among the contrasts under ambient (~ 1500 µmole m⁻² s⁻¹) and low light intensities (~ 300 µmole m⁻² s⁻¹).

Data suggest that triose phosphate utilization (TPU) limitation acts as a switch between carbon and nitrogen assimilation. Under ambient conditions, the high GPC genotype effectively managed the flow of electrons to carbon assimilation and diverted the excess electrons to other sinks. The diversion of extra electrons, particularly to nitrite reduction, was boosted by increased substrates from higher uptake, transport, and metabolism of nitrogen in leaves. In low GPC genotype, under ambient condition, excess electrons were quenched in the form of heat.

The current study suggests that an efficient utilization of electrons by adopting a switch called TPU limitation coupled with better uptake of nitrogen and remobilization efficiency can be a promising genotype for breeders to develop a high yielding variety complemented with high GPC.