Nutritional Quality and Grain Yield of NAM-RNAi Wheat Under Abiotic Stress.

Grain Protein Content (GPC)-B1 (also called NAM-B1) and its paralogs are transcription factors of the No Apical Meristem (NAM) superfamily.  Non-functional alleles predominate in commercial wheat;  addition of functional copies of GPC has been shown to accelerate senescence and increase grain concentrations of N, Fe, and Zn.  However, when all functional copies of GPC/NAM in hexaploid wheat were down-regulated by RNA interference (TaNAM-RNAi), senescence was substantially delayed, and grain concentrations of N, Fe, and Zn were decreased.  Our objectives were to: a) determine whether the delayed senescence phenotype of the NAM-RNAi line would be advantageous under abiotic stresses of drought, heat, and low N, and b) determine the effects of abiotic stress on grain nutritional quality.  To assess the interaction between growing conditions and the TaNAM-RNAi phenotype, greenhouse experiments compared accumulation and remobilization of nitrogen and minerals in TaNAM-RNAi and control plants at a range of applied N and reduced post-anthesis irrigation under both optimal and hot greenhouse conditions.  Under both temperature conditions, TaNAM-RNAi plants generally accumulated greater quantities of N and minerals on a whole-plant basis, but accumulated less N per plant in seed.  Mineral nutrient contents of grain (per plant) were similar in TaNAM-RNAi and control plants.  Thus nutrient harvest indices were nearly universally lower for TaNAM-RNAi plants relative to the control.  Significant treatment effects were observed for N fertilization and applied irrigation in both experiments.  However, the TaNAM-RNAi and control plants responded similarly to treatments, both under optimal and hot growing conditions.  Therefore, the expression of the senescence processes conditioned by endogenous NAM genes appears to be largely independent of temperature, fertility, and post-anthesis moisture.  Tillering and yield was greatly reduced under heat stress, but grain mineral concentrations remained constant, suggesting that wheat grown under high temperatures has similar nutritional value.