Abstract
Foxtail millet (FM) [Setaria italica (L.) Beauv.] is a grain and forage crop well adapted to nutrient-poor soils. To date little is known how FM adapts to low nitrogen (LN) at the morphological, physiological, and molecular levels. Using the FM variety Yugu1, we found that LN led to lower chlorophyll contents and N concentrations, and higher root/shoot and C/N ratios and N utilization efficiencies under hydroponic culture. Importantly, enhanced biomass accumulation in the root under LN was in contrast to a smaller root system, as indicated by significant decreases in total root length; crown root number and length; and lateral root number, length, and density. Enhanced carbon allocation toward the root was rather for significant increases in average diameter of the LN root, potentially favorable for wider xylem vessels or other anatomical alterations facilitating nutrient transport. Lower levels of IAA and CKs were consistent with a smaller root system and higher levels of GA may promote root thickening under LN. Further, up-regulation of SiNRT1.1, SiNRT2.1, and SiNAR2.1 expression and nitrate influx in the root and that of SiNRT1.11 and SiNRT1.12 expression in the shoot probably favored nitrate uptake and remobilization as a whole. Lastly, more soluble proteins accumulated in the N-deficient root likely as a result of increases of N utilization efficiencies. Such “excessive” protein-N was possibly available for shoot delivery. Thus, FM may preferentially transport carbon toward the root facilitating root thickening/nutrient transport and allocate N toward the shoot maximizing photosynthesis/carbon fixation as a primary adaptive strategy to N limitation.
Highlights
Nitrogen (N) is an essential macronutrient for plant growth, development, and production
Legumes enhance N uptake through nodulation and N fixation (Postgate, 1998), while many non-legume crops, i.e., maize, enhance root growth for N forage when grown in local low nitrogen (LN) environment (Wang et al, 2003; Chun et al, 2005a)
Plants were harvested after 1 week of the LN treatment
Summary
Nitrogen (N) is an essential macronutrient for plant growth, development, and production. As an essential component of nucleic acids and proteins, N actively participates in most physiological and biological processes in crop production including photosynthesis, carbohydrate allocation, root patterning, and flower development, and signifies itself as a critical macronutrient controlling crop yield and quality (Stitt, 1999; Miller and Cramer, 2004). N concentrations in the soil solution, as nitrate and ammonium, range from 100 μM to 10 mM This heterogeneity and dynamic variations of N concentrations lead plants to sense external N availability and respond via hierarchical morphological, physiological, and molecular adaptations (Glass, 2003; Miller et al, 2007; Garnett et al, 2009). Different adaptive strategies may have arisen in different crop species over evolution
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