Phenotyping nitrogen responses in maize and finger millet using root morphometrics and biosensor analysis
Cereal crops can acclimate to better utilize low and high soil-available nitrogen (N), often the most limiting soil nutrient. Finger millet (Eleusine coracana Gaertn.) is an ancient crop grown by subsistence farmers in Africa and South Asia, adapted to low N input conditions. The roots have not been studied for acclimation traits such as altered root number, length, or root hair characteristics. It was hypothesized that drastic root morphometric acclimation would be observed upon extreme N limitation. N limitation caused declines in multiple root traits including crown root number, which was coordinated with decreased tillering, demonstrating whole plant acclimation to low N. Field pot experiments showed finger millet was able to grow to maturity and produce grain heads in the absence of deliberately added N. In contrast to finger millet, modern maize (Zea mays L.) varieties are adapted to high N inputs and require the ability to assimilate high volumes of inorganic N into important amino acids, particularly glutamine (Gln), for transport to supply growing tissues. Free Gln can be measured inexpensively by the luminescent output of GlnLux, a genetically engineered whole-cell microbial biosensor for Gln. It was hypothesized that GlnLux would allow for previously unreported spatial/temporal mapping of N assimilation. The results represent the most detailed analysis of Gln in any plant, and show that Gln accumulation is governed by multi-factorial interactions including position along the leaf blade, leaf age, N application rate, and N uptake duration. GlnLux was also hypothesized to have potential in the field as a novel diagnostic for early-to-mid season soil N availability. When sampled at stage V6 or later, GlnLux glutamine output consistently correlated with the N application rate, but also end-season yield, and grain N content. Yield prediction ability was minimally equivalent to SPAD chlorophyll, and outperformed GreenSeekerTM NDVI. Additionally, depleting soil N via overplanting increased GlnLux resolution to an earlier stage (V5), suggesting a novel means to improve plant N testing early in the growing season. These studies have provided a combination of novel tools and information for crop improvement and management in both low and high input cereals.