Biosensor-based spatial and developmental mapping of maize leaf glutamine at vein-level resolution in response to different nitrogen rates and uptake/assimilation durations

Background: The amino acid glutamine (Gln) is a primary transport form of nitrogen in vasculature following root uptake, critical for the location/timing of growth in maize and other cereals. Analytical chemistry methods do not permit in situ analysis of Gln, including visualization within the vascular network. Their cost and tissue requirement are barriers to exploring the complexity of Gln dynamics. We previously reported a biosensor, GlnLux, which can measure relative Gln levels inexpensively with tiny amounts of tissue. Results: Here, maize seedlings were given different N rates for multiple uptake/assimilation durations, after which > 1500 leaf disk extracts were analyzed. A second technique permitted in situ imaging of Gln for all leaves sampled simultaneously. We demonstrate that multifactorial interactions govern Gln accumulation involving position within each leaf (mediolateral/proximodistal), location of leaves along the shoot axis, N rate, and uptake duration. In situ imaging localized Gln in leaf veins for the first time. A novel hypothesis is that leaf Gln may flow along preferential vascular routes, for example in response to mechanical damage or metabolic needs. Conclusions: The GlnLux technology enabled the most detailed map of relative Gln accumulation in any plant, and the first report of in situ Gln at vein-level resolution. The technology might be used with any plant species in a similar manner.