Comparison of physiological, biochemical and proteomic responses to nitrate or amino acids availability in maize

It is well known that plants can take up organic nitrogen (N) in the form of amino acids. However, their contribution to N nutrition in crops is still uncertain. In recent years, several studies have pointed out that distinct amino acids affect plants differently. However, little is known about their effects on plant metabolism, especially when available in mixtures such as in soils. This work aimed to study the effects on the growth and the biochemical, physiological and proteomic responses of maize (Zea mays L.) plants grown in a hydroponic system in the presence of different N forms: amino acids and nitrate. Eight amino acids were provided to the roots as a mixture at composition and concentrations mimicking soil conditions (total concentration 250 μM). The treated plants were compared with control plants provided with 250 μM of nitrate. Plants were analyzed after five and seven days of hydroponic growth to follow both early responses and later metabolic adjustments. Plants supplied with amino acids showed a slower biomass accumulation. Moreover, the amino acid-treated plants were characterized by a decreased root/shoot ratio and a shorter but thicker root system. The metabolic status of plants was affected by the nutritional conditions, as suggested by the contents in leaves and roots of nitrate, ammonium and key metabolites, such as reducing sugars and sucrose. The N treatments influenced the amino acid accumulation and composition in roots, xylem sap and leaves, as revealed by the LC-ESI-MS analysis. In detail, the root profiles of plants supplied with amino acids showed an upsurge in the levels of uncharged and basic amino acids. Interestingly, under both nutritional conditions glutamine and alanine were the main amino acids in the xylem sap, and the leaf amino acid compositions were similar. In order to clarify the meaning of these metabolic adjustments, we conducted a comparative proteomic analysis of roots and leaves through one-dimensional Gel Liquid Chromatography Mass-Spectrometry (1D GeLC-MS/MS) technique. This approach enabled the identification and quantification of 605 and 481 proteins in roots and leaves, respectively. In both organs, the N forms differently affected the abundance of proteins involved not only in nitrate and amino acid metabolism, but also in carbon and energy metabolism, stress and hormonal responses, thus suggesting a modulation of multiple metabolic responses. However, the N treatment mainly affected the root proteome, while the leaf proteome was highly influenced by the plant growth stage. All together, these analyses highlighted different roles played by roots and leaves in response to N inputs. This work will improve the knowledge about the roles of biochemical pathways involved in amino acid metabolism, providing information that could be useful for enhancing the sustainability of crop production.