An integrative approach to study the responses of Zea mays L. to organic nitrogen provided to roots in the form of amino acids

Although the capacity to take up organic nitrogen (N) in the form of amino acids is widespread among plants, their relevance for N nutrition in crops has been little investigated compared to inorganic N sources. In the last years, several studies have pointed out that distinct amino acids exert different effects on plant morphology and physiology. However, less is known about their effects on plant metabolism, especially when supplied to the roots at composition and concentration mimicking soil conditions. In this respect, this work aimed to investigate the capacities of crops, such as maize (Zea mays L.), to utilize amino acids as N nutrients through an approach that integrates morphological, physiological, biochemical and proteomic evaluations. Maize seedlings were grown by a hydroponic system for three days in the absence of N and then were provided for further four days with 250 μM of a mixture of eight amino acids [Glutamic acid (50 μM), Glutamine (50 μM), Glycine (50 μM), Alanine (50 μM), Aspartic acid (12.5 μM), Arginine (12.5 μM), Lysine (12.5 μM) and Threonine (12.5 μM)], or with 250 μM of inorganic N in the form 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 continued to grow over the experiment, although less than those supplied with nitrate. Therefore, plants utilized both the N sources for growth. In detail, amino-acid treated plants were characterized by a decreased root/shoot ratio and a shorter but thicker root system than nitrate-treated plants. The responses at a nutritional and metabolic level significantly differed, as suggested by changes in the plant content of nitrate, ammonium and key metabolites such as reducing sugars and sucrose. The LC-ESI-MS analysis of amino acid composition in roots, xylem sap and leaves revealed that the roots of plants supplied with amino acids showed an upsurge in the levels of uncharged and basic amino acids. Interestingly, glutamine and alanine were the major amino acids translocated through xylem sap for both the nutritional conditions, while the proportion of amino acids in leaves was more constant over time. Overall, these results highlight the different roles played by roots and leaves in response to N inputs and suggest that roots perceived and adapted promptly to the kind of N available and influenced N translocation to the shoot. In order to clarify the meaning of these metabolic adjustments, large-scale analysis of the total proteome fraction of roots and leaves based on one-dimensional Gel Liquid Chromatography-Mass Spectrometry (1D GeLC-MS/MS) is ongoing. This study will improve the knowledge about plant responses to different N inputs and plant metabolic use of amino acids, providing information that could be useful for enhancing the sustainability of crop production.