Physiological and transcriptional characterization of response to N-starvation in roots of two maize inbred lines with different nitrogen use efficiency

Nitrogen (N) is the element required in greatest amounts by plants after carbon (C) and it is a primary component of nucleic acids, proteins, co-enzymes, phytohormones, chlorophyll and also secondary metabolites which plays extremely important roles for plant life. The bioavailability this element to roots is therefore a crucial factor for plant growth and consequently the use of fertilizers is required to agricultural systems. In most soils, NH4+ and NO3- are the predominant sources of N that are available for plant nutrition. Although the average NH4+ concentrations in soils are often 10-1000 times lower than those of NO3-, this difference does not necessarily reflect the uptake ratio of each N source. The characteristics of root NO3- uptake have been extensively studied. Less information is on the contrary available for NH4+. Previous works performed in rice, spruce and Arabidopsis have revealed the existence of two transport systems for NH4+ with high (HATS) and low (LATS) affinity. Since information regarding molecular aspects of NH4+ uptake in maize is very limited, as a first purpose of this work we characterized some biochemical aspects of NH4+ uptake in seedlings of two maize inbred lines (Lo5 and T250). These two lines were identified in field experiments as a high (Lo5) and low (T250) nitrogen use efficiency (NUE), lines respectively. As far as, the uptake of N mineral forms, the two lines were previously characterized for the difference in HATS and LATS for NO3-. The analysis of kinetics parameters of NH4+ uptake here determined showed a lower Km for the high-NUE line. The influence of pH on the uptake rate on both HATS and LATS was also evaluated showing that the uptake rate is not dependent from H+ availability. Differences between Lo5 and T250 in the uptake rate of the two inorganic N-forms during the growth without N source were analyzed. NH4+ uptake rate increased during N deprivation with a steeper profile in Lo5 whilst NO3- uptake rate tended to decrease in both lines. When the uptake rates were analyzed in the contemporary presence of NO3- and NH4+ in the uptake solution with a 100:1 ratio, the NH4+ uptake rates showed similar levels to those of and NO3-. The two lines were also characterized for their differences in root transcriptional profile during N deprivation through microarray analysis. Data analysis highlighted that 112 transcripts were differentially expressed between Lo5 and T250 at 0, 1 and 4 days of N deprivation, while 85 and 646 transcripts were differentially expressed both at 0 and 1 days and both at 1 and 4 days, respectively. The annotation of these differentially expressed transcripts and the study of their behaviour in the two lines strongly support the idea that the high NUE line responds to N deprivation though a stronger expression of genes known as involved in the molecular mechanisms mediating the response to the absence of the macronutrient in roots relative to the low NUE line (T250).