An integrated "omics" approach to the characterization of maize (Zea mays L.) mutants deficient in the expression of two genes encoding cytosolic glutamine synthetase

Abstract

Background: To identify the key elements controlling grain production in maize, it is essential to have an integrated view of the responses to alterations in the main steps of nitrogen assimilation by modification of gene expression. Two maize mutant lines (gln1.3 and gln1.4), deficient in two genes encoding cytosolic glutamine synthetase, a key enzyme involved in nitrogen assimilation, were previously characterized by a reduction of kernel size in the gln1.4 mutant and by a reduction of kernel number in the gln1.3 mutant. In this work, the differences in leaf gene transcripts, proteins and metabolite accumulation in gln1.3 and gln1.4 mutants were studied at two key stages of plant development, in order to identify putative candidate genes, proteins and metabolic pathways contributing on one hand to the control of plant development and on the other to grain production.

Results: The most interesting finding in this study is that a number of key plant processes were altered in the gln1.3 and gln1.4 mutants, including a number of major biological processes such as carbon metabolism and transport, cell wall metabolism, and several metabolic pathways and stress responsive and regulatory elements. We also found that the two mutants share common or specific characteristics across at least two or even three of the "omics" considered at the vegetative stage of plant development, or during the grain filling period.

Conclusions: This is the first comprehensive molecular and physiological characterization of two cytosolic glutamine synthetase maize mutants using a combined transcriptomic, proteomic and metabolomic approach. We find that the integration of the three "omics" procedures is not straight forward, since developmental and mutant-specific levels of regulation seem to occur from gene expression to metabolite accumulation. However, their potential use is discussed with a view to improving our understanding of nitrogen assimilation and partitioning and its impact on grain production.

Figure 1

Functional categories of metabolites, proteins and gene transcripts isolated from the leaves of maize gln1.3 and gln1.4 mutant plants, exhibiting differences in their level of accumulation. Pie charts show the number of metabolites, proteins and transcripts for each functional class identified in the three “omics” experiments exhibiting an increase or a decrease in the leaves of the gln1.3 and gln1.4 mutants at the vegetative (V) and mature (M) stage of leaf development. For each mutant and each developmental stage, the total number of changes is indicated on the left side of the pie chart.

Figure 2

Schematic representation of the main biological changes occurring in the leaves of the gln1.3 and gln1.4 mutants. On the top of the figure is shown a schematic representation of leaf N management in maize during the developmental cycle. During vegetative growth (V), N is taken up by the roots and assimilated to build up plant cellular structures (green arrow). After flowering the N accumulated in the vegetative parts of the plant is remobilized and translocated to the developing kernels. At the same time (yellow arrow), which corresponds to the grain filling period (M), about half of the N that is translocated to the developing kernels is taken up after flowering to contribute to storage protein deposition until the kernels reach maturity. During these two main phases of plant development the large arrows indicate the different biological functions that exhibit the most significant decrease (blue arrow) or increase (red arrows) in the gln1.3 and gln1.4 mutants. The physiological impact of the two mutations is indicated below the green and yellow arrows in italics, which corresponds to the two main phases of leaf N management before and after flowering.