Omics Approaches to Decipher Nitrogen Response in Bread Wheat

Abstract

Nitrogen (N) plays significant role to improve above ground biomass, grain yield, grain production and grain protein content. It is used for synthesis of amino acids, signaling molecules and storage molecules as well as being essential for number of metabolic processes. The synthetic nitrogen fertilizer improves crop performance and yield related traits but most of crops absorb merely 30–50% of applied N fertilizer, depending on the environment, plant genotype and soil type. More than 50% of applied N fertilizer is not utilized by crops and lost into environment ultimately leading to ecosystems’ destabilization. Even in intensive farming systems, total crop production has not been improved according to chemical fertilizer application rate and leads to low NUE and environmental pollution. These facts highlight the potentials and challenges of improving global food security while implementing novel strategies not only to improve crop yield but also to reduce N inputs is concurrent in future. The nitrogen uptake, utilization and remobilization in wheat needs to be further explored at agro-physiological, biochemical, and molecular levels for introgression in future breeding programs. Our first study aimed to unravel the genetic composition of nitrogen response in a diverse germplasm consisting of landraces, green revolution, post green revolution, elite cultivars, and CIMMYT advance cultivars using 90K SNP array by employing general linear model, mixed linear model, and fixed and random model circulating probability unification based genome-wide association mapping. Seventy two significant marker trait associations were selected for gene identification conferring chlorophyll content, normalized difference vegetation index, flag leaf area, plant height, tiller number, grain yield, biomass, harvest index, grains per spike and nitrogen agronomic efficiency. Genes corresponding to the significant MTAs were retrieved as candidate genes, including members of the transcription factor families and protein kinases. The second study aimed to identify the major grain yield components and root traits and their level of contribution for yield maximization under variable N supplies through multiple linear regression and building their path model using LISREL software. It computes multiple linear regression (MLR) to show the interaction between independent (grain yield components and root traits) and dependent (grain yield) variables in the form of direct effect DRSML QAU xii (DE), indirect effect (IE) and total effect (TE). The tiller number, days to maturity, nitrogen use efficiency and root length showed high correlations and direct effects on GY under variable N application. Multiple linear regression (MLR) analysis by building path model is an effective way to predict improvement in grain yield as it showed the intensity of association between two or more yield related traits and indicated relative importance of each trait. The third study aimed to demonstrate the impact of nitrogen use efficiency to mitigate terminal heat stress in bread wheat under variable nitrogen applications. Nitrogen (N) deficiency and heat stress (HS) are major abiotic stresses that affect the quantity and quality of wheat grains. Twelve wheat varieties were evaluated in 2016–2017 and 2017–2018 at the National Agricultural Research Centre (NARC), Islamabad, Pakistan. The experiment was divided into three sets, i.e., N120 (120 kg N/ha), N60 (60 kg N/ha) and N0 (0 kg N/ha), based on the nitrogen fertilizer application. The strong positive correlation of RSI and RNDVI with grain yield at R2 = 0.73 and R2 = 0.49 suggest that these parameters can be used as efficient and precise selection criteria for identifying nitrogen-use-efficient wheat varieties under terminal heat-stress conditions. This work will help the researchers to identify and develop nitrogen-use efficient and thermos-tolerant wheat cultivars by minimizing the negative impacts of heat stress at the anthesis stage. The fourth study aimed to demonstrate how related NAM genes control nitrogen remobilization at the molecular level in bread wheat. We carried out a comparative transcriptomic study at seven time points (3, 7, 10, 13, 15, 19 and 26 days after anthesis) in wild type and NAM RNA interference (RNAi) lines with reduced NAM gene expression. Approximately 2.5 times more genes were differentially expressed in WT than NAM RNAi during this early senescence time course (6,508 vs 2,605 genes). In both genotypes, differentially expressed genes were enriched for GO terms related to photosynthesis, hormones, amino acid transport and nitrogen metabolism. However, nitrogen metabolism genes including glutamine synthetase (GS1 and GS2), glutamate decarboxylase (GAD), glutamate dehydrogenase (GDH) and asparagine synthetase (ASN1) showed stronger or earlier differential expression in WT than in NAM RNAi plants, consistent with higher nitrogen remobilisation. The current thesis reports fundamental knowledge of molecular basis of nitrogen response in bread wheat.