The Physiological And Molecular Mechanisms Of Nitrogen Use Efficiency In Cotton

Worldwide nitrogen(N)is the most important and limiting factor of crop production as it is the main component of several macromolecules,metabolites,and signaling compounds that are necessary for plant growth and productivity.Cotton(Gossypium hirsutum)is the leading fiber crop grown worldwide for its natural fiber.The high input in cotton production,especially N fertilizer is the major challenge for agriculturist.The excessive use of N fertilizers not only increases the cost of production but also causes environmental pollution.The cultivation of cotton genotypes with a substantial root system and high genetic potentials for nitrogen use efficiency(NUE)would best address these problems.However,such a genotype with high NUE has not been identified and developed because of limited knowledge regarding the mechanism and traits contributing to NUE.A large genotypic variation existed in cotton genotypes for NUE and related traits like N uptake,transport,and assimilation.However,the underlying molecular mechanisms involved in the genotypic difference is still to be elucidated.Therefore,it is an immense need to understand the underlying molecular mechanism of NUE in cotton and to develop N-efficient cotton genotypes.The current study was designed to investigate the genotypic variations for morphological,physiological,agronomic,and biochemical traits in contrasting N-efficient cotton genotypes under various N conditions.Meanwhile,we also studied the possible physiological(seedling and reproductive stage)and molecular mechanisms of NUE in cotton genotypes.The main results obtained are summarized as follows:A large genotypic variation was found in root related traits,that are positively correlated with nitrogen utilization efficiency(NUtE),whereas shoot related traits and N assimilating enzymes were positively correlated with nitrogen uptake efficiency(NUpE).Root related traits are important under low N condition,while shoot related traits and N assimilating enzymes under moderate to high N conditions for genotype selection.N-efficient cotton genotype(CCRI-69)showed higher values for NUE related traits like shoot dry matter,NUpE,NUtE,and N metabolism at both low and normal N concentrations.Among the N assimilating enzymes,a large genotypic variation was observed for glutamine synthetase that may be considered a potential trait for improving NUE.Thus,these traits could be used as potential indicators for genotypes identification/screening at low(0.2-0.25 mM)and normal N concentrations(2-2.5 mM)for NUE.Under low nitrogen,N-efficient cotton genotype(CCRI-69)exports a great amount of carbohydrates from shoots to roots that induce root morphological traits for better N uptake and assimilation.The profound root system developed due to more carbohydrates accumulation might be the feedback mechanism in N-efficient genotypes in response to low N conditions at seedling stage.Photosynthetic attributes and sucrose metabolism in the cotton subtending leaves showed great importance in identifying the genotypic variation across the growth stages.Similarly,the upregulation of N metabolism,non-structural carbohydrate contents like sucrose and fructose as well as sucrose synthesizing enzyme(SPS)in the cotton subtending leaves may be the possible reason of high yield in N-efficient cotton genotype(CCRI-69).Thus,C/N metabolism and balance in cotton subtending leaves proved to be a key physiological mechanism of NUE at reproductive stage to achieve high yield.Like physiological traits,a large genotypic difference was observed at the molecular level,where N-efficient cotton genotype(CCRI-69)showed upregulation of the genes responsible for amino acid,carbon and nitrogen metabolism.It was concluded that efficient N absorption and assimilation by roots and more energy-producing by the shoots/leaves may contribute to high NUE.The study also provides the uncover hub genes for amino acid,carbon,and nitrogen metabolism,benefiting the breeding of N-efficient cotton genotypes.The transcript levels of genes responsible for nutrient transporters,photosynthetic pathways,antioxidants,phytohormones,and transcription factors in N-efficient cotton genotype(CCRI-69)were significantly high than N-inefficient genotype(XLZ-30),indicating that these pathways are very important for NUE.Moreover,the transcription factors like WRKY and AP2/ERF in the roots and AP2/ERF and MYB in the shoots playing a key role in regulating the genes responsible for high NUE in N-efficient cotton genotype(CCRI-69).Based on the identified hub genes,we deduced that high N uptake,less energy consumption in N metabolism,more energy/biomass production in photosynthetic system,high antioxidant and hormonal activity as well as transcription regulation system may contribute to high NUE in cotton.Overall,N-efficient cotton genotype exports a great amount of carbohydrates from shoots to roots at the seedling stage and from cotton subtending leaves to sink at the reproductive stage that supports better root system and high yield,respectively.Moreover,efficient N uptake through N transporters,enhanced photosynthetic activity,improved carbon and nitrogen metabolism via enzymatic activities,high antioxidants and hormonal activity under N starvation or deficiency as well as transcription regulation of genes responsible for these pathways could be the possible mechanism of high NUE in cotton.