Abstract
Nitrogen (N) fertilizers are a major agricultural input where more than 100 million tons are supplied annually. Cereals are particularly inefficient at soil N uptake, where the unrecovered nitrogen causes serious environmental damage. Sorghum bicolor (sorghum) is an important cereal crop, particularly in resource-poor semi-arid regions, and is known to have a high NUE in comparison to other major cereals under limited N conditions. This study provides the first assessment of genetic diversity and signatures of selection across 230 fully sequenced genes putatively involved in the uptake and utilization of N from a diverse panel of sorghum lines. This comprehensive analysis reveals an overall reduction in diversity as a result of domestication and a total of 128 genes displaying signatures of purifying selection, thereby revealing possible gene targets to improve NUE in sorghum and cereals alike. A number of key genes appear to have been involved in selective sweeps, reducing their sequence diversity. The ammonium transporter (AMT) genes generally had low allelic diversity, whereas a substantial number of nitrate/peptide transporter 1 (NRT1/PTR) genes had higher nucleotide diversity in domesticated germplasm. Interestingly, members of the distinct race Guinea margaritiferum contained a number of unique alleles, and along with the wild sorghum species, represent a rich resource of new variation for plant improvement of NUE in sorghum.
Highlights
Nitrogen (N) is the most important nutrient required for plant growth and development and is often supplied in the form of synthetic N fertilizers
Genes were grouped based on their gene family, where analysis was focused on the five transporter protein families, NRT1/peptide transporters (PTR), NRT2, NRT3, AMT1, and AMT2; genes involved in N assimilation by two reductases Nitrate Reductase (NR) and Nitrite Reductase (NiR), and conversions into glutamine and glutamate by Glutamine Synthetase (GS), Glutamine Oxoglutarate Aminotransferase (GOGAT), and Glutamate Dehydrogenase (GDH); and within the aspartate and asparagine cycling involving AST, Asparagine Synthetase (AS), ASPG, and Ammonia Lyase (AAL)
Nitrate requires assimilation into ammonium through the enzymatic roles of NR (5 genes) and NiR (30 genes), ammonium can be converted into organic amino acid glutamine by GS (49 genes), and further into glutamate by GOGAT (15 genes) where GDH (3 genes) balances levels of glutamate and ammonium
Summary
Nitrogen (N) is the most important nutrient required for plant growth and development and is often supplied in the form of synthetic N fertilizers. Their intensive commercial use and generation from fossil fuels, means N fertilizers have a high economic volatility and are often the most expensive additive used in achieving high yielding crops. Cereals are inherently inefficient at N uptake resulting in only ∼40% of total N applied being harvested in the grain (Peoples et al, 1995; Raun and Johnson, 1999; Sylvester-Bradley and Kindred, 2009). Population growth is most accentuated in developing countries, including SubSaharan Africa, where there are limited resources for agriculture and input-intensive crops are already displaying depleting yields (Cardinale, 2012)
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