Abstract
BackgroundIn order to grow, plants rely on soil nutrients which can vary both spatially and temporally depending on the environment, the soil type or the microbial activity. An essential nutrient is nitrogen, which is mainly accessible as nitrate and ammonium. Many studies have investigated transport genes for these ions in Arabidopsis thaliana and recently in crop species, including Maize, Rice and Barley. However, in most crop species, an understanding of the participants in nitrate and ammonium transport across the soil plant continuum remains undefined.ResultsWe have mapped a non-exhaustive set of putative nitrate and ammonium transporters in maize. The selected transporters were defined based on previous studies comparing nitrate transport pathways conserved between Arabidopsis and Zea mays (Plett D et. al, PLOS ONE 5:e15289, 2010). We also selected genes from published studies (Gu R et. al, Plant and Cell Physiology, 54:1515-1524, 2013, Garnett T et. al, New Phytol 198:82-94, 2013, Garnett T et. al, Frontiers in Plant Sci 6, 2015, Dechorgnat J et. al, Front Plant Sci 9:531, 2018). To analyse these genes, the plants were grown in a semi-hydroponic system to carefully control nitrogen delivery and then harvested at both vegetative and reproductive stages. The expression patterns of 26 putative nitrogen transporters were then tested. Six putative genes were found not expressed in our conditions. Transcripts of 20 other genes were detected at both the vegetative and reproductive stages of maize development. We observed the expression of nitrogen transporters in all organs tested: roots, young leaves, old leaves, silks, cobs, tassels and husk leaves. We also followed the gene expression response to nitrogen starvation and resupply and uncovered mainly three expression patterns: (i) genes unresponsiveness to nitrogen supply; (ii) genes showing an increase of expression after nitrogen starvation; (iii) genes showing a decrease of expression after nitrogen starvation.ConclusionsThese data allowed the mapping of putative nitrogen transporters in maize at both the vegetative and reproductive stages of development. No growth-dependent expression was seen in our conditions. We found that nitrogen transporter genes were expressed in all the organs tested and in many cases were regulated by the availability of nitrogen supplied to the plant. The gene expression patterns in relation to organ specificity and nitrogen availability denote a speciality of nitrate and ammonium transporter genes and their probable function depending on the plant organ and the environment.
Highlights
In order to grow, plants rely on soil nutrients which can vary both spatially and temporally depending on the environment, the soil type or the microbial activity
In order to study the expression profiles of maize nitrogen transporter genes, plants were grown using a semi-hydroponic system in the glasshouse that enabled full-plant growth with the ability to access aerial and root samples
Plants were harvested at a vegetative (V7) and reproductive stage (R1) of growth from which RNA was extracted from a range of tissues that included roots, old leaves, young leaves, cobs, silks, tassels and husk leaves (Fig. 1)
Summary
Plants rely on soil nutrients which can vary both spatially and temporally depending on the environment, the soil type or the microbial activity. An essential nutrient is nitrogen, which is mainly accessible as nitrate and ammonium. Plants not capable of fixing N2 absorb it through their roots mainly in the form of the inorganic ions, nitrate (NO3−) in aerobic soils and ammonium (NH4+) in acidic soils and wetlands. The excessive and inefficient use of nitrogen fertilisers, coupled with the low absorption capacity of crops, results in leaching of NO3− after rainfall or irrigation events [9]. This causes contamination of ground water and in many cases excessive algal growth in rivers and deltas leading to eutrophication and subsequently death of aquatic life. A better understanding of nitrogen uptake and distribution within the plant is important for genetically engineering improvements in nitrogen use efficiency and nitrogen utilisation for yield and quality in crop species
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