Abstract
BackgroundNitrogen (N) is a key macronutrient required for plant growth and development. In this study, watermelon plants were grown under hydroponic conditions at 0.2 mM N, 4.5 mM N, and 9 mM N for 14 days.ResultsDry weight and photosynthetic assimilation at low N (0.2 mM) was reduced by 29 and 74% compared with high N (9 mM). The photochemical activity (Fv/Fm) was also reduced from 0.78 at high N to 0.71 at low N. The N concentration in the leaf, stem, and root of watermelon under low N conditions was reduced by 68, 104, and 108%, respectively compared with 9 mM N treatment after 14 days of N treatment. In the leaf tissues of watermelon grown under low N conditions, 9598 genes were differentially expressed, out of which 4533 genes (47.22%) were up-regulated whereas, 5065 genes (52.78%) were down-regulated compared with high N. Similarly in the root tissues, 3956 genes were differentially expressed, out of which 1605 genes were up-regulated (40.57%) and 2351 genes were down-regulated (59.43%), compared with high N. Our results suggest that leaf tissues are more sensitive to N deficiency compared with root tissues. The gene ontology (GO) analysis showed that the availability of N significantly affected 19 biological processes, 8 cell component metabolic pathways, and 3 molecular functions in the leaves; and 13 biological processes, 12 molecular functions, and 5 cell component metabolic pathways in the roots of watermelon. The low affinity nitrate transporters, high affinity nitrate transporters, ammonium transporters, genes related with nitrogen assimilation, and chlorophyll and photosynthesis were expressed differentially in response to low N. Three nitrate transporters (Cla010066, Cla009721, Cla012765) substantially responded to low nitrate supply in the root and leaf tissues. Additionally, a large number of transcription factors (1365) were involved in adaptation to low N availability. The major transcription factor families identified in this study includes MYB, AP2-EREBP, bHLH, C2H2 and NAC.ConclusionCandidate genes identified in this study for nitrate uptake and transport can be targeted and utilized for further studies in watermelon breeding and improvement programs to improve N uptake and utilization efficiency.
Highlights
Nitrogen (N) is a key macronutrient required for plant growth and development
Response to growth and physiological traits under low N conditions Plant growth The results showed that fresh weight and dry weight of watermelon was significantly affected by different levels of N (Figs. 1, 2a-d)
The transcriptome data showed that watermelon leaf is more sensitive to low nitrate supply compared with watermelon root
Summary
Nitrogen (N) is a key macronutrient required for plant growth and development. Nitrogen (N) is a major component of amino acids, proteins, nucleic acid, chlorophyll and hormones [1]. It is a key macronutrient required for plant growth and development of watermelon. N is absorbed by the roots in the form of nitrate (NO3−) and ammonium (NH4+) through the nitrate and ammonium transporters. NH4+ is converted to glutamine, glutamate, or glutamate dehydrogenease These synthesized N compounds serve as precursor for amino acids, proteins, and other N-containing metabolites that are utilized for plant growth and development [4, 5]. N starvation reduces leaf area and photosynthetic assimilation capacity leading to reduced plant growth, dry matter accumulation and yield [7, 8]
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