Abstract
Nitrogen (N) is an essential nutrient for plant growth and development. Plant species respond to N fluctuations and N sources, i.e., ammonium or nitrate, differently. Masson pine (Pinus massoniana Lamb.) is one of the pioneer plants in the southern forests of China. It shows better growth when grown in medium containing ammonium as compared to nitrate. In this study, we had grown masson pine seedlings in medium containing ammonium, nitrate, and a mixture of both, and performed comparative transcriptome and proteome analyses to observe the differential signatures. Our transcriptome and proteome resulted in the identification of 1593 and 71 differentially expressed genes and proteins, respectively. Overall, the masson pine roots had better performance when fed with a mixture of ammonium and nitrate. The transcriptomic and proteomics results combined with the root morphological responses suggest that when ammonium is supplied as a sole N-source to masson pine seedlings, the expression of ammonium transporters and other non-specific NH4+-channels increased, resulting in higher NH4+ concentrations. This stimulates lateral roots branching as evidenced from increased number of root tips. We discussed the root performance in association with ethylene responsive transcription factors, WRKYs, and MADS-box transcription factors. The differential analysis data suggest that the adaptability of roots to ammonium is possibly through the promotion of TCA cycle, owing to the higher expression of malate synthase and malate dehydrogenase. Masson pine seedlings managed the increased NH4+ influx by rerouting N resources to asparagine production. Additionally, flavonoid biosynthesis and flavone and flavonol biosynthesis pathways were differentially regulated in response to increased ammonium influx. Finally, changes in the glutathione s-transferase genes suggested the role of glutathione cycle in scavenging the possible stress induced by excess NH4+. These results demonstrate that masson pine shows increased growth when grown under ammonium by increased N assimilation. Furthermore, it can tolerate high NH4+ content by involving asparagine biosynthesis and glutathione cycle.
Highlights
Nitrogen (N) is the building block of fundamental biological molecules and a key nutrient for plant growth and development
The data on the growing masson pine seedlings in different N sources, i.e., T1: NO3−/NH4+: 100/0 mM, T2: NO3−/NH4+: 50/50 mM, and T3: NO3−/NH4+: 0/100 mM, for 30 days showed that T3 had the highest no. of root tips and longest roots
We found that DEGs and DEPs related to tricarboxylic cycle (TCA) cycle, i.e., malate synthase (MS) and MD, were abundant in T3 as well as T2, clearly indicating that under ammonium nutrition, masson pine root adapts by the promotion of TCA flux mode to sustain C skeleton availability (Figure 7)
Summary
Nitrogen (N) is the building block of fundamental biological molecules and a key nutrient for plant growth and development. Both N-sources have similar diffusion coefficients but in soil, they behave differently due to negative ion charge and viscosity of the soil and other complex soil properties. Plants adapt their root morphology in response to N-source to optimize N absorption. Different plant species have demonstrated preference to ammonium, e.g., Picea glauca and Pinus radiate [6,7] These reports suggested that relatively slower growth in nitrate rich environments was possibly due to highly atrophied transport systems for the ion. The rich ammonium content in the forest soils due to low nitrification potential could be a driving factor for the preference [8]
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