Abstract
Wheat growth and nitrogen (N) uptake gradually decrease in response to high NH4+/NO3– ratio. However, the mechanisms underlying the response of wheat seedling roots to changes in NH4+/NO3– ratio remain unclear. In this study, we investigated wheat growth, transcriptome, and proteome profiles of roots in response to increasing NH4+/NO3– ratios (Na: 100/0; Nr1: 75/25, Nr2: 50/50, Nr3: 25/75, and Nn: 0/100). High NH4+/NO3– ratio significantly reduced leaf relative chlorophyll content, Fv/Fm, and ΦII values. Both total root length and specific root length decreased with increasing NH4+/NO3– ratios. Moreover, the rise in NH4+/NO3– ratio significantly promoted O2– production. Furthermore, transcriptome sequencing and tandem mass tag-based quantitative proteome analyses identified 14,376 differentially expressed genes (DEGs) and 1,819 differentially expressed proteins (DEPs). The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis indicated that glutathione metabolism and phenylpropanoid biosynthesis were the main two shared enriched pathways across ratio comparisons. Upregulated DEGs and DEPs involving glutathione S-transferases may contribute to the prevention of oxidative stress. An increment in the NH4+/NO3– ratio induced the expression of genes and proteins involved in lignin biosynthesis, which increased root lignin content. Additionally, phylogenetic tree analysis showed that both A0A3B6NPP6 and A0A3B6LM09 belong to the cinnamyl-alcohol dehydrogenase subfamily. Fifteen downregulated DEGs were identified as high-affinity nitrate transporters or nitrate transporters. Upregulated TraesCS3D02G344800 and TraesCS3A02G350800 were involved in ammonium transport. Downregulated A0A3B6Q9B3 is involved in nitrate transport, whereas A0A3B6PQS3 is a ferredoxin-nitrite reductase. This may explain why an increase in the NH4+/NO3– ratio significantly reduced root NO3–-N content but increased NH4+-N content. Overall, these results demonstrated that increasing the NH4+/NO3– ratio at the seedling stage induced the accumulation of reactive oxygen species, which in turn enhanced root glutathione metabolism and lignification, thereby resulting in increased root oxidative tolerance at the cost of reducing nitrate transport and utilization, which reduced leaf photosynthetic capacity and, ultimately, plant biomass accumulation.
Highlights
IntroductionPlants have evolved sophisticated root regulatory mechanisms and N transport systems for soil N uptake (Luo et al, 2020)
Wheat (Triticum aestivum L.) is one of the most important cereal foods worldwide
We found that a high NH4+/NO3− ratio significantly increased root lignin content (Figure 10), likely because reactive oxygen species (ROS) directly modify the structure of the root cell wall by their involvement in lignin formation (Tsukagoshi, 2016)
Summary
Plants have evolved sophisticated root regulatory mechanisms and N transport systems for soil N uptake (Luo et al, 2020). Previous studies showed that urea was directly absorbed by urea transporters, such as DUR3 and MIP (Wang et al, 2012; Liu et al, 2015), it is widely accepted that plants absorb mainly ammonium or nitrate generated by microbial conversion or via their soil application as fertilizers (Witte, 2011). Two soil nitrate and ammonium uptake systems have been identified in higher plants, namely, the high-affinity transport system (HATS) and the low-affinity transport system (LATS) (Kiba and Krapp, 2016; Gojon, 2017; Plett et al, 2018). N transporters have been reported to have specific effects on lateral root initiation (Remans et al, 2006; Motte et al, 2019), there are few reports on the effects of different NH4+/NO3− ratios on changes in N transport and uptake and metabolism in roots of young wheat seedlings
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