Diazotrophic Cyanobacteria are Associated With a Low Nitrate Resupply to Surface Waters in Lake Tanganyika

In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria exploit the abundant phosphate and fix nitrogen after other phytoplankton taxa have consumed the available nitrate. However, it remains less clear, which mechanisms favour diazotrophic cyanobacteria under more heavily stratified conditions with lower levels of excess phosphate and persistent nitrate-depletion. Here, we collected profiles of physicochemical parameters, nutrients and photo-pigments, as well as the medium- to large-sized phytoplankton community during two lake-wide cruises to elucidate to what extent the abundance of diazotrophic cyanobacteria in Lake Tanganyika may be controlled by the nitrate resupply through the thermocline into the euphotic zone. At stations where nitrate was depleted, but phosphate remained available near the surface, high densities of diazotrophic cyanobacteria were associated with a low nitrate supply to surface waters. Our data provide first support for two conceptual scenarios, where the relative position of the thermocline and the euphotic depth may create a functional niche for diazotrophic cyanobacteria: when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline, diazotrophic cyanobacteria, comprising Dolichospermum and Anabaenopsis, are key players in the medium-to large-sized phytoplankton community. By contrast, a thermocline located within the euphotic zone allows for a rapid vertical transport of nitrate for a thriving nitrate-assimilating phytoplankton community that evidently outcompetes diazotrophic cyanobacteria. This study highlights that, under nitrogen-depleted conditions, diazotrophic cyanobacteria can also grow in response to a reduced nutrient resupply to the productive surface waters.

The Rice NRT2.4 Alters Metabolism and Nitrogen Uptake through Root Modulation

Purpose The expression patterns of the NRT2 genes have been well described, however, it is not well known the role of OsNRT2.4 in root growth. In this study we thus aimed at investigating the role of high-affinity NO3- transport OsNRT2.4 in NO3--regulated and the modulation of root growth.Methods Through the gene silencing technique amiRNA-mediated we successfully obtained osnrt2.4 knockdown lines to study the role of OsNRT2.4 on root growth under low nitrate conditions. We performed real time RT-PCR analysis to investigate the relative gene expression level in root and shoot, soluble metabolites, and measurement of root system.Results Knockdown of OsNRT2.4 did not affect rice growth. In comparison with wild-type (WT) plants showed that knockdown of OsNRT2.4 inhibited root formation under low NO3- supply. We demonstrated that the mutant lines had significantly increased NO3- uptake than WT plants when growth in different nitrate supply; osnrt2.4 knockdown lines showed an alteration in nitrogen metabolism, and this affected the root growth. The downregulation of OsNRT2.4 enhanced the expression of genes response of low external NO3- concentrations.Conclusion Herein we provide new insights in OsNRT2.4 functions. Our data demonstrated that OsNRT2.4 plays a role in root growth, nitrogen metabolic pathway and probably have functions in nitrate transport from root to shoot under low nitrate availability in rice.

Genome-wide analysis of the soybean root transcriptome reveals the impact of nitrate on alternative splicing

In plants, nitrate acts not only as a signaling molecule that affects plant development but also as a nutrient. The development of plant roots, which directly absorb nutrients, is greatly affected by nitrate supply. Alternative gene splicing plays a crucial role in the plant stress response by increasing transcriptome diversity. The effects of nitrate supply on alternative splicing (AS), however, have not been investigated in soybean roots. We used high-quality high-throughput RNA-sequencing data to investigate genome-wide AS events in soybean roots in response to various levels of nitrate supply. In total, we identified 355 nitrate-responsive AS events between optimal and high nitrate levels (NH), 335 nitrate-responsive AS events between optimal and low nitrate levels (NL), and 588 nitrate-responsive AS events between low and high nitrate levels (NLH). RI and A3SS were the most common AS types; in particular, they accounted for 67% of all AS events under all conditions. This increased complex and diversity of AS events regulation might be associated with the soybean response to nitrate. Functional ontology enrichment analysis suggested that the differentially splicing genes were associated with several pathways, including spliceosome, base excision repair, mRNA surveillance pathway and so on. Finally, we validated several AS events using reverse transcription–polymerase chain reaction to confirm our RNA-seq results. In summary, we characterized the features and patterns of genome-wide AS in the soybean root exposed to different nitrate levels, and our results revealed that AS is an important mechanism of nitrate-response regulation in the soybean root.

Root Foraging in Soybean (Glycine max) under Nitrogen Deprivation

Nitrate is one of the key sources of nitrogen in natural and agricultural soils. The distribution and concentration of nitrate determine root system architecture in plants. Soybean (Glycine max L) is one of the key leguminous crops, while farmers rarely apply nitrogen in soybean crops except for a starter nitrogen dose at the time of sowing. However, the effects of severe deficiency nitrate on early seedling establishment of soybean before nodulation are not yet studied. Therefore, this study evaluated the effects of high dose of nitrate (54.3 mM) and its deprivationon (0 mM) on the root system architecture of soybean during seedling establishment. Results showed that the root traits including primary root length, fresh biomass, total length, surface area, tips, forks, and its crossings were significantly higher under no nitrate condition than nigh nitrate condition except for root volume, its dry biomass and diameter. Shoot growth attributes such as shoot length, shoot fresh biomass, shoot dry biomass, single leaf area, soil-plant analysis development value, and photosynthesis was significantly decreased while leaf dry mass per area was increased significantly under no nitrate condition. Furthermore, high nitrate supply significantly enhanced the content of nitrate in root tissue, but there was no significant difference between low and optimal nitrate supply. In summary, this study indicated that soybean root system architecture adopts a foraging strategy under nitrogen deprived environment. © 2021 Friends Science Publishers

Contribution of Particulate and Mineral-Associated Organic Matter to Potential Denitrification of Agricultural Soils

Water-extractable organic carbon (WEOC) is considered as the most important carbon (C) source for denitrifying organisms, but the contribution of individual organic matter (OM) fractions (i.e., particulate (POM) and mineral-associated (MOM)) to its release and, thus, to denitrification remains unresolved. Here we tested short-time effects of POM and MOM on potential denitrification and estimated the contribution of POM- and MOM-derived WEOC to denitrification and CO2 production of three agricultural topsoils. Suspensions of bulk soils with and without addition of soil-derived POM or MOM were incubated for 24 h under anoxic conditions. Acetylene inhibition was used to determine the potential denitrification and respective product ratio at constant nitrate supply. Normalized to added OC, effects of POM on CO2 production, total denitrification, and its product ratios were much stronger than those of MOM. While the addition of OM generally increased the (N2O + N2)-N/CO2-C ratio, the N2O/(N2O + N2) ratio changed differently depending on the soil. Gas emissions and the respective shares of initial WEOC were then used to estimate the contribution of POM and MOM-derived WEOC to total CO2, N2O, and N2O + N2 production. Water-extractable OC derived from POM accounted for 53–85% of total denitrification and WEOC released from MOM accounted for 15–47%. Total gas emissions from bulk soils were partly over- or underestimated, mainly due to nonproportional responses of denitrification to the addition of individual OM fractions. Our findings show that MOM plays a role in providing organic substrates during denitrification but is generally less dominant than POM. We conclude that the denitrification potential of soils is not predictable based on the C distribution over POM and MOM alone. Instead, the source strength of POM and MOM for WEOC plus the WEOC’s quality turned out as the most decisive determinants of potential denitrification.