A New Perspective on the Role of Glutamine Synthetase in Nitrogen Remobilization in Wheat (Triticum aestivum L.)

Glutamine synthetase (GS), a key enzyme in plant nitrogen metabolism, is closely related to nitrogen remobilization. However, how GS isoforms participate in nitrogen remobilization remains unclear. Here, the spatiotemporal expression of the TaGS gene family after anthesis was investigated, and the results showed that TaGS1;1 was mainly encoded by TaGS1;1-6A, while the other isozymes were mainly encoded by TaGS localized on the A and D subgenomes. TaGS1;2-4A/4D had the highest expression level, especially in rachis and peduncle. Furthermore, immunofluorescence showed TaGS1;2 was located in the phloem of rachis and peduncle. GUS (β-glucuronidase) staining confirmed that ProTaGS1;2-4A/4D::GUS activity was mainly present in the vascular system of leaves, roots, and petal of Arabidopsis. Ureides, an important transport form of nitrogen, were mainly synthesized in flag leaves and transported to grains through the phloem of peduncle and rachis during grain filling. TaAAH, which encodes the enzyme that degrades ureides to release NH4+, had a higher expression in rachis and peduncle and was synchronized with the increase in NH4+ concentration in phloem, indicating that NH4+ in phloem is from ureide degradation. Taking the above into account, TaGS1;2, which is highly expressed in the phloem of peduncle and rachis, may participate in N remobilization by assimilating NH4+ released from ureide degradation.

Ethephon-Induced Ethylene Enhances Protein Degradation in Source Leaves, but Its High Endogenous Level Inhibits the Development of Regenerative Organs in Brassica napus

To investigate the regulatory role of ethylene in the source-sink relationship for nitrogen remobilization, short-term effects of treatment with different concentrations (0, 25, 50, and 75 ppm) of ethephon (2-chloroethylphosphonic acid, an ethylene inducing agent) for 10 days (EXP 1) and long-term effects at 20 days (Day 30) after treatment with 100 ppm for 10 days (EXP 2) on protein degradation and amino acid transport in foliar sprayed mature leaves of Brassica napus (cv. Mosa) were determined. In EXP 1, endogenous ethylene concentration gradually increased in response to the treated ethephon concentration, leading to the upregulation of senescence-associated gene 12 (SAG12) expression and downregulation of chlorophyll a/b-binding protein (CAB) expression. Further, the increase in ethylene concentration caused a reduction in protein, Rubisco, and amino acid contents in the mature leaves. However, the activity of protease and expression of amino acid transporter (AAP6), an amino acid transport gene, were not significantly affected or slightly suppressed between the treatments with 50 and 75 ppm. In EXP 2, the enhanced ethylene level reduced photosynthetic pigments, leading to an inhibition of flower development without any pod development. A significant increase in protease activity, confirmed using in-gel staining of protease, was also observed in the ethephon-treated mature leaves. Ethephon application enhanced the expression of four amino acid transporter genes (AAP1, AAP2, AAP4, and AAP6) and the phloem loading of amino acids. Significant correlations between ethylene level, induced by ethephon application, and the descriptive parameters of protein degradation and amino acid transport were revealed. These results indicated that an increase in ethylene upregulated nitrogen remobilization in the mature leaves (source), which was accompanied by an increase in proteolytic activity and amino acid transport, but had no benefit to pod (sink) development.

Moderate DNA methylation changes associated with nitrogen remobilization and leaf senescence in Arabidopsis

SummaryThe lifespan of plants and tissues is restricted by environmental and genetic components. Following the transition to reproductive growth, leaf senescence ceases cellular life in monocarpic plants to remobilize nutrients to storage organs.We observed altered leaf to seed ratios, faster senescence progression and enhanced nitrogen remobilization from the leaves in two methylation mutants (ros1 and the triple dmr1/2 cmt3 knockout).DNA methylation in wild type Col-0 leaves initially moderately declined with progressing leaf senescence, predominantly in the CG context, while the ultimate phase of leaf discoloration was associated with moderate de novo methylation of cytosines, primarily in the CHH context.Relatively few differentially methylated regions, including one in the ROS1 promoter linked to the down-regulation of ROS1, were present, but these were unrelated to known senescence-associated genes.Differential methylation patterns were identified in transcription factor binding sites, such as the W-boxes that are targeted by WRKYs, which impaired transcription factor binding when methylated in vitro.Mutants that are defective in DNA methylation showed distinct nitrogen remobilization, which was associated with altered patterns of leaf senescence progression. But moderate methylome changes during leaf senescence were not specifically associated with up-regulated genes during senescence.

Effect of Nitrogen Fertilizer Applications on Nitrogen Remobilization and Grain Yield in Soybean (Glycine max [L] Merril) in the Savelugu-Nanton Municipality in Northern Region of Ghana

Two experiments were conducted in the Savelugu-Nanton Municipality of the Northern Region of Ghana in 2012 and 2013 to assess the effect of N fertilizer on growth, N remobilization and grain yield of three local varieties of soybean (Glycine max [L] Merill). The experiments were a 3 x 4 factorial laid in Randomized Complete Block Design (RCBD) with four replications. Factor A was soybean varieties (Jenguma, Quarshie, Ahotor); Factor B was 0, 15, 30 and 45 kg N ha-1. The experimental fields were planted manually on the flat by drilling and later thinned to 2 plants hill-1 at 0.50 m × 0.10 m with a population of about 400 000 plants ha-1. Growth and yield parameters measured were plant height, nodule number plant-1, nodule dry weight plant-1, percent nodule effectiveness, number of pods plant-1, number of seeds pod-1, 100 seed weight, harvest index and grain yield. The results showed that the control recorded lower figures in all growth parameters. Nitrogen remobilization was also observed in all plots, which indicate that soybean needs greater levels of N during grain filling. Again, N remobilization and soybean yield were highest in the 45 kg N ha-1 treatment compared to the other treatments. However, considering the overall yields, farmers in the study area should be advised to adopt starter N fertilization of soybean for higher yields as the soils are highly degraded in soil fertility.

Getting more bark for your buck: nitrogen economy of deciduous forest trees

This article comments on: Li G, Lin R, Egekwu C, Blakeslee J, Lin J, Pettengill E, Murphy AS, Peer WA, Islam N, Babst BA, Gao F, Komarov S, Tai Y-C, Coleman GD. 2020. Seasonal nitrogen remobilization and the role of auxin transport in poplar trees. Journal of Experimental Botany 71, 4512–4530.