Autophagy mediates grain yield and nitrogen stress resistance by modulating nitrogen remobilization in rice

Autophagy, a conserved cellular process in eukaryotes, has evolved to a sophisticated process to dispose of intracellular constituents and plays important roles in plant development, metabolism, and efficient nutrients remobilization under suboptimal nutrients conditions. Here, we show that OsATG8b, an AUTOPHAGY-RELATED8 (ATG8) gene in rice, was highly induced by nitrogen (N) starvation. Elevated expression of OsATG8b significantly increased ATG8 lipidation, autophagic flux, and grain yield in rice under both sufficient and deficient N conditions. Overexpressing of OsATG8b could greatly increase the activities of enzymes related to N metabolism. Intriguingly, the 15N-labeling assay further revealed that more N was remobilized to seeds in OsATG8b-overexpressing rice, which significantly increased the N remobilization efficiency (NRE), N harvest index, N utilization efficiency (NUE), and N uptake efficiency (NUpE). Conversely, the osatg8b knock-out mutants had the opposite results on these characters. The substantial transcriptional changes of the overexpressed transgenic lines indicated the presence of complex signaling to developmental, metabolic process, and hormone, etc. Excitingly, the transgenic rice under different backgrounds all similarly be boosted in yield and NUE with OsATG8b overexpression. This work provides an excellent candidate gene for improving N remobilization, utilization, and yield in crops simultaneously.

Dynamics of biologically fixed N in legume-cereal rotations: a review

Agronomically significant N yield responses of cereals following grain legumes compared with cereal monoculture are frequently measured. The positive N response of the cereal has been attributed to the transfer of biologically fixed N, to N-sparing under the antecedent legume, and to less immobilisation of nitrate during the decomposition of legume residues. Methods for estimating the transfer of biologically fixed N in rotations, and for separating the N benefit into fixed N and non-fixed N components, are reviewed. Available data indicate that both sources of N contribute to the N benefit. The role of the grain legume in the gain or drain of soil N is evaluated by considering the balance between symbiotic dependence and N harvest index, as well as long-term changes in total soil N. Several 15N-based techniques for direct estimation of inputs of biologically fixed N to the soil N pool are reviewed. N balances in grain legume-cereal rotations may be positive or negative depending on the legume species, symbiotic performance, and agronomic factors.

Growth, yield and composition of four winter cereals. II. Nitrogen and carbohydrate economy

A field experiment with 3 cultivars each of wheat, rye, triticale and barley, grown at a density of about 320 plants/m, was conducted in 1986 on a fertile clay soil at East Flevoland, Netherlands. N at 120 kg/ha for wheat and triticale and 60 kg/ha for rye and barley was split-dressed in 2 applications. N yield was highest in wheat (196 kg/ha) and lowest in rye (123 kg/ha). The amounts taken up were influenced by the N rate. The triticale cv. Lasko and the barley cv. Marinka had a higher N-uptake than the other triticale and barley cultivars. N harvest index (i.e. the ratio of N in grains and N in above-ground DM at final harvest) was lowest in rye and highest in barley. N concentration in plant organs (grains, chaff, leaves, stems and roots) was higher in wheat and triticale than in rye and barley. This was probably caused by the difference in the level of N application. N use efficiency, expressed as grain DM production/kg N taken up, was 53 in wheat, 68 in rye, 50 in triticale and 61 in barley. In all species, the largest reserves of water-soluble carbohydrates (WSC) were found in the stems. Rye allocated more dry matter to stem growth before flowering than wheat, triticale and barley. Averaged over these cereals, 26% of WSC, produced before flowering, was used for redistribution and respiration during grain production.

Sealed-tube combustion for natural 15N abundance estimation of N2 fixation and application to supernodulating soybean mutants

Measurement of biological N2 fixation by the natural 15N abundance technique is based on the fact that soil N is usually enriched in 15N compared with atmospheric N2. The technique has been limited by the care required to prevent isotope fractionation in the Kjeldahl method. We describe a sealed-tube combustion technique that reduces isotope fractionation problems. Plant samples were combusted at 850 °C in Vycor tubes with Cu, CuO, CaO, and Ag foil, and N2 gas was admitted directly into the mass spectrometer. A δ15N value of 5.43 ± 0.01 (SE, n = 11) was obtained for a nonnodulating soybean seed sample. This method was used to estimate N2 fixation by soybean cv. Bragg and three supernodulating mutants, nts246, nts382, and nts1007, in the field. Average percent N derived from the atmosphere was 70 ± 4% in vegetative parts at physiological maturity and 72 ± 2% in the grain at harvest. No significant differences were found. The N harvest index averaged 77%; thus, despite the high percent N derived from the atmosphere, these soybean crops would have been net exporters of soil N. The natural 15N abundance method consistently gave estimates of N2 fixation that were 60–90 kg N ha−1 higher than the N difference method. Key words: sealed-tube combustion, natural 15N abundance, supernodulating soybean mutants, biological N2 fixation.

Effects of light intensity on nitrogen economy of spring barley (Hordeum distichum L.).

In a pot experiment in a phytotron, barley cv. Trumpf seedlings were grown at a light intensity of 6.50 MJ/msuperscript 2 for 40 d and then at light intensities of 6.50, 4.33 or 1.86 MJ/msuperscript 2 with a 14-h day. 134 mg N/plant was applied in 3 dressings at 2 d before emergence and 26 and 48 d after emergence. N uptake was only slightly affected by light intensity. DM yield/plant increased with increasing light intensity. 101 mg DM/mg N was obtained at a light intensity of 1.86 MJ/msuperscript 2 and 175 mg DM/mg N at 6.50 MJ. Higher light intensity accelerated leaf senescence and shortened the photosynthetically active period by restricting N concn. N harvest index was reduced at the lowest light intensity. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Differences in grain growth, crop photosynthesis and distribution of assimilates between a semi-dwarf and a standard cultivar of winter wheat.

The crop performance of semi-dwarf wheat cv. (Maris Hobbit) was compared with a standard-ht. cv. (Lely) at various levels of N supply. The grain yields of Maris Hobbit were considerably higher due to a higher number of grains and a heavier grain wt. Owing to the higher grain yield and a lower stem wt. the harvest index of Maris Hobbit was higher than that of Lely (0.47 and 0.40, resp.). The content of water-soluble carbohydrates in the stems of both cv. appeared to be very high until 3 wk after anthesis, despite the occurrence of low light intensities. Lely used more assimilates for structural stem material than did Maris Hobbit. Quantity and date of N application greatly affected grain number, but affected grain wt. to a lesser extent. Thus within each cv. grain number/m2 was the main determinant of grain yield. Late N dressings promoted photosynthetic production, grain wt. and CP content of the grain. The low CP contents of the grain were attributed to the low temp. during the grain-filling period. The distribution of N within the plant was only slightly influenced by N dressings and cv. differences. N harvest index ranged from 0.74 to 0.79. Grain N was derived from the vegetative organs (63-94%) and from uptake after anthesis (6-37%). The importance of carbohydrate and N economy for grain yield are discussed. (Abstract retrieved from CAB Abstracts by CABI’s permission)