Relationship of Nitrogen Deficiency-Induced Leaf Senescence with ROS Generation and ABA Concentration in Rice Flag Leaves

Nitrogen (N) deficiency is one of the critical environmental factors that induce leaf senescence, and its occurrence may cause the shorten leaf photosynthetic period and markedly lowered grain yield. However, the physiological metabolism underlying N deficiency-induced leaf senescence and its relationship with the abscisic acid (ABA) concentration and reactive oxygen species (ROS) burst in leaf tissues are not well understood. In this paper, the effect of N supply on several senescence-related physiological parameters and its relation to the temporal patterns of ABA concentration and ROS accumulation during leaf senescence were investigated using the premature senescence of flag leaf mutant rice (psf) and its wild type under three N treatments. The results showed that N deficiency hastened the initiation and progression of leaf senescence, and this occurrence was closely associated with the upregulated expression of 9-cis-epoxycarotenoiddioxygenase genes (NCEDs) and with the downregulated expression of two ABA 8′-hydroxylase isoform genes (ABA8ox2 and ABA8ox3) under LN treatment. Contrarily, HN supply delayed the initiation and progression of leaf senescence, concurrently with the suppressed ABA biosynthesis and relatively lower level of ABA concentration in leaf tissues. Exogenous ABA incubation enhanced ROS generation and MDA accumulation in a dose-dependent manner, but it decreased the activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in detached leaf. These results suggested that the participation of ABA in the regulation of ROS generation and N assimilating/remobilizing metabolism in rice leaves was strongly responsible for induction of leaf senescence by N deficiency.

     

丛枝菌根真菌和根瘤菌对白三叶氮同化的影响

为揭示丛枝菌根真菌(AMF)和根瘤菌在白三叶氮(N)同化中的作用,该研究对白三叶进行单一或联合接种隐类球囊霉(Paraglomus occultum)和三叶草根瘤菌(Rhizobium trifolii),分析其对白三叶的生长、光合作用、叶片N和氨基酸含量以及N同化相关酶活性的影响。结果表明:(1)单一接种AMF或根瘤菌以及联合接种AMF和根瘤菌均显著增加了白三叶的株高、匍匐茎长度、叶片数、地上部生物量、总生物量、叶绿素b和总叶绿素含量、稳态光量子效率和叶片N含量,这种增强效应是联合接种>单一AMF>单一根瘤菌>未接种处理。(2)联合接种AMF和根瘤菌显著增加了白三叶叶片中丙氨酸、精氨酸、天冬酰胺、天冬氨酸、谷氨酰胺、谷氨酸和组氨酸的含量,显著提升了叶片N同化相关酶如硝酸还原酶、亚硝酸还原酶、谷氨酰胺合成酶、谷氨酸合成酶、谷氨酸脱氢酶、天冬酰胺合成酶和天冬氨酸转氨酶的活性,显著促进AMF对白三叶根系的侵染。综上认为,联合接种AMF和根瘤菌通过激活N同化相关酶活性有效促进N同化,产生更多氨基酸,进一步促进白三叶植株生长; 联合接种AMF和根瘤菌具有协同作用,有效促进了白三叶的N同化。     

Inoculation with an enhanced N2‐fixing Bradyrhizobium japonicum strain (USDA110) does not alter soybean (Glycine max Merr.) response to elevated [CO2]

This study tested the hypothesis that inoculation of soybean (Glycine max Merr.) with a Bradyrhizobium japonicum strain (USDA110) with greater N₂ fixation rates would enhance soybean response to elevated [CO₂]. In field experiments at the Soybean Free Air CO₂ Enrichment facility, inoculation of soybean with USDA110 increased nodule occupancy from 5% in native soil to 54% in elevated [CO₂] and 34% at ambient [CO₂]. Despite this success, inoculation with USDA110 did not result in greater photosynthesis, growth or seed yield at ambient or elevated [CO₂] in the field, presumably due to competition from native rhizobia. In a growth chamber experiment designed to study the effects of inoculation in the absence of competition, inoculation with USDA110 in sterilized soil resulted in nodule occupation of >90%, significantly greater ¹⁵N₂ fixation, photosynthetic capacity, leaf N and total plant biomass compared with plants grown with native soil bacteria. However, there was no interaction of rhizobium fertilization with elevated [CO₂]; inoculation with USDA110 was equally beneficial at ambient and elevated [CO₂]. These results suggest that selected rhizobia could potentially stimulate soybean yield in soils with little or no history of prior soybean production, but that better quality rhizobia do not enhance soybean responses to elevated [CO₂].     

Carbon and nitrogen assimilation and partitioning in soybeans exposed to low root temperatures

Low root temperature effects on vegetative growth of soybean (Harosoy 63 × Rhizobium japonicum USDA 16) were examined in 35 day old plants exposed to temperatures of 15°C (shoots at 25°C) for an 11 day period. Duing this period various aspects of C and N assimilation and partitioning were monitored including shoot night and nodulated root respiration, C and N partitioning to six plant parts, C₂H₂ reduction, H₂ evolution, leaf area, transpiration, net photosynthesis, and N₂ fixation. The low temperature treatment resulted in a decrease in the net rate of N₂ fixation but nitrogenase relative efficiency increased. In response, the plant retained N in the tissues of the nodulated root and decreased N partitioning to young shoot tissues, thereby inducing the remobilization of N from older leaves, and reducing leaf area development. The leaf area specific rate of net photosynthesis was not affected over the study period; however, shoot and nodulated root respiration declined. Consequently, C accumulated in mature leaves and stems, partly in the form of increased starch reserves. Three possibilities were considered for increasing low temperature tolerance in nodulated soybeans: (a) decrease in temperature optima for nitrogenase, (b) increased development of nodules and N₂ fixation capacity at low temperature, and (c) alterations in the pattern of C and N partitioning in response to low temperature conditions.     

Phosphorus nutrition influence on leaf senescence in soybean

Remobilization of mineral nutrients from leaves to reproductive structures is a possible regulatory factor in leaf senescence. The relationship between P remobilization from leaves of soybean (Glycine max [L.] Merr. cv McCall) during reproductive development and leaf senescence was determined by utilizing soil P treatments that supplied deficient, optimum, and supraoptimum soil P levels. The soil P treatments simulated field conditions, being initiated at the time of planting with no subsequent addition or removal of P. It was hypothesized that P deficiency would accelerate leaf senescence and that supraoptimum P nutrition would delay the timing or rate of leaf senescence relative to plants grown with optimum P. Supraoptimum soil P led to a two- to fourfold increase in leaf P concentration compared with optimum P, and during senescence there was no net P remobilization from leaves for this treatment. Leaf P concentration was similar for plants grown at optimum or deficient soil P, and there was significant net P remobilization from leaves of both treatments in one of the two experiments. As indicated by changes in leaf N, carbon dioxide exchange rate, ribulose 1,5-bisphosphate carboxylase/oxygenase activity, and chlorophyll concentration, leaf senescence patterns were similar for all soil P treatments. Thus, it can be concluded that leaf senescence was not affected by either P deficiency or enhanced leaf P concentration resulting from supraoptimum soil P. The results suggest that P nutrition in general, and specifically P remobilization from leaves, does not exert any regulatory control on the process of leaf senescence.     

Demand and supply of N in seed production of soybean (<Emphasis Type="Italic">Glycine max</Emphasis>) at different N fertilization levels after flowering

Nitrogen (N) has been suggested as a determinant of seed production especially in species with high seed N content. Assuming that seed yield was determined as the balance between N demand and supply for seed production, we studied the effect of N fertilization after flowering on soybean (Glycine max L. Merr.) yield. Seed N concentration was nearly constant irrespective of N fertilization, indicating that seed production was proportional to the amount of N available for seed growth. N demand for seed production was analyzed as the product of seed number, the rate of N filling in individual seeds, and the length of the reproductive period. N fertilization increased seed number and the reproductive period, but did not influence the N filling rate. Seed number was positively correlated with dry mass productivity after flowering. Three N sources were distinguished: mineral N uptake, symbiotic N₂ fixation and N remobilization from vegetative body. N fertilization increased N uptake and N remobilization, but lowered N₂ fixation. We concluded that N availability in the reproductive period determined seed yield directly through increasing N supply for seed growth and indirectly through increasing seed N demand with enhanced plant dry mass productivity.     

Effects of N fertilization on N2 fixation and N balances of soybean grown after lowland rice

We report a study in northern Thailand to examine the effects of fertilizer N, applied both to paddy rice and to a subsequent soybean crop on symbiotic and yield characteristics of soybean and on the differences between inputs of fixed N₂ and the removal of N as harvested product. Treatments were a factorial arrangement of 0, 100 and 300 kg N ha^-1 applied to the rice (designated R0, R100 and R300, respectively), and 0,25 and 50 kg N ha^-1, applied as starter fertilizer to the soybean (S0, S25 and S50, respectively).Nitrogen applied to the rice increased rice yields by up to 74% but proportions recovered by the rice were low (45% [R100] and 14% [R300]). The rice N treatments had only marginal effects on soybean nodulation (up to 17% reduction in early growth) and above-ground dry matter (up to 9% increase). Effects on soybean seed yield and total N₂ fixed were insignificant. Starter N, applied to the soybean at sowing, also marginally reduced nodulation and enhanced above-ground dry matter. Total N₂ fixed was unaffected but seed yield was increased by up to 6%. For all treatments, total above-ground N ranged from 145 to 179 kg ha^-1 with 72 to 85% (122 and 140 kg ha^-1) derived from N₂ fixation. When harvested product consisted of seed only, differences between inputs of fixed N₂ and removals of seed N were close to zero (-10 to+9 kg N ha^-1) with little effect of fertilizer N. The N balances were reduced by an average of 18 kg N ha^-1 when straw was included as harvested product. We concluded that N applied to the rice and to the following soybean was inefficiently used by those crops and had only marginal effects of symbiotic activity of the soybean. Furthermore, the benefit of the N₂ fixing soybean in this system was to slow the decline of, rather than enhance, the N fertility of the soil     

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin syn‐thesis, i.e. targeted to senescing leaves and negatively auto‐regulated (P_SAG12–IPT), thus preventing developmental abnormalities. Plants were grown with growth‐limiting nitrogen supply. Compared to the wild‐type, endogenous levels of free zeatin (Z)‐ and Z riboside (ZR)‐type cytokinins were increased up to 15‐fold (total ZR up to 100‐fold) in senescing leaves, and twofold in younger leaves of P_SAG12–IPT. In these plants, the senescence‐associated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more (¹⁵N‐labelled) N than younger leaves, associated with reduced shoot N accumulation (–60%) and a partially inverted canopy N profile in P_SAG12–IPT plants. While root N accumulation was not affected, N translocation to non‐senescing leaves was progressively reduced. We discuss potential consequences of these modified sink–source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals.     

Transgenic rice with low sucrose-phosphate synthase activities retain more soluble protein and chlorophyll during flag leaf senescence

We investigated whether changes in sucrose-phosphate synthase (EC 2.4.1.14, SPS) activity could alter N remobilization during leaf senescence. Transgenic rice (Oryza sativa L. cv. Nipponbare) with low SPS activities and wild-type rice plants were grown with basal N (1.0 mM NH₄NO₃) until the late vegetative stage. Subsequently, half of the plants were transferred to a low N (0.1 mM NH₄NO₃) condition to accelerate leaf senescence, and the others were continuously grown with basal N. With low N supply, the amounts of chlorophyll and soluble protein in flag leaf blades decreased after anthesis in both the low SPS plants and wild-type plants, although the decrease was less in the low SPS plants. Panicle weights were significantly lower in the low SPS plant than in the wild-type plant. These results suggest that the remobilization of N from flag leaves was diminished by suppressing the development of reproductive sinks in the low SPS plant.     

Role of N2 fixation in the soybean N credit in maize production

Many studies have shown that maize (Zea mays L.) requires less fertilizer N for optimum yield when grown in rotation with soybean [Glycine max (L.) Merr] than when grown in monoculture, which is referred to as the `soybean N credit' in the maize growing areas of the United States. Because the specific source of this soybean N credit is unclear, our objective was to determine the role of nodules and N₂ fixation as a contributing source of the soybean N credit. Our research approach was designed to separate the effect of symbiotic N₂ fixation from other rotational effects, as the treatments included: maize grown after nodulated (N₂ fixing) soybean and maize grown after non-nodulated (non N₂ fixing) soybean. A separate experiment examined maize grown after maize. For each previous crop, maize was grown the following year with varying rates of fertilizer applied N. In both years, the yield differences between nodulated and non-nodulated soybean as the previous crop were much smaller than the apparent yield decrease associated with continuous maize. Although small in magnitude, maize following non-nodulated soybean accumulated less total N, was paler in leaf color, and yielded less than maize following nodulated soybean in the more favorable year of 1999, while most of these differences were not observed in 2000. These findings indicate that soybean nodules and N₂ fixation, while having a certain role, are not the major determinants of the soybean N credit.     

Phenolic acids: Possible agents of modifying N2-fixing symbiosis through rhizobial alteration?

Phenolic acids, low molecular weight phenolics, are precursors of a variety of antimicrobial compounds, root signalling molecules, and phytoalexins that play an important role in plant defence responses. In agro ecosystem, a large amount of litter is turned over during the cropping season, fallow period and land preparation. This releases a flush of phenolic acids, amounts of which exceed very much the quantities released in root exudation. In rhizobial inoculation of legumes, these phenolic acids, depending on the concentration, may affect the persistence of rhizobia in the soil and their symbiotic efficiency, in terms of N₂ fixation. The present study evaluates the effects of different concentrations of four phenolic acids (protocatechuic, p-coumaric, ferulic and vanillic) on population size of four rhizobial strains (Bradyrhizobium elkanii SEMIA 5019, B. japonicum TAL 102 and TAL 620, and Azorhizobium caulinodans ORS 571). Culture media with different concentrations of phenolic acids in the presence or absence of manitol were used to evaluate rhizobial population size on day 6. Rhizobial total proteins were extracted and electrophoresed on polyacrylamide gels. Further, the effects of phenolic acid-affected rhizobia on N₂ fixing capacity were also investigated by inoculating two of those strains to soybean. Phenolic acid-treated B. elkanii SEMIA 5019 and B. japonicum TAL 102 were inoculated to soybean, and plant growth, N accumulation and nodule dry weight were assessed in a pot experiment. The population size of TAL 102 was induced when the culture medium was supplied with different phenolic acids as the sole carbon source. In many cases, the presence of manitol in the medium masked the differential effects of phenolic acids on the rhizobial population size. All four phenolic acids used in our study suppressed the population size of TAL 620. Strain ORS 571 showed low population size at low concentrations followed by a growth recovery at high phenolic acid concentrations. Strain SEMIA 5019 treated with 0.03 mM ferulic acid produced the highest increase in shoot growth of soybean, (ca. 65%). Treating strain SEMIA 5019 with 9 mM protocatechuic acid produced the largest decrease in nodule dry weight (ca. 50%) without any significant changes in shoot N accumulation. P-coumaric acid, even at 0.12 mM, could stimulate the N₂ fixing activity of SEMIA 5019, whereas the same concentration reduced the effectiveness of TAL102 in a soybean-rhizobium symbiosis. Phenolic acid interactions with rhizobia led to biochemical, and hence physiological changes, resulting in an alteration in their symbiotic ability. Different leguminous plants secrete different phenolic compounds other than phenolic acids during root exudation. Further studies should therefore be conducted to evaluate the effects of those compounds on the symbiosis. It is concluded from this study that the effect of phenolic acids is concentration and structure dependant, and strain-specific. The effect will also be pH dependant. Thus, phenolic acids are possible agents for modifying the legume-rhizobial symbiosis.     

Fixation of [13N]N2 and transfer of fixed nitrogen in the Anthoceros-Nostoc symbiotic association

The initial product of fixation of [¹³N]N₂ by pure cultures of the reconstituted symbiotic association between Anthoceros punctatus L. and Nostoc sp. strain ac 7801 was ammonium; it accounted for 75% of the total radioactivity recovered in methanolic extracts after 0.5 min and 14% after 10 min of incubation. Glutamine and glutamate were the primary organic products synthesized from [¹³N]N₂ after incubation times of 0.5–10 min. The kinetics of labeling of these two amino acids were characteristic of a precursor (glutamine) and product (glutamate) relationship. Results of inhibition experiments with methionine sulfoximine (MSX) and diazo-oxonorleucine were also consistent with the assimilation of N₂-derived NH ₄ ⁺ by Anthoceros-Nostoc through the sequential activities of glutamine synthetase (EC 6.3.1.2) and glutamate synthase (EC 1.4.7.1), with little or no assimilation by glutamate dehydrogenase (EC 1.3.1.3). Isolated symbiotic Nostoc assimilated exogenous ¹³NH ₄ ⁺ into glutamine and glutamate and their formation was inhibited by MSX, indicating operation of the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway: However, relative to free-living cultures, isolated symbiotic Nostoc assimilated 80% less exogenous ammonium into glutamine and glutamate, implying that symbiotic Nostoc could assimilate only a fraction of N₂-derived NH ₄ ⁺ . This implication was tested by using Anthoceros associations reconstituted with wild-type or MSX-resistant strains of Nostoc incubated with [¹³N]N₂ in the presence of MSX. The results of these experiments indicated that, in situ, symbiotic Nostoc assimilated about 10% of the N₂-derived NH ₄ ⁺ and that NH ₄ ⁺ was made available to Anthoceros tissue where it was apparently assimilated by the GS-GOGAT pathway. Since less than 1% of the fixed N₂ was lost to the suspension medium, it appears that transfer of NH ₄ ⁺ from symbiont to host tissue was very efficient in this extracellular symbiotic association.Abbreviations DON 6-diazo-5-oxo-l-norleucine - GDH glutamate dehydrogenase - GOGAT glutamate synthase - GS glutamine synthetase - MSX l-methionine-dl-sulfoximine     

Acid-Base Regulation by Azolla spp. with N2 as Sole N Source and with Supplementation by NH+4 or NO−3

Diazotrophic cultures of three species of Azolla (Az. caroliniana, Az. microphylla, Az. pinnata) symbiotic with Anabaena azollae accumulated some 0.10–0.24 mol organic anion per mol N assimilated (0.010–0.018 mol organic anion per mol C assimilated), with a corresponding efflux of 0.05–0.11 mol H⁺ per mol N assimilated (0.006–0.009 mol H⁺ per mol C assimilated). These values are lower than those found for terrestrial diazotrophic vascular plants; this may be related to the decreased possibility of increasing Fe and P availability by rhizosphere acidification in a free-floating plant. Modification of the organic anion content, and of the quantity (and direction) of H⁺ exchange with the medium, with 5 mol m^?3 NH⁺₄ or NO^?₃ added to diazotrophic cultures, are consistent with substantial N acquisition from combined N as well as N₂ assimilation. This conclusion is consistent with previously published work with ¹⁵N.     

Relationships between Carbon Assimilation, Partitioning, and Export in Leaves of Two Soybean Cultivars

To evaluate leaf carbon balance during rapid pod-fill in soybean (Glycine max [L.] Merrill), measurements were made of CO₂ assimilation at mid-day and changes in specific leaf weight, starch, and sucrose concentrations over a 9-hour interval. Assimilate export was estimated from CO₂ assimilation and leaf dry matter accumulation. Chamber-grown `Amsoy 71' and `Wells' plants were subjected on the day of the measurements to one of six photosynthetic photon flux densities in order to vary CO₂ assimilation rates.

Rate of accumulation of leaf dry matter and rate of export both increased as CO₂ assimilation rate increased in each cultivar.

Starch concentrations were greater in Amsoy 71 than in Wells at all CO₂ assimilation rates. At low CO₂ assimilation rates, export rates in Amsoy 71 were maintained in excess of 1.0 milligram CH₂O per square decimeter leaf area per hour at the expense of leaf reserves. In Wells, however, export rate continued to decline with decreasing CO₂ assimilation rate. The low leaf starch concentration in Wells at low CO₂ assimilation rates may have limited export by limiting carbon from starch remobilization.

Both cultivars exhibited positive correlations between CO₂ assimilation rate and sucrose concentration, and between sucrose concentration and export rate. Carbon fixation and carbon partitioning both influenced export rate via effects on sucrose concentration.

     

Interactions between elevated CO2 and N2-fixation determine soybean yield—a test using a non-nodulated mutant

Elevated CO₂ increases seed production more in plant species that form a symbiotic association with N₂–fixing bacteria than in species without such association. We studied the mechanism of the increase of seed production with elevated CO₂ using nodulated soybean (Glycine max cv. Enrei) and its non-nodulated isogenic line (cv. En1282). Increase in seed production with elevated CO₂ was observed in nodulated Enrei but was not in non-nodulated En1282. The increase in seed production in Enrei was explained by the increase in the rate of dry mass production during the reproductive period. This increase was associated with the increase in N assimilation in the reproductive period and the seed N concentration that remained the same as that at ambient CO₂. Dry mass production and nitrogen assimilation did not increase in the vegetative phase in both lines. These results accorded with the amount of nodules in Enrei that increased at elevated CO₂ especially after flowering. We conclude that the increase in N assimilation in the reproductive period would be the key for increasing soybean yield in the future high-CO₂ world.     

Short communication: Screening of native and foreign Bradyrhizobium japonicum strains for high N2 fixation in soybean

Four local rhizobia isolates selected after two screening experiments and five USDA Bradyrhizobium japonicum strains were estimated for N₂ fixation in soybean using the ¹⁵N isotope dilution technique. Strain USDA 110 was superior to the local isolates in nodulation and N₂ fixation when inoculated onto soybean cv TGX 1497-ID in a Nigerian soil and could therefore be used as an inoculant for enhanced N₂ fixation in soybean in Nigeria.     

Shade light interaction with salicylic acid in regulating growth of sun (alpine) and shade (prairie) ecotypes of Stellaria longipes

Magnesium (Mg) deficiency in plants is a widespread problem, affecting productivity and quality in agriculture. The mechanism of Mg deficiency inducing antioxidant enzyme activities has not been elucidated in rice. We examined the relationship among abscisic acid (ABA), H₂O₂, and antioxidant enzymes in the leaves of rice seedlings grown under conditions of Mg deficiency. The expression of OsRab16A, an ABA responsive gene, was used to determine the content of ABA. Mg deficiency resulted in increased ABA content in leaves of rice seedlings. The production of H₂O₂ was examined by 3,3-diaminobenzidine staining and a colorimetric method. Mg deficiency also induced H₂O₂ production in leaves, which was blocked by dipehnyleneiodonium chloride (DPI), an NADPH oxidase inhibitor. Tungstate (Tu), an ABA biosynthesis inhibitor, was effective in reducing Mg deficiency-increased ABA content, as well as Mg deficiency-induced H₂O₂ production. Both Tu and DPI were effective in reducing Mg deficiency-induced activities of superoxide dismutase, ascorbate peroxidase, glutathione reductase, and catalase in the leaves. Mg deficiency-induced ABA accumulation may trigger increased production of H₂O₂, which may involve plasma-membrane NADPH oxidase, and, in turn, up-regulates the activities of antioxidant enzymes in leaves of rice seedlings.     

The presence of nodules on legume root systems can alter phenotypic plasticity in response to internal nitrogen independent of nitrogen fixation

All higher plants show developmental plasticity in response to the availability of nitrogen (N) in the soil. In legumes, N starvation causes the formation of root nodules, where symbiotic rhizobacteria fix atmospheric N₂ for the host in exchange for fixed carbon (C) from the shoot. Here, we tested whether plastic responses to internal [N] of legumes are altered by their symbionts. Glasshouse experiments compared root phenotypes of three legumes, Medicago truncatula, Medicago sativa and Trifolium subterraneum, inoculated with their compatible symbiont partners and grown under four nitrate levels. In addition, six strains of rhizobia, differing in their ability to fix N₂ in M. truncatula, were compared to test if plastic responses to internal [N] were dependent on the rhizobia or N₂‐fixing capability of the nodules. We found that the presence of rhizobia affected phenotypic plasticity of the legumes to internal [N], particularly in root length and root mass ratio (RMR), in a plant species‐dependent way. While root length responses of M. truncatula to internal [N] were dependent on the ability of rhizobial symbionts to fix N₂, RMR response to internal [N] was dependent only on initiation of nodules, irrespective of N₂‐fixing ability of the rhizobia strains.     

GhTZF1 regulates drought stress responses and delays leaf senescence by inhibiting reactive oxygen species accumulation in transgenic Arabidopsis

Redox homeostasis is important for plants to be able to maintain cellular metabolism, and disrupting cellular redox homeostasis will cause oxidative damage to cells and adversely affect plant growth. In this study, a cotton CCCH-type tandem zinc finger gene defined as GhTZF1, which was isolated from a cotton cell wall regeneration SSH library in our previous research, was characterized. GhTZF1 was predominantly expressed during early cell wall regeneration, and it was expressed in various vegetative and reproductive tissues. The expression of GhTZF1 was substantially up-regulated by a variety of abiotic stresses, such as PEG and salt. GhTZF1 also responds to methyl jasmonate (MeJA) and H₂O₂ treatment. Overexpression of GhTZF1 enhanced drought tolerance and delayed drought-induced leaf senescence in transgenic Arabidopsis. Subsequent experiments indicated that dark- and MeJA-induced leaf senescence was also attenuated in transgenic plants. The amount of H₂O₂ in transgenic plants was attenuated under both drought conditions and with MeJA-treatment. The activity of superoxide dismutase and peroxidase was higher in transgenic plants than in wild type plants under drought conditions. Quantitative real-time PCR analysis revealed that overexpression of GhTZF1 reduced the expression of oxidative-related senescence-associated genes (SAGs) under drought conditions. Overexpression of GhTZF1 also enhanced oxidative stress tolerance, which was determined by measuring the expression of a set of antioxidant genes and SAGs that were altered in transgenic plants during H₂O₂ treatment. Hence, we conclude that GhTZF1 may serve as a regulator in mediating drought stress tolerance and subsequent leaf senescence by modulating the reactive oxygen species homeostasis.