Coordination between Bradyrhizobium and Pseudomonas alleviates salt stress in soybean through altering root system architecture

It is a well accepted strategy to improve plant salt tolerance through inoculation with beneficial microorganisms. However, its underlying mechanisms still remain unclear. In the present study, hydroponic experiments were conducted to evaluate the effects of Bradyrhizobium japonicum USDA 110 with salt-tolerant Pseudomonas putida TSAU1 on growth, protein content, nitrogen, and phosphorus uptake as well as root system architecture of soybean (Glycine max L.) under salt stress. The results indicated that the combined inoculation with USDA 110 and TSAU1 significantly improved plant growth, nitrogen and phosphorus contents, and contents of soluble leaf proteins under salt stress compared to the inoculation with the symbiont alone or compared to un-inoculated ones. The root architectural traits, like root length, surface area, project area, and root volume; as well as nodulation traits were also significantly increased by co-inoculation with USDA 110 and TSAU1. The plant-growth promoting rhizobacteria (PGPR) P. putida strain TSAU1 could improve the symbiotic interaction between the salt-stressed soybean and B. japonicum USDA 110. In conclusion, inoculation with B. japonicum and salt-tolerant P. putida synergistically improved soybean salt tolerance through altering root system architecture facilitating nitrogen and phosphorus acquisition, and nodule formation.     

Root architectural responses of Betula lenta to spatially heterogeneous ammonium and nitrate

Inorganic soil nitrogen is often heterogeneously distributed, both spatially and in form (ammonium versus nitrate). Here we present information on the architecture of black birch (Betula lenta L.) root systems exposed to homogeneous and heterogeneous nitrogen environments. The major effects on root architecture were at the whole root system level in response to heterogeneity of nitrogen form rather than the effect of local of local nitrate or ammonium supply on local root growth. In the heterogeneous treatment, plant root systems had greater link lengths and more simple branching patterns. Root architectural responses to heterogeneous nitrogen, independent of localized responses to patches, suggest that in a seedling of B. lenta whole plant integration of its environment may override local control of root growth.     

Is Fe deficiency rather than P deficiency the cause of cluster root formation in Casuarina species?

When subjected, directly (through nutritional deficiencies) or indirectly (through alkaline constraints leading to such deficiencies) to nutrient deficiencies, certain plants respond by developing special root structures called cluster roots. This phenomenon can be considered as an ecophysiological response to a specific nutrient deficiency enabling plants to enhance nutrient uptake. Experiments conducted on an alkaline and an acid soil showed that Casuarina glauca (Sieber ex Spreng.) produced cluster roots only in the alkaline soil and not in the acid soil. In addition, iron (Fe) and phosphorus (P) deficiencies were examined separately or together to determine their effect on cluster root formation in C. glauca seedlings grown hydroponically. Results from experiments carried out on three Casuarina species (C. glauca, C. cunninghamiana Miq. and C. equisetifolia L.) indicated that Fe is involved in cluster root formation. In nutrient media lacking P but containing Fe, no cluster roots formed while seedlings receiving P and lacking Fe developed cluster roots. When incubated on chrome-azurol S-agar on blue plates (CAS assay), a technique used routinely to detect the production of siderophores by micro-organisms, the root system of Fe-deficient plants exhibited orange halos around cluster roots, indicating production of a ferric-chelating agent. It is concluded that the capacity of cluster roots of C. glauca to chelate Fe allows the plant to grow normally on alkaline soils.     

Boron efficiency in oilseed rape: II. Development of a rapid lab-based screening technique

The level of genotypic variation in tolerance to low boron (B) supply was investigated in solution culture grown, 10 day old (D10) oilseed rape (Brassica napus L.) plants, by using a rapid screening technique whereby root length, root elongation rate and total root dry weight were used to indicate plant response to B. Root length proved more reliable in determining genotype responses, and was used to characterise a total of 61 genotypes, of which Huashuang 2, Nangchang rape, Huashuang 1 and Zhongyou 821, and to a lesser extent, Zheyou 2, Dunkeld, Xinza 2, Nangjin 2051, 92-58, 92-13, and Awassa 115 exhibited some form of tolerance to low B supply. The genotypic rankings based on this early vegetative response corroborated with field based B-efficiency. The results demonstrate the expression of the B-efficiency mechanism in the early vegetative stages of plant growth, and establish the value of root length as a selection criterion for B-efficiency in oilseed rape. This revised version was published online in June 2006 with corrections to the Cover Date.     

Regulation of plasma membrane aquaporins by inoculation with a Bacillus megaterium strain in maize (Zea mays L.) plants under unstressed and salt-stressed conditions

It is documented that some plant-growth-promoting rhizobacteria (PGPR) enhance plant salt tolerance. However, as to how PGPR may influence two crucial components of plant salt tolerance such as, root hydraulic characteristics and aquaporin regulation has been almost unexplored. Here, maize (Zea mays L.) plants were inoculated with a Bacillus megaterium strain previously isolated from a degraded soil and characterized as PGPR. Inoculated plants were found to exhibit higher root hydraulic conductance (L) values under both unstressed and salt-stressed conditions. These higher L values in inoculated plants correlated with higher plasma membrane type two (PIP2) aquaporin amount in their roots under salt-stressed conditions. Also, ZmPIP1;1 protein amount under salt-stressed conditions was higher in inoculated leaves than in non-inoculated ones. Hence, the different regulation of PIP aquaporin expression and abundance by the inoculation with the B. megaterium strain could be one of the causes of the different salt response in terms of root growth, necrotic leaf area, leaf relative water content and L by the inoculation treatment.     

Estimates of root system topology of five plant species grown at steady-state nutrition

Results from a controlled growth-analysis experiment were used to illustrate some methods for measuring and describing root system topology. The experiment was performed in a nutrient solution system with an exponential nutrient supply and steady-state growth, to achieve well-defined levels of whole-plant nutrient status. Five naturally coexisting grassland species were included: The slow-growing forbs Polygala vulgaris L. and Crepis praemorsa (L.) F. L. Walth., and the grass Danthonia decumbens (L.) DC. were compared with the more common, fast-growing grasses Agrostis capillaris L. and Dactylis glomerata L. The most marked difference in morphological indices was a much higher specific root length in the grasses than in the forbs, which implies thinner roots. In contrast to the conclusions of previous studies, an index of the topology for the grasses was very similar to that for the forbs. The specific root lenght and link length apparently vary more between species and nutrient levels than topology does, and may therefore be more ecologically important. The only clear plastic response to growth-limiting nitrogen supply was a markedly increased link length in P. vulgaris. There were also indications that nitrogen limitation led to more herringbone-like root systems in P. vulgaris and C. praemorsa. In general, there was a clear tendency for the estimates of topology to change with plant size, which may make many topological indices, especially those based on regression slopes, very difficult to interpret. Until interactions with plant size, other morphological parameters and among-plant competition can be properly understood, the relevance of root topology for plant performance remains unclear. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of the <Emphasis Type="Italic">ipt</Emphasis> gene expression on wheat tolerance to root flooding

The effect of enhanced cytokinin synthesis due to expression of the ipt gene from Agrobacterium tumefaciens on plant tolerance to root flooding was studied. Transgenic wheat (Triticum aestivum L.) plants carrying the ipt gene were more tolerant to flooding than wild-type plants. The effect of transformation was manifested in the higher yield and less growth inhibition during flooding. The measurements of activities of antioxidant enzymes, superoxide dismutase and catalase, as well as MDA content during flooding revealed differences between wild-type and transgenic plants that correlated with their tolerance. These results point to the protective role of cytokinins during wheat root flooding.     

Role of the Rice BAHD Acyltransferase Gene OsAt10 in Plant Cold Stress Tolerance

The rice (Oryza sativa L.) BAHD acyltransferase gene OsAt10 affects growth and metabolism of cells and regulates cell response to environmental stress. However, influence of the OsAt10 gene on low-temperature stress tolerance has not been evaluated in plant cells. Here, cell suspension cultures of plant species Arabidopsis (Arabidopsis thaliana L.), cotton (Gossypium hirsutum L.), white pine (Pinus strobus L.), and rice (Oryza sativa L.) were used to generate transgenic cell lines via Agrobacterium tumefaciens-mediated genetic transformation to examine the effects of OsAt10 on cold stress tolerance. OsAt10 transgenic cell lines of A. thaliana, G. hirsutum, P. strobus, and O. sativa were confirmed by molecular analyses including Southern blotting ND northern blotting, following by physiological and biochemical analyses under cold stress. The experimental results demonstrated that growth rate, cell viability, lipid peroxidation, ion leakage, antioxidative enzyme activity, polyamines level, and cell morphology were changed in transgenic cells under cold stress, compared to the controls. In transgenic A. thaliana cells, overexpression of the OsAt10 gene increases expression of polyamines biosynthesis genes under cold stress. In transgenic A. thaliana plants, overexpression of the OsAt10 gene increased cold stress tolerance by regulating expression of stress marker genes, TBARS content, ion leakage level, antioxidant enzymes activity, and polyamines content, indicating that the OsAt10 gene could be economically important for improving low-temperature stress tolerance in plants.

     

Salinity Tolerance in Brassica Oilseeds

Brassica oilseed species now hold the third position among oilseed crops and are an important source of vegetable oil. The most common Brassica oil-seed crops grown for commercial purposes are rape seeds, (Brassica campestris L. and B. napus L.) and mustards (B. juncea (L.) Czern. & Coss. and B. carinata A.Br.). The other Brassica species such as B. nigra (L.) Koch and B. tournefortii Gouan are grown on a very small scale. Brassica napus, B. juncea, and B. carinata are amphidiploids, whereas B. campestris and B. nigra are diploid. Most of the Brassica species have been categorized as moderately salt tolerant, with the amphidiploid species being the relatively salt tolerant in comparison with the diploid species. Due to the higher salt tolerance of the amphidiploids, it has been suggested that their salt tolerance has been acquired from the A (B. campestris) and C (B. oleracea L.) genomes. However, significant inter- and intraspecific variation for salt tolerance exists within brassicas, which can be exploited through selection and breeding for enhancing salt tolerance of the crops. There are contrasting reports regarding the response of these species to salinity at different plant developmental stages, but in most of them it is evident that they maintain their degree of salt tolerance consistently throughout the plant ontogeny. The pattern of uptake and accumulation of toxic ions (Na⁺ and Cl^?), in tissues of plants subjected to saline conditions appears to be mostly due to mechanism of partial ion exclusion (exclusion of Na⁺ and/or Cl^?) in most of the species, although ion inclusion in some cases at intraspecific levels has also been observed. Maintenance of high tissue K⁺/Na⁺ and Ca^{2 +}/Na⁺ ratios has been suggested as an important selection criterion for salt-tolerance in brassicas. Osmotic adjustment has also been reported in Brassica plants subjected to saline conditions, but particularly to a large extent in salt-tolerant species or cultivars. The roles of important organic osmotica such as total soluble sugars, free amino acids, and free proline, which are central to osmotic adjustment, have been discussed. In canola, B. napus, no positive relationship has been observed between salt tolerance and erucic acid content of seed oil in different cultivars. Furthermore, glucosinolate content of the seed meal in canola generally increases with an increase in salt level of the growth medium. This review highlights the responses of potential Brassica crops to soil salinity from the whole plant to the molecular level. It also describes the efforts made during the past millennium in uncovering the mechanism(s) of salinity tolerance of these crops both at the whole plant and cellular levels. The important selection criteria, which are used by researchers to enhance the degree of salinity tolerance in brassicas, are summarized. In addition, the vital role of genetic engineering and molecular biology approaches to the improvement of salt tolerance in brassicas is emphasized.     

Physiological Mechanisms of <Emphasis Type="Italic">Solanum tuberosum</Emphasis> L. Plants’ Tolerance to Chloride Salinity

The mechanisms of potato (Solanum tuberosum L.) plants’ tolerance to chloride salinity were investigated in cv. Lugovskoi regionalized in Russia. Regenerated plants were produced in vitro from apical meristem and grown on half-strength Murashige and Skoog medium (0.5 MS) using a hydroponic unit in controlled-climate conditions. At the age of six weeks, the plants were exposed to salt stress (50–150 mM NaCl, 7 days). Plant response to salt stress was estimated by growth parameters (fresh and dry biomass of the aboveground and underground parts of plants, linear dimensions of shoot and root, area of leaf surface, and number of stolons) and physiological characteristics (level of photosynthetic pigments, accumulation of sodium, potassium, and calcium ions in the aboveground and underground parts of plants, content of proline, activity of antioxidant enzymes, plant tissue hydration, osmotic potential, and POL). It was found that, in response to salinity, the plants of potato, cv. Lugovskoi, showed a considerable inhibition of growth processes, reduction in chlorophyll a content, and suppression of stolon formation, which points to a rather low salinity tolerance of the cultivar. At the same time, under weak or moderate salt stress, the plants preserved water homeostasis owing to effective osmoregulation, actively accumulated proline that acted as a stress protector, and showed hardly any signs of oxidative stress. It was assumed that low salt tolerance of this cultivar depends on the inability of its root system to retain sodium ions and ensure selective ion transport to the aboveground part of the plant and on inefficiency of the system of sodium ions’ removal from the cytoplasm of leaf cells and their compartmentalization in the central vacuole with the purpose of reducing their toxic effect. The obtained results may be useful for working out a technique of improving salt tolerance of this cultivar by the methods of molecular genetics.     

Organic Anion Exudation by Lowland Rice (Oryza sativa L.) at Zinc and Phosphorus Deficiency

The objectives of this paper were to determine (1) if lowland rice (Oryza sativa L.) plants respond similarly to low zinc (Zn) and phosphorus (P) availability by increased root exudation of low-molecular weight organic anions (LMWOAs) and (2) if genotypic variation in tolerance to low soil supply of either Zn or P is related to LMWOA exudation rates. Exudation of LMWOAs can increase bioavailability of both Zn and P to the plant, through partly similar chemical mechanisms. We used seven lowland rice genotypes and showed in two experiments that genotypes that grow relatively well on a soil with low Zn availability also grow well on a sparingly soluble Ca-phosphate (r = 0.80, P = 0.03). We measured exudation rates of LMWOAs on nutrient solution and found that both Zn and P deficiency induced significant increases. Among the LMWOAs detected oxalate was quantitatively the most important, but citrate is considered more effective in mobilizing Zn. Citrate exudation rates correlated with tolerance to low soil levels of Zn (P=0.05) and P (P = 0.07). In a low-Zn-field we found an increased biomass production at higher plant density, which is supportive for a concentration-dependent rhizosphere effect on Zn bioavailability such as LMWOA exudation. We, for the first time, showed that tolerance to low Zn availability is related to the capacity of a plant to exude LMWOAs and confirmed that exudation of LMWOAs must be regarded a multiple stress response.     

Study of the contribution of the shoot and/or root ofAlnus sp. in the compatibility between the host and a Sp+ Frankia strain using a grafting technique

Two alder species,Alnus glutinosa (L.) Gaertn. andAlnus incana (L) Moench, were inoculated with a Sp⁺ Frankia homogenate obtained fromA. incana root nodules. This inoculum formed effective nodules on the original host plant and ineffective nodules onA. glutinosa. Grafts between the two alder species were made to determine which part of the plant is involved in this phenomenon. The results obtained indicate that the compatibility between Alnus andFrankia is restricted to the root system.     

Tolerance and antibiosis resistance to cabbage root fly in vegetable Brassica species

Four accessions of the wild species Brassica fruticulosa Cyrillo (Brassicaceae) were studied in order to identify its tolerance and antibiosis resistance to the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae), in comparison to a widely cultivated cauliflower cultivar and a rapid cycling Brassica oleracea L. line. Antibiosis was prominent, as the insects reared on resistant accessions showed reduced individual pupal weight, total pupal weight, adult dry weight, and the longest average fly eclosion time. Host plant resistance, however, did not affect the sex ratio of adult flies. A study of the root architecture of plants with and without root fly inoculation revealed differences in the structure within B. oleracea accessions. A long main root and a high number of lateral roots appeared to be important characteristics for a Brassica type, with a higher tolerance level to cabbage root fly attack.     

Variation in response of accessions of minor millets,Pennisetum americanum (L.) Leeke (pearl millet) and Eleusine coracana (L.) Gaertn (finger millet), and Eragrostis tef (Zucc.) Trotter (tef) to salinity in early seedling growth

The response to increasing NaCl concentration of seedlings of 25 accessions of Ethiopian land races of each of Pennisetum americanum (L.) Leeke (pearl millet) and Eleusine coracana (L.) Gaertn (finger millet), and 15 accessions of Eragrostis tef (Zucc.) Trotter (tef), was examined after two week's growth in NaCl solution culture. Although increasing NaCl concentration significantly reduced seedling root lengths, there was considerable variation within, and between accessions within each species.Analysis based upon a non-linear least square inversion method, using root length data, revealed significant differences in accessions of P. americanum and E. tef on the basis of the estimated salinity threshold, C _t , the NaCl concentrations at which root length begins to decrease. C _t did not differ significantly between E. coracana accessions. Estimates of C₅₀ and C₀, mininum concentrations causing a 50% decrease in root length, and zero root growth respectively, revealed differences between and within accessions for all three species. Overall, finger millet was more tolerant than tef, which was more tolerant than pearl millet. There is clear evidence that differences in tolerance are genetically based from broad sense heritability estimates.     

Stimulation of adventitious root formation in non-woody stem cuttings by uridine

Uridine strongly stimulated adventitious root formation in stem cuttings of sunflower (Helianthus annuus L.), mung bean (Vigna radiata L.) and common bean (Phaseolus vulgaris L.). A dose response curve of uridine induced rooting showed that the optimum concentration of uridine was 0.1 µM. At all concentrations employed, uridine had no significant effect on root elongation. The rooting response of stem cuttings to the optimal concentration of indole-3-butyric acid (10 µM) in combination with 0.1 µM uridine did not significantly differ from their response to either of these compounds when applied alone. However, the rooting response of the cuttings to sub-optimal IBA (0.01 µM) was significantly stimulated by uridine. These findings suggested that uridine may have stimulated rooting by increasing the sensitivity of the rooting tissue to auxin.     

Long-term culturing of plants with aseptic roots Determination of rape root exudates

Abstract A plant culture system allowing growth of aseptic roots has been designed. One version of the system comprises vessels developed for plant root-microorganism interaction studies in series. A second version has been built for measurements of different physiological parameters of the shoot and root system during growth periods of at least 2 months. The system was tested by determining soluble organic carbon glucose and sucrose in the root exudates of rape plants (Brassica napus L.) grown for 3 weeks in the culture system. The plants were cultivated with a sterile or a non-sterile root system, or with a root system infected with Verticillium dahliae Kleb.     

Root bacterial endophytes confer drought resistance and enhance expression and activity of a vacuolar H+-pumping pyrophosphatase in pepper plants

It has been previously shown that the transgenic overexpression of the plant root vacuolar proton pumps H⁺-ATPase (V-ATPase) and H⁺-PPase (V-PPase) confer tolerance to drought. Since plant-root endophytic bacteria can also promote drought tolerance, we hypothesize that such promotion can be associated to the enhancement of the host vacuolar proton pumps expression and activity. To test this hypothesis, we selected two endophytic bacteria endowed with an array of in vitro plant growth promoting traits. Their genome sequences confirmed the presence of traits previously shown to confer drought resistance to plants, such as the synthesis of nitric oxide and of organic volatile organic compounds. We used the two strains on pepper (Capsicuum annuum L.) because of its high sensitivity to drought. Under drought conditions, both strains stimulated a larger root system and enhanced the leaves' photosynthetic activity. By testing the expression and activity of the vacuolar proton pumps, H⁺-ATPase (V-ATPase) and H⁺-PPase (V-PPase), we found that bacterial colonization enhanced V-PPase only. We conclude that the enhanced expression and activity of V-PPase can be favoured by the colonization of drought-tolerance-inducing bacterial endophytes.     

Localized alteration in lateral root development in roots colonized by an arbuscular mycorrhizal fungus

 The morphological responses of root systems to localized colonization by endophytes is not well understood. We examined the responses of lateral roots to the arbuscular mycorrhizal (AM) fungus Gigaspora margarita Becker & Hall inoculated locally into the soil. Peanut (Arachis hypogaea L.) and pigeon pea (Cajanus cajan (L.) Millsp.) were examined. Root boxes filled with nutrient-poor soil in were inoculated in one half with the fungus and in the other half with a sterilized inoculum. Responses were apparent after 30 days but not after 20 days. Overall, lateral root development was more advanced in inoculated soil. This was clearly observed for 2nd- and 3rd-order lateral roots, but less clear for 1st-order lateral roots in both species, although percentage of colonized root length was higher in 1st-order lateral roots. Whilst in peanut the responses were clearly evident at the level of lateral roots initiated on more proximal parts of the tap root axis, they occurred on more distal parts in pigeon pea. We conclude that plants under nutrient-poor conditions give priority to mycorrhizal roots when partitioning assimilation products within the root system. Thus, AM formation may induce local morphological alteration of root systems. Accepted: 29 August 1996     

A comparison of ferric-chelate reductase and chlorophyll and growth ratios as indices of selection of quince,pear and olive genotypes under iron deficiency stress

Quince (Cydonia oblonga Mill.), pear (Pyrus communis L.) and olive (Olea europaea L.) genotypes were evaluated for their tolerance to iron deficiency stress by growing young plants in three types of aerated nutrient solutions: (1) with iron, (2) without iron or (3) low in iron and with 10 mM bicarbonate. Plants were obtained either from rooted softwood cuttings or from germination of seeds. The degree of tolerance was evaluated with several indices: (1) the chlorophyll content, (2) the root Fe³⁺ reducing capacity and (3) the whole plant relative growth. Fifteen hours before Fe³⁺ reducing capacity determination, iron was applied to the roots of plants with iron-stress, since this method resulted in increasing the reductase activity. All quince and pear genotypes increased the root Fe³⁺ reducing capacity when grown in the treatments for iron-stress, in relation to control plants of the same genotypes. In olive cultivars, the Fe³⁺ reducing capacity was lower in the iron-stress treatments than in the control one. Studying the relationship between relative growth and chlorophyll content for each genotype under iron-stress, in relation to both indices in control plants, a classification of species and genotypes was established. According to that, most olive cultivars and some pear rootstocks and cultivars appear more iron-efficient than quince rootstocks. Our study shows that in some woody species, determining root Fe³⁺ reducing capacity is not the best method to establish tolerance to iron deficiency stress.