Root antioxidant responses of two Pisum sativum cultivars to direct and induced Fe deficiency

The contribution of antioxidant defence systems in different tolerance to direct and bicarbonate-induced Fe deficiency was evaluated in two pea cultivars (Kelvedon, tolerant and Lincoln, susceptible). Fe deficiency enhanced lipid peroxidation and H2 O2 concentration in roots of both cultivars, particularly in the sensitive one grown under bicarbonate supply. The results obtained on antioxidant activities (SOD, CAT, POD) suggest that H2 O2 accumulation could be due to an overproduction of this ROS and, at the same time, to a poor capacity to detoxify it. Moreover, under bicarbonate supply the activity of POD isoforms was reduced only in the sensitive cultivar, while in the tolerant one a new isoform was detected, suggesting that POD activity might be an important contributor to pea tolerance to Fe deficiency. The presence of bicarbonate also resulted in stimulation of GR, MDHAR and DHAR activities, part of the ASC-GSH pathway, which was higher in the tolerant cultivar than in the sensitive one. Overall, while in the absence of Fe only slight differences were reported between the two cultivars, the adaptation of Kelvedon to the presence of bicarbonate seems to be related to its greater ability to enhance the antioxidant response at the root level.

The effect of steepness of temporal resource gradients on spatial root allocation

Plants are able to discriminately allocate greater biomass to organs that grow under higher resource levels. Recent evidence demonstrates that split-root plants also discriminately allocate more resources to roots that grow under dynamically improving nutrient levels, even when their other roots grow in richer patches. Here, we further tested whether, besides their responsiveness to the direction of resource gradients, plants are also sensitive to the steepness of environmental trajectories. Split-root Pisum sativum plants were grown so that one of their roots developed under constantly-high nutrient levels and the other root was subjected to dynamically improving nutrient levels of variable steepness. As expected, plants usually allocated a greater proportion of their biomass to roots that developed under constantly high resource availability; however, when given a choice, they allocated greater biomass to roots that initially experienced relatively low but steeply improving nutrient availabilities than to roots that developed under continuously-high nutrient availability. Such discrimination was not observed when the roots in the poor patch experienced only gentler improvements in nutrient availability. The results are compatible with the notion that responsiveness to the direction and steepness of environmental gradients could assist annual plants to increase their performance by anticipating resource availabilities foreseeable before the end of their growing season. The results exemplify the ability of plants to integrate and utilize environmental information and execute adaptive behaviours which, until recently, were attributed only to animals with central nervous systems.

Stromal Hsp70 is important for protein translocation into pea and Arabidopsis chloroplasts

Hsp70 family proteins function as motors driving protein translocation into mitochondria and the endoplasmic reticulum. Whether Hsp70 is involved in protein import into chloroplasts has not been resolved. We show here Arabidopsis thaliana knockout mutants of either of the two stromal cpHsc70s, cpHsc70-1 and cpHsc70-2, are defective in protein import into chloroplasts during early developmental stages. Protein import was found to be affected at the step of precursor translocation across the envelope membranes. From solubilized envelope membranes, stromal cpHsc70 was specifically coimmunoprecipitated with importing precursors and stoichiometric amounts of Tic110 and Hsp93. Moreover, in contrast with receptors at the outer envelope membrane, cpHsp70 is important for the import of both photosynthetic and nonphotosynthetic proteins. These data indicate that cpHsc70 is part of the chloroplast translocon for general import and is important for driving translocation into the stroma. We further analyzed the relationship of cpHsc70 with the other suggested motor system, Hsp93/Tic40. Chloroplasts from the cphsc70-1 hsp93-V double mutant had a more severe import defect than did the single mutants, suggesting that the two proteins function in parallel. The cphsc70-1 tic40 double knockout was lethal, further indicating that cpHsc70-1 and Tic40 have an overlapping essential function. In conclusion, our data indicate that chloroplasts have two chaperone systems facilitating protein translocation into the stroma: the cpHsc70 system and the Hsp93/Tic40 system.

Rhizosphere bacteria containing 1-aminocyclopropane-1-carboxylate deaminase increase yield of plants grown in drying soil via both local and systemic hormone signalling

Decreased soil water availability can stimulate production of the plant hormone ethylene and inhibit plant growth. Strategies aimed at decreasing stress ethylene evolution might attenuate its negative effects. An environmentally benign (nonchemical) method of modifying crop ethylene relations - soil inoculation with a natural root-associated bacterium Variovorax paradoxus 5C-2 (containing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase that degrades the ethylene precursor ACC), was assessed with pea (Pisum sativum) plants grown in drying soil. Inoculation with V. paradoxus 5C-2, but not with a transposome mutant with massively decreased ACC deaminase activity, improved growth, yield and water-use efficiency of droughted peas. Systemic effects of V. paradoxus 5C-2 included an amplified soil drying-induced increase of xylem abscisic acid (ABA) concentration, but an attenuated soil drying-induced increase of xylem ACC concentration. A local bacterial effect was increased nodulation by symbiotic nitrogen-fixing bacteria, which prevented a drought-induced decrease in nodulation and seed nitrogen content. Successfully deploying a single bacterial gene in the rhizosphere increased yield and nutritive value of plants grown in drying soil, via both local and systemic hormone signalling. Such bacteria may provide an easily realized, economic means of sustaining crop yields and using irrigation water more efficiently in dryland agriculture.

Symbiotic N2 fixation activity in relation to C economy of Pisum sativum L. as a function of plant phenology

The relationships between symbiotic nitrogen fixation (SNF) activity and C fluxes were investigated in pea plants (Pisum sativum L. cv. Baccara) using simultaneous 13C and 15N labelling. Analysis of the dynamics of labelled CO2 efflux from the nodulated roots allowed the different components associated with SNF activity to be calculated, together with root and nodule synthetic and maintenance processes. The carbon costs for the synthesis of roots and nodules were similar and decreased with time. Carbon lost by turnover, associated with maintenance processes, decreased with time for nodules while it increased in the roots. Nodule turnover remained higher than root turnover until flowering. The effect of the N source on SNF was investigated using plants supplied with nitrate or plants only fixing N2. SNF per unit nodule biomass (nodule specific activity) was linearly related to the amount of carbon allocated to the nodulated roots regardless of the N source, with regression slopes decreasing across the growth cycle. These regression slopes permitted potential values of SNF specific activity to be defined. SNF activity decreased as the plants aged, presumably because of the combined effects of both increasing C costs of SNF (from 4.0 to 6.7 g C g-1 N) and the limitation of C supply to the nodules. SNF activity competed for C against synthesis and maintenance processes within the nodulated roots. Synthesis was the main limiting factor of SNF, but its importance decreased as the plant aged. At seed-filling, SNF was probably more limited by nodule age than by C supply to the nodulated roots.

Aluminum toxicity is associated with mitochondrial dysfunction and the production of reactive oxygen species in plant cells

Potential mechanisms of Al toxicity measured as Al-induced inhibition of growth in cultured tobacco cells (Nicotiana tabacum, nonchlorophyllic cell line SL) and pea (Pisum sativum) roots were investigated. Compared with the control treatment without Al, the accumulation of Al in tobacco cells caused instantaneously the repression of mitochondrial activities [monitored by the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide and the uptake of Rhodamine 123] and, after a lag of about 12 h, triggered reactive oxygen species (ROS) production, respiration inhibition, ATP depletion, and the loss of growth capability almost simultaneously. The presence of an antioxidant, butylated hydroxyanisol, during Al treatment of SL cells prevented not only ROS production but also ATP depletion and the loss of growth capability, suggesting that the Al-triggered ROS production seems to be a cause of ATP depletion and the loss of growth capability. Furthermore, these three late events were similarly repressed in an Al-tolerant cell line (ALT301) isolated from SL cells, suggesting that the acquisition of antioxidant functions mimicking butylated hydroxyanisol can be a mechanism of Al tolerance. In the pea root, Al also triggered ROS production, respiration inhibition, and ATP depletion, which were all correlated with inhibition of root elongation. Taken together, we conclude that Al affects mitochondrial functions, which leads to ROS production, probably the key critical event in Al inhibition of cell growth.

Aluminum toxicity is associated with mitochondrial dysfunction and the production of reactive oxygen species in plant cells

Potential mechanisms of Al toxicity measured as Al-induced inhibition of growth in cultured tobacco cells (Nicotiana tabacum, nonchlorophyllic cell line SL) and pea (Pisum sativum) roots were investigated. Compared with the control treatment without Al, the accumulation of Al in tobacco cells caused instantaneously the repression of mitochondrial activities [monitored by the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide and the uptake of Rhodamine 123] and, after a lag of about 12 h, triggered reactive oxygen species (ROS) production, respiration inhibition, ATP depletion, and the loss of growth capability almost simultaneously. The presence of an antioxidant, butylated hydroxyanisol, during Al treatment of SL cells prevented not only ROS production but also ATP depletion and the loss of growth capability, suggesting that the Al-triggered ROS production seems to be a cause of ATP depletion and the loss of growth capability. Furthermore, these three late events were similarly repressed in an Al-tolerant cell line (ALT301) isolated from SL cells, suggesting that the acquisition of antioxidant functions mimicking butylated hydroxyanisol can be a mechanism of Al tolerance. In the pea root, Al also triggered ROS production, respiration inhibition, and ATP depletion, which were all correlated with inhibition of root elongation. Taken together, we conclude that Al affects mitochondrial functions, which leads to ROS production, probably the key critical event in Al inhibition of cell growth.

Analysis of the raffinose family oligosaccharide pathway in pea seeds with contrasting carbohydrate composition

Raffinose family oligosaccharides (RFOs) are synthesized by a set of galactosyltransferases, which sequentially add galactose units from galactinol to sucrose. The accumulation of RFOs was studied in maturing seeds of two pea (Pisum sativum) lines with contrasting RFO composition. Seeds of the line SD1 accumulated stachyose as the predominant RFO, whereas verbascose, the next higher homolog of stachyose, was almost absent. In seeds of the line RRRbRb, a high level of verbascose was accumulated alongside with stachyose. The increase in verbascose in developing RRRbRb seeds was associated with galactinol-dependent verbascose synthase activity. In addition, a galactinol-independent enzyme activity was detected, which catalyzed transfer of a galactose residue from one stachyose molecule to another. The two enzyme activities synthesizing verbascose showed an optimum at pH 7.0. Both activities were almost undetectable in SD1. Maximum activity of stachyose synthase was about 4-fold higher in RRRbRb compared with SD1, whereas the activities of galactinol synthase and raffinose synthase were only about 1.5-fold higher in RRRbRb. The levels of galactinol synthase and stachyose synthase activity were reflected by steady-state levels of corresponding mRNAs. We suggest that the accumulation of verbascose in RRRbRb was controlled by a coordinated up-regulation of the last steps of verbascose biosynthesis.

Characterization of plant growth promoting rhizobacteria isolated from polluted soils and containing 1-aminocyclopropane-1-carboxylate deaminase

Fifteen bacterial strains containing 1-aminocyclopropane-1-carboxylate (ACC) deaminase were isolated from the rhizoplane of pea (Pisum sativum L.) and Indian mustard (Brassica juncea L.) grown in different soils and a long-standing sewage sludge contaminated with heavy metals. The isolated strains were characterized and assigned to various genera and species, such as Pseudomonas brassicacearum, Pseudomonas marginalis, Pseudomonas oryzihabitans, Pseudomonas putida, Pseudomonas sp., Alcaligenes xylosoxidans, Alcaligenes sp., Variovorax paradoxus, Bacillus pumilus, and Rhodococcus sp. by determination of 16S rRNA gene sequences. The root elongation of Indian mustard and rape (Brassica napus var. oleifera L.) germinating seedlings was stimulated by inoculation with 8 and 13 isolated strains, respectively. The bacteria were tolerant to cadmium toxicity and stimulated root elongation of rape seedlings in the presence of 300 microM CdCl2 in the nutrient solution. The effect of ACC-utilising bacteria on root elongation correlated with the impact of aminoethoxyvinylglycine and silver ions, chemical inhibitors of ethylene biosynthesis. A significant improvement in the growth of rape caused by inoculation with certain selected strains was also observed in pot experiments, when the plants were cultivated in cadmium-supplemented soil. The biomass of pea cv. Sparkle and its ethylene sensitive mutant E2 (sym5), in particular, was increased through inoculation with certain strains of ACC-utilising bacteria in pot experiments in quartz sand culture. The beneficial effect of the bacteria on plant growth varied significantly depending on individual bacterial strains, plant genotype, and growth conditions. The results suggest that plant growth promoting rhizobacteria containing ACC deaminase are present in various soils and offer promise as a bacterial inoculum for improvement of plant growth, particularly under unfavourable environmental conditions.

Transient expression of a pea MAP kinase gene induced by gibberellic acid and 6-benzyladenine in unpollinated pea ovaries

PsMAPK3, a new MAP kinase cDNA, was cloned from ovaries of Pisum sativum L. Expression of PsMAPK3 is at low basal levels in unpollinated ovaries but it is rapidly induced by gibberellic acid (peak at 30 min) and 6-benzyladenine (peak at 45 min). Both treatments promoted the development of a parthenocarpic fruit. In situ hybridization localized PsMAPK3 mRNA in ovules. The transcript was additionally detected in the mesocarp when it is expanding in response to the treatments. These observations suggest that gibberellins and cytokinins regulate PsMAPK3 mRNA levels in pea ovary shortly after fruit set is induced.

Effect of biocontrol strains of Trichoderma on plant growth, Pythium ultimum polulations, soil microbial communities and soil enzyme activities

Five strains of Trichoderma with known biocontrol activities were assessed for their effect upon pea growth and their antagonistic activity against large Pythium ultimum inocula. The effect of Trichoderma inocula upon the indigenous soil microflora and soil enzyme activities in the presence and absence of Pythium is assessed. In the absence of Pythium, Trichoderma strain N47 significantly increased the wet shoot weight by 15% but did not significantly affect the dry weight, whilst strains T4 and N47 significantly increased the root weights by 22% and 80%) respectively. Strains TH1 and N47 resulted in significantly greater root lengths. Pythium inoculation significantly reduced the root length and the number of lateral roots and nodules, and significantly increased the root and rhizosphere soil fungal populations. Pythium inoculation significantly reduced the plant wet and dry shoot weights and significantly increased the wet and the dry shoot/root ratio. All the Trichoderma strains reduced the number of lesions caused by Pythium and increased the number of lateral roots. The effect of the Pythium on emergence and shoot growth was significantly reduced by all the Trichoderma strains except strain To10. Inoculation with Trichoderma strains TH1 and T4 resulted in significantly greater wet root weights (62% and 57%, respectively) in the presence of Pythium compared to the Pythium control. Strain N47 significantly increased the shoot/root ratio compared to the Pythium control. Inoculation with Trichoderma strains T4, T12 and N47 significantly reduced Pythium populations. Pythium increased the activity of C, N and P cycle enzymes, whilst four Trichoderma strains reduced this effect, indicating reduced plant damage and C leakage. Overall, strains T4 and N47 had the greatest beneficial characteristics, as both these strains improved plant growth in the absence of Pythium and reduced plant damage in the presence of Pythium. The dual properties of these strains improve the commercial application, giving them an advantage over single action inocula, especially in the absence of plant pathogens.

Effect of pectin methylesterase gene expression on pea root development

Expression of an inducible gene with sequences common to genes encoding pectin methylesterase (PME) was found to be tightly correlated, both spatially and temporally, with border cell separation in pea root caps. Partial inhibition of the gene's expression by antisense mRNA in transgenic pea hairy roots prevented the normal separation of root border cells from the root tip into the external environment. This phenotype was correlated with an increase in extracellular pH, reduced root elongation, and altered cellular morphology. The translation product of the gene exhibited PME activity in vitro. These results are consistent with the long-standing hypothesis that the demethylation of pectin by PME plays a key role in cell wall metabolism.