Intrinsic morphology and spatial distribution of non-structural carbohydrates contribute to drought resistance of two mulberry cultivars

• Drought is one of the most adverse environmental stresses limiting plant growth and productivity. However, the underlying mechanisms regarding metabolism of non-structural carbohydrates (NSC) in source and sink organs are still not fully elucidated in woody trees.
• Saplings of mulberry cv Zhongshen1 and Wubu were subjected to a 15-day progressive drought stress. NSC levels and gene expression involved in NSC metabolism were investigated in roots and leaves. Growth performance and photosynthesis, leaf stomatal morphology, and other physiological parameters were also analysed.
• Under well-watered conditions, Wubu had a higher R/S, with higher NSC in leaves than in roots; Zhongshen1 had a lower R/S with higher NSC in roots than leaves. Under drought stress, Zhongshen1 showed decreased productivity and increased proline, abscisic acid, ROS content and activity of antioxidant enzymes, while Wubu sustained comparable productivity and photosynthesis. Interestingly, drought resulted in decreased starch and slightly increased soluble sugars in leaves of Wubu, accompanied by notable downregulation of starch-synthesizing genes and upregulation of starch-degrading genes. Similar patterns in NSC levels and relevant gene expression were also observed in roots of Zhongshen1. Concurrently, soluble sugars decreased and starch was unchanged in roots of Wubu and leaves of Zhongshen1. However, gene expression of starch metabolism in roots of Wubu was unaltered, but in leaves of Zhongshen1 starch metabolism was more activated.
• These findings revealed that intrinsic R/S and spatial distribution of NSC in roots and leaves concomitantly contribute to drought resistance in mulberry.

     

The accumulation of trehalose in nodules of several cultivars of common bean (Phaseolus vulgaris) and its correlation with resistance to drought stress

Nine cultivars of common bean were grown in the presence of a natural microflora without exogenous rhizobial inoculation. Nodules were harvested 30 days post planting (early flowering stage) and the presence of trehalose determined. Amounts present varied according to cultivar and were between 0.20 and 1.63 mg g⁻¹ nodule dry weight. Rhizobial strains were isolated from the nodules of three selected cultivars (Canario 101, Flor de Mayo Bajio and Flor de Mayo 38). Trehalose levels in nodules produced after either mixed strain reinfection, or after axenic homologous reinfection or after axenic cross‐reinfection could be manipulated by applying drought stress. Mixed reinfection nodules from stressed plants accumulated between two and six times the trehalose concentration found in non‐stressed control plants. After axenic cross‐reinfection up to 48‐fold increases in nodule trehalose content were recorded during drought stress. Those cultivars exhibiting high nodule trehalose levels and/or a high degree of trehalose stimulation in response to drought stress also exhibited a high leaf relative water content and were also the most drought resistant. During drought stress nodule trehalase levels rose only slightly.     

The role of RAPD markers in breeding for disease resistance in common bean

Diseases are regarded as the leading constraint to increased common bean (Phaseolus vulgaris L.) production worldwide. The range in variability and complexity among bean pathogens can be controlled with different single gene and quantitative resistance sources. Combining these resistance sources into commercial cultivars is a major challenge for bean breeders. To assist breeders, a major effort to identify RAPD markers tightly linked to different genes was undertaken. To date, 23 RAPD and five SCAR markers linked to 15 different resistance genes have been identified, in addition to QTL conditioning resistance to seven major pathogens of common bean. We review the feasibility of using marker-assisted selection (MAS) to incorporate disease resistance into common bean. Indirect selection of single resistance genes in the absence of the pathogen and the opportunity afforded breeders to pyramid these genes to improve their longevity and retain valuable hypostatic genes is discussed. The role of markers linked to the QTL controlling complex resistance and the potential to combine resistance sources using marker based selection is reviewed. Improving levels of selection efficiency using flanking markers, repulsion-phase linkages, co-dominant marker pairs, recombination-facilitated MAS and SCAR markers is demonstrated. Marker-assisted selection for disease resistance in common bean provides opportunities to breeders that were not feasible with traditional breeding methods.     

Differential accumulation of mRNAs in drought-tolerant and susceptible common bean cultivars in response to water deficit

The physiological response to drought was measured in two common bean varieties with contrastive susceptibility to drought stress. A subtractive cDNA library was constructed from the two cultivars, Phaseolus vulgaris'Pinto Villa' (tolerant) and 'Carioca' (susceptible). 18 cDNAs displayed protein-coding genes associated with drought, cold and oxidative stress, signal transduction, plant defense, chloroplast function and unknown function. A cDNA coding for an aquaporin (AQP) was selected for further analyses. The open reading frames (ORFs) of AQPs from 'Pinto Villa' and 'Carioca' were compared and despite their similarity, accumulated differentially in the plant organs, as demonstrated by Northern blot and in situ hybridization. A phylogenetic analysis of the deduced amino acid sequence with other AQPs suggested a tonoplast-located protein. Under drought conditions, the levels of AQP mRNA from the susceptible cultivar decreased to undetectable levels; by contrast, 'Pinto Villa' mRNA was present and restricted the phloem tissue. This would allow 'Pinto Villa' to maintain vascular tissue functions under drought stress.     

Gas exchange parameters, water relations and carbohydrate partitioning in leaves of field-grown Prunus domestica following fruit removal

The effect of fruit removal on gas exchange, water relations, chlorophyll and non-structural carbohydrate content of leaves from mature, field-grown plum trees ( Prunus domestica L. cv. Stanley) was determined over 2 consecutive growing seasons. Removal of fruits during stage II of fruit development decreased CO₂ assimilation rate within 24 h from 12.6 to 8.5 μmol m^-2 s^-1 in 1986, and from 12.1 to 10.2 μmol m^-2 s^-1 in 1987. Depression of net photosynthesis persisted for at least 5 days and was greatest in the early afternoon. Recovery of the CO₂ assimilation rate to pretreatment levels coincided in defruited trees with vegetative growth that was more than 5-fold that of fruiting trees in the first 6 weeks after fruit removal in 1986. Estimated photorespiration was similar in both fruiting and defruited trees. The stomatal contribution to the decrease of CO₂ assimilation rate, calculated from assimilation/intercellular CO₂ curves, ranged from 31 to 46%. Defruiting did not affect leaf water potential, but decreased leaf osmotic potential. Leaf levels of chlorophyll, fructose, glucose, sorbitol and sucrose were not affected by defruiting, whereas starch content increased up to 51% in leaves of defruited trees within 24 h after fruit removal. However, because of the small starch pool present in plum leaves (<1.9% dry weight) it is unlikely that starch accumulation was responsible for the observed decline in CO₂ assimilation rate after fruit removal. The decrease of CO₂ assimilation rate is discussed in relation to the hypothesis of assimilate demand regulating photosynthesis through a feedback mechanism.     

Effects of gibberellic acid on patterns of carbohydrate distribution and acid invertase activity in Phaseolus vulgaris

The application of gibberellic acid (GA₃,10 μ M ) as a root drench to 16-day-old plants of Phaseolus vulgaris L. cv. Masterpiece stimulated growth of the stem internodes and reduced root growth. GA₃ treatment did not affect the export of ¹⁴C from a primary leaf to which [¹⁴C]-sucrose was applied, but greatly increased upward translocation to the elongation region of the stem at the expense of transport to the hypocotyl and root system. The observed changes in the patterns of growth and [¹⁴C]-labelled assimilate distribution were correlated with an increase in the specific activity of acid invertase in the elongating stem internodes and a decrease in invertase activity in the hypocotyl and root. Sucrose concentration in the elongating internodes fell substantially after treatment with GA₃ while the concentration of hexose sugars increased. We suggest that by stimulating acid invertase synthesis in the elongating internodes, GA₃ acts to establish a more favourable sucrose gradient between these sinks and source leaves. Under source-limiting conditions this, in turn, will lead to a reduced rate of assimilate translocation to competing sinks in the root system.     

Multiple paternity in common bean (Phaseolus vulgaris L., Fabaceae)

We report on two field experiments that were conducted in 1991 and 1992 at the South Coast Extension and Research Center, Irvine, CA, to study the incidence of multiple paternity in the common bean (Phaseolus vulgaris L.). Hypocotyl color and shikimate dehydrogenase (Skdh) isozymes were used as genetic markers. The white-seeded cultivar ‘Ferry Morse 53’ (FM 53) was used as the female parent. This cultivar is homozygous recessive (pp) for hypocotyl color. The pollen source parents were three homozygous dominant (PP) purple-hypocotyled, black-seeded cultivars. Three cultivars, ‘ICA Pijao,’ G4459, and the maternal parent FM 53, are of Mesoamerican origin and homozygous for the fast (F) allele at the Skdh locus. The other cultivar, Black Valentine, is of Andean origin and is homozygous for the slow (S) allele at the Skdh locus. Overall, 6 125 pods were obtained from 57 and 111 plants harvested individually in 1991 and 1992, respectively. All progeny, 28938 seeds, were scored for hypocotyl color at the seedling stage. The purple-hypocotyled seedlings were genotyped for the Skdh locus to identify their pollen parents. Multiple paternity was identified in all the pods with hybrid seeds (i.e., those of intercultivar crosses) at 5.8% and 8.1% in 1991 and 1992, respectively. All multiply sired pods produced both nonhybrid and hybrid seeds. As many as three successful fathers per pod were identified, but the number of markers limited measuring higher levels of multiple paternity. Most multiply sired pods (≈70%) were filled by nonhybrid seeds plus a single hybrid seed. Ovule position effect within multiply sired pods was inferred from the nonrandom distribution of hybrid seeds within a pod. On average, hybrid seeds occurred more frequently in ovules in position 7 (most basal) and in position 1 (most stylar) than in ovules in the middle positions of the pod.     

Modelling partitioning between structure and storage in sugar beet: Effects of drought and soil nitrogen

In the UK sugar beet is grown on contrasting soils that vary both in their nutritional status and water-holding capacities. Water shortage and low nitrogen reduce canopy growth and dry matter production, which is compensated in part by an increase in the fraction of assimilates partitioned to storage. Conversely, high nitrogen and ample water encourage growth of the canopy, increase assimilation of carbon dioxide but reduce the proportion of assimilates stored as sugar. This paper sets out to examine simple relationships between sugar yield, total dry matter and soil nitrogen in rain-fed and irrigated sugar beet crops (Beta vulgaris L.) from 46 field experiments spanning 12 years and a range of soil types, in order to improve prediction of sugar yields. Two partitioning functions were fitted to the data. The first represents a useful alternative formulation of the allometric growth function that overcomes some of the difficulties in the interpretation of the parameters. This model adequately described the seasonal progress of sugar yield (Y) in relation to total dry matter (W), but it was difficult to postulate biological mechanisms as to how the parameters should vary in relation to varying soil nitrogen or to drought. The second partitioning function, given by Y = W − (1/k) log(1 + kW), also described the data well, but had the more useful parameter, k, the decay rate of the fraction of assimilates partitioned to structural matter. This was shown to be greater in crops which had experienced significant drought and was inversely proportional to the amount of nitrogen taken up by the crops. Relationships between k and amounts of nitrogen fertilizer applied and/or amounts of residual nitrogen in the soil at sowing, however, were more variable. These could be improved by additionally taking account of soil type and rainfall following nitrogen fertilizer application in late spring. The models are a useful extension to yield forecasting models because they provide a simple means of estimating sugar yield from total dry matter in relation to factors that affect partitioning of assimilates such as drought and soil nitrogen availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

Physiological and agronomical responses of common bean subjected to tryptophol

With the increasing global demand for food, fuel and fibre, the use of plant growth regulators in agriculture has become an agricultural practice aimed to improve physiological and productive responses. Our work aimed to evaluate the effect of tryptophol (Tol), a precursor of auxin, on common bean (Phaseolus vulgaris L.). The experiment was conducted in pots under greenhouse conditions, where we evaluated the Tol effect on bean crop under two different application forms: T_Soil – soil application of Tol (4.10^?4 mg L^?1) and T_Leaf – leaf tryptophol application (4.10^?4 mg L^?1), plus a reference treatment (0 mg L^?1 of Tol). We analysed the variables: shoot fresh and dry matter; root dry matter, area and volume; leaf macro and micronutrients; CO₂ net assimilation rate (A); stomatal conductance (g_S); internal CO₂ concentration (C_I); foliar transpiration (E); photosynthetic pigment content and some crop production attributes. The application of Tol through the foliar pathway proved to be more advantageous because it improved the shoot fresh and dry matter, increased the root volume and area, favoured less foliar transpiration and improved the length of pods, while the application of Tol in soil induced higher nitrogen accumulation in leaves. Our observations allow the characterization of Tol as a bioactive metabolite, suggesting an important potential for use in agricultural systems.     

Bruchid resistance of common bean lines having an altered seed protein composition

 Arcelin seed proteins of common bean (Phaseolus vulgaris L.) are toxic to one of the most damaging pests of bean seeds, Zabrotes subfasciatus (Boheman), but they appear to have little effect on another important bean pest, Acanthoscelides obtectus (Say), when introduced into standard cultivars by backcrossing. With the goal of increasing arcelin concentration to improve resistance, we modified seed-protein composition by introducing a null allele for the major seed protein, phaseolin, into lines (SMARC1, 2 and 4) or three phytohemagglutinin types (SMPHA lines). These lines were tested for resistance to both insects by measuring percentage insect emergence (%E) and days-to-adult emergence (DAE). For SMARC lines, arcelin type was the most important factor in resistance levels, with SMARC1 lines being most resistant, SMARC2 lines intermediate, and SMARC4 lines the least resistant to both bruchids. Additionally, the absence of phaseolin was a significant factor in the resistance of SMARC lines to A. obtectus. SMARC1 lines without phaseolin had half the percentage insect emergence of lines with phaseolin. SMARC1 lines with an altered seed composition had the highest levels of resistance to both bruchids of any large-seeded line reported to-date. Received: 2 April 1997 / Accepted: 20 May 1997     

Influence of salinity and abscisic acid on the O2 uptake by N2-fixing nodules of common bean

The effects of NaCl and ABA on the respiration of N₂-fixing nodules were analysed in common bean (Phaseolus vulgaris) inoculated with Rhizobium tropici the reference strain CIAT899. Shoot and nodule growth was more inhibited by NaCl than root growth. The O₂ uptake by nodulated roots at 21 kPa O₂ was significantly inhibited by salinity. Raising pO₂ stimulated nodule respiration more under NaCl treatment than for the control, although it did not compensate totally for the inhibitory effect of NaCl. Short NaCl application was less destructive than long term application. Also, the external application of ABA inhibited nodule respiration, and this inhibition was partly compensated by raising pO₂.     

Polymorphism of a complex resistance gene candidate family in wild populations of common bean (Phaseolus vulgaris) in Argentina: comparison with phenotypic resistance polymorphism

Fifteen populations of wild bean (Phaseolus vulgaris), located in three provinces in Argentina, were analysed for their polymorphism for a complex resistance gene candidate (RGC) family clustered on the genome and for resistance phenotypes to strains of Colletotrichum lindemuthianum. Results indicate that RGC polymorphism is high. Population structure obtained for markers related to resistance was compared to population structure obtained for RAPD markers in order to infer the evolutionary forces driving polymorphism for resistance in wild populations at both molecular and phenotypic levels. Hierarchical analysis of differentiation showed that, within provinces, populations were differentiated for RAPD as well as for molecular and phenotypic markers of resistance. In contrast, provinces were differentiated only for RAPD and RGC markers and not for resistance phenotypes. The discrepancies found between diversity structures for molecular markers (RAPD and RGCs) and for resistance phenotypes suggest an effect of selection and indicate that diversity for resistance may not be driven by the same selective forces at the molecular and phenotypic levels. We further discuss whether specific selective forces are exerted on RGC markers and underline the importance of spatial scale of analysis for demonstrating an impact of selection.     

Molecular characterisation of wild populations and landraces of common bean from northwestern Argentina

Abstract

Information on the variability of wild bean populations and landraces is essential to set conservation strategies and design breeding programmes aimed at enlarging the genetic base of commercial beans. Nineteen Argentinean common bean landraces and wild populations were characterised and their diversity was analysed by means of inter‐simple sequence repeat (ISSR) markers and seed proteins. Populations were successfully identified as belonging to the Andean gene pool of origin by phaseolin electrophoresis, whereas ISSR markers revealed high levels of inter‐ and intra‐population variability. Four of 10 primers produced polymorphic and reproducible DNA profiles, which were used to generate UPGMA (unweighted pair group method with arithmetical averages) and NJ (neighbour‐joining) trees. ISSR markers revealed a high level of variability both within wild bean populations and landraces. Genetic variability of wild samples was associated with their geographic distribution. By contrast, landraces were clustered, at least to some degree, based on their seed colour and shape, showing no clear discrimination among sites. The results presented here suggest that, to a certain extent, hybridisation between wild beans and landraces occurs in the wild, a hypothesis that needs to be tested through further analyses.     

Cytokinin-mediated source/sink modifications improve drought tolerance and increase grain yield in rice under water-stress

Drought is the major environmental factor limiting crop productivity worldwide. We hypothesized that it is possible to enhance drought tolerance by delaying stress-induced senescence through the stress-induced synthesis of cytokinins in crop-plants. We generated transgenic rice (Oryza sativa) plants expressing an isopentenyltransferase (IPT) gene driven by P(SARK) , a stress- and maturation-induced promoter. Plants were tested for drought tolerance at two yield-sensitive developmental stages: pre- and post-anthesis. Under both treatments, the transgenic rice plants exhibited delayed response to stress with significantly higher grain yield (GY) when compared to wild-type plants. Gene expression analysis revealed a significant shift in expression of hormone-associated genes in the transgenic plants. During water-stress (WS), P(SARK)::IPT plants displayed increased expression of brassinosteroid-related genes and repression of jasmonate-related genes. Changes in hormone homeostasis were associated with resource(s) mobilization during stress. The transgenic plants displayed differential expression of genes encoding enzymes associated with hormone synthesis and hormone-regulated pathways. These changes and associated hormonal crosstalk resulted in the modification of source/sink relationships and a stronger sink capacity of the P(SARK)::IPT plants during WS. As a result, the transgenic plants had higher GY with improved quality (nutrients and starch content).     

Molecular diversity in Ligurian local races of common bean (Phaseolus vulgaris L.)

Abstract

Eight varieties of Ligurian common beans (Phaseolus vulgaris L.) were analysed using molecular approaches. Results were compared with two commercial cultivars (‘Cannellino’ and ‘Borlotto’). Data suggest that all Ligurian bean varieties have a low genetic variability and are very close to the commercial varieties. In particular, the three ‘Bianco’ varieties showed a molecular affinity, probably due to their common genomic origin.     

Row orientation effects on FR/R light ratio, growth and development of field-grown bush bean

Bush bean ( Phaseolus vulgaris L. cv. Tenderette) plants were grown in north-south (N-S) and east-west (E-W) rows under field conditions to test effects of row orientation on reflected far-red (FR) light patterns and on shoot size and edible bean productivity. Soil water and nutrients were adequate. Plants in N-S rows received slightly higher ratios of FR relative to red (R) light, because of heliotropic movement of the leaves. Plants in N-S rows partitioned more dry matter to shoots and edible green beans than those in E-W rows. We conclude that row orientation of broadleaf plants can affect the FR/R light ratio and the phytochrome-mediated regulation of plant development under field conditions.     

Determination of traits associated with leafhopper (Empoasca fabae and Empoasca kraemeri) resistance and dissection of leafhopper damage symptoms in the common bean (Phaseolus vulgaris)

The typical presentation of potato leafhopper injury in beans includes necrosis at the leaf margins (leaf burn or hopperburn), and downward curling or “cupping” of the leaves. To evaluate potato leafhopper damage a visual score that combines the overall severity of leaf burn, leaf curling and stunting symptoms is usually used. Nonetheless, it may be useful to evaluate these symptoms separately since they may be the result of separate mechanisms of damage, controlled by separate genes. A population of 108 recombinant inbred lines (RILs), derived from a cross between a leafhopper‐susceptible Ontario cultivar (Berna) and a resistant line (EMP 419) were scored for injury after natural infestation with Empoasca fabae in Canada and Empoasca kraemeri in Colombia. Leaf burn and leaf curl were significantly rank‐correlated (0.37–0.74, P<0.001) in all environments. However, several RILs consistently exhibited high scores for leaf curl but low values for leaf burn, which suggests that genetic dissection of these characters may be possible. Indeterminate growth habit was associated with slightly lower damage scores in Colombia and Ontario, Canada (P<0.05) while white‐seeded lines had lower damage scores in Colombia (P<0.05). The resistant parental line had significantly lower nymph counts than did the susceptible parent. A positive relationship between damage scores and nymph counts was also observed in the F₃ families and the F_5:6 RILs.     

Thigmomorphogenesis: Ethylene evolution and its role in the changes observed in mechanically perturbed bean plants

Mechanical perturbations, in the form of either rubbing or wounding, cause ethylene evolution from bean internodes ( Phaseolus vulgaris L. cv. Cherokee Wax). This evolution begins 45 to 60 min after perturbation or wounding and peaks about 2 h later. Maximal thigmomorphogenesis occurs if internodes are perturbed when they are 10 mm or less in length. Maximal ethylene evolution, however, occurs in longer internodes. When one internode is perturbed, ethylene evolution is not observed from other internodes even though they respond thigmomorphogenetically by exhibiting decreased elongation. Ethylene evolution is apparently a result of increased 1-amino-cyclopropane-1-carboxylic acid (ACC) production after perturbation. Inhibitors of ACC and ethylene synthesis block increased radial growth but not reduced elongation. Ethylene may therefore be only one of several factors causing thigmomorphogenesis.