Sunflower Roots Research Articles

Sunflower (Helianthus annuus L.) response to broomrape (Orobanche cernua Loefl.) parasitism: induced synthesis and excretion of 7-hydroxylated simple coumarins.

The interaction of the parasitic plant Orobanche cernua with resistant and susceptible cultivars of Helianthus annuus L. was investigated. Using different bioassays to evaluate the early stages of the parasite life cycle (germination, attachment, penetration, and establishment), differences were observed between O. cernua-resistant and O. cernua-susceptible sunflower varieties. Germination of O. cernua seeds in the presence of resistant sunflower roots was approximately half that of germination in the presence of susceptible roots, and germinated seeds displayed enhanced browning symptoms. Parasite radicles or host-tissue around the contact point turned brown after O. cernua attachment to sunflower roots, especially in the resistant varieties. These observations suggested the possible accumulation of toxic compounds as a defence strategy in the resistant sunflower varieties. Sunflower 7-hydroxylated simple coumarins may play a defensive role against O. cernua parasitism by preventing successful germination, penetration and/or connection to the host vascular system. This hypothesis is supported by the following data: (i) coumarins inhibited the in vitro germination of O. cernua seeds induced by the strigol analogue GR(24) and caused a browning reaction in germinated seeds and (ii) resistant sunflowers accumulated higher levels of coumarins in roots and excreted greater amounts than susceptible varieties in response to O. cernua infection.

Several Mechanisms are Involved in Resistance of Helianthus to Orobanche cumana Wallr.

Orobanche cumana(broomrape) poses a serious threat to sunflower cultivation in many countries since there is no effective method of controlling the pathogen. It is known that wild species of the genusHelianthus show resistance to different pathogens. Our goal was to identify resistant wild species and derived interspecific hybrids (wild Helianthus×sunflower) and to characterize the resistance mechanisms involved. After identification of key stages (germination, fixation, tubercule formation and development) in the resistance of Helianthus to O. cumana, several germplasms were tested. Development of a hydroponic co-culture system allowed rapid screening of new Helianthus germplasms and a more accurate study of the resistance phenomenon. Several resistance mechanisms, such as cell wall deposition, vessel occlusion and broomrape cellular disorganization were demonstrated in the resistant sunflower genotype LR1 by histological observation of the host-parasite interface. In contrast, no reaction was observed in the susceptible cultivated genotype 2603. H. debilis debilis(the wild parent of LR1) exhibited an encapsulation layer which prevented intrusion of the parasite into the cortex. Severe reduction in the root and apex biomass of sunflower was observed in the susceptible line (2603) indicating strong competition between broomrape and sunflower sinks. This large loss of sunflower biomass was compensated by broomrape biomass. In resistant genotypes LR1 and 92B6, the effect of broomrape infestation on sunflower biomass was much more limited.14CO2radiolabelling showed differences between resistant and susceptible sunflower genotypes in nutrient flux from the host to broomrape.

Rooting Front and Water Depletion Depths in Grain Sorghum and Sunflower

Rooting front advance and rooting depth are factors to consider in selecting a crop for effective use of stored soil water. Our objective was to compare rooting front development (advance rate and maximum depth) and water depletion depths of grain sorghum [Sorghum bicolor (L.) Moench] and sunflower (Helianthus annuus L.) in a field experiment. The study was near Manhattan, KS on Eudora silt loam soil (coarse‐silty, mixed, superactive, mesic Fluventic Hapludolls). Root development was quantified through multiple field samplings and the core‐break method. Water content was measured to 3.05 m by neutron attenuation. Rooting front depths of the two crops were similar from emergence to mid‐June [0 to 20 d after emergence (DAE)], but from late June, sunflower roots were deeper than sorghum roots. Maximum rooting depths were 1.85 m in sorghum and 2.49 m in sunflower (means of 2 yr). From 20 to 60 DAE, rooting front depth increased 25 and 41 mm d−1 in sorghum and sunflower, respectively. From 60 to 90 DAE, rooting front depth increased 8 and 6 mm d−1 in sorghum and sunflower, respectively. Net seasonal water depletion was greater in sunflower plots than in sorghum plots (by 103 mm in 1985 and 112 mm in 1986). Sunflower roots advanced downward from 0 to 60 DAE at a rate and depth 46 and 35% greater, respectively, than that of sorghum. The faster advance rate and deeper depth of rooting aid sunflower in drought avoidance and use of water from deeper soil depths.

Immunohistochemical localisation of ubiquitin and the proteasome in sunflower (Helianthus annuus cv. Giganteus).

This study provides an immunohistochemical demonstration of the involvement of the ubiquitin- and proteasome-dependent pathway during differentiation and organogenesis in plants. The localisation of ubiquitin and the proteasome was studied in meristems, leaves, stems and roots of sunflower (Helianthus annuus L. cv. Giganteus). By using a new technique that enhances very low antigen signals, we obtained information on the structural distribution of the ubiquitin- and proteasome-dependent pathway, and of the importance of this pathway during organogenesis and plant development. Ubiquitin and the proteasome showed overall similarities in their cellular localisation. The highest antigenic signal was observed in the root and shoot apical meristems, in leaf primordia and vascular tissue. The cambium showed less expression than the apical meristems. During adventitious root formation in cuttings, no sign of increased expression was observed within dedifferentiating tissue, but as organogenesis progressed, the antigenic signal of ubiquitin and the proteasome gradually increased in the developing roots. Comparison of immunochemical results and Western blots demonstrated that important changes in the cellular antigen signal could only be detected by immunochemistry.

Allelopathic activity of Mexican sunflower [Tithonia diversifolia (Hemsl.) A. Gray] in soil under natural field conditions and different moisture conditions

The allelopathic activity of Mexican sunflower [Tithonia diversifolia (Hemsl.) A. Gray] in soil under natural field conditions and the effect of water stress on the growth and allelopathic activity of this plant were investigated. Seed germination, shoot growth and root growth of tested plant species in soil collected from a field where Mexican sunflower had been grown for 5 years were less than those in soil from an area without the plant. Growth of young leaves, mature leaves, senescent leaves, stem and roots of Mexican sunflower was reduced with decrease in soil moisture level. The allelopathic activity of water extracts (per dry weight of starting material) from each part of the plants grown at low soil moisture levels was greater than that of the water extracts from the same part of the plants grown at high soil moisture levels. The allelopathic activity was found in the soil from the pots previously planted with Mexican sunflower to a similar extent (per plant) at different soil moisture levels. These results suggested that, in the field, under water stress conditions, the growth of Mexican sunflower was reduced but the plants contained a greater amount of allelopathic substance(s) per dry weight than in the absence of water stress. It was considered that allelopathic activity of Mexican sunflower in soil was maintained to a similar extent under various soil moisture conditions in natural fields.

Geographic and Host Range of Meloidogyne spp. in North Central Mexico.

A disease survey carried out in 1998, 1999, and 2000 in the states of Aguascalientes, San Luis Potosí, and Zacatecas revealed the dispersal of Meloidogyne spp in this region of Mexico. Pepper (Capsicum annuum L.) Mirasol type plants showing general chlorosis, root rot, and galls were observed in central Zacatecas and western San Luis Potosí. Dry bean (Phaseolus vulgaris L.) plants (Landrace Flor de Mayo) collected in western San Luis Potosí and Aguascalientes also showed root galls. Roots of squash (Cucurbita spp) and sunflower (Helianthus annuus L.) plants that showed galled roots were found under dryland conditions in northern Zacatecas. Nursery peach (Prunus persica L.) plantlets with no foliar symptoms but showing severe root galling were detected in Zacatecas. Perineal patterns of Meloidogyne females obtained from those galled roots were coincident with those of M. incognita according to pictoral keys (1). This is the first report of M. incognita affecting these hosts in that region of the country. Alfalfa (Medicago sativa) plants collected in Aguascalientes showed galls caused by Meloidogyne spp; this is the first report of this nematode affecting alfalfa in the state. Volunteer onion (Allium cepa L., 'Grano Blanco') plants growing in a squash field in eastern Zacatecas had galled roots; a few Meloidogyne spp. females were obtained from small galls. This is the first report of the root-knot nematode affecting onion plants in north central México. Onion is known to be a host for several species of this nematode (2). Stunted, chlorotic squash plants had roots severely galled by Meloidogyne spp, but pepper crops growing in the same field in previous years showed general chlorosis, reduced size, and poor yield did not have root galls.

Cell-type-specific expression of plant cytochrome c mRNA in developing flowers and roots.

We have used RNA in situ hybridization to analyze the expression of transcripts encoding cytochrome c in different tissues and organs of sunflower (Helianthus annuus). Although northern-blot hybridization experiments indicate that the relative abundance of transcripts does not vary greatly, we have detected important changes in localization during flower development. Enhanced expression is observed in floral meristems as soon as they are discernible from the central portion of the capitulum containing the inflorescence meristem. As flowers develop, labeling is observed in all developing floral organ primordia. Later in development, expression in petals is reduced, and only the central portion of the flower becomes labeled. During the process of stamen formation, hybridization signals were obtained mainly in anthers. Less developed flowers at this stage showed expression through the archesporial tissue. During meiosis, the label was observed mainly in tapetal cells. Specific expression patterns, similar to those obtained for sunflower, were observed when Arabidopsis flowers were analyzed with a homologous cytochrome c probe. Specific patterns of expression were also observed in young sunflower roots. In this case, enhanced expression was detected in developing endodermis and pericycle and in protoxylem initials. We conclude that cell-specific mechanisms operate to regulate the abundance of cytochrome c encoding transcripts in different plant tissues. The overlap between the expression patterns of the nuclear encoded cytochrome c gene and some mitochondrial genes suggests the existence of coordinated mechanisms of expression.

Effect of dichlorophenolindophenol, dichlorophenolindophenol-sulfonate, and cytochrome c on redox capacity and simultaneous net H+/K+ fluxes in aeroponically grown seedling roots of sunflower (Helianthus annuus L.): new evidence for a plasma membrane CN(-)-resistant redox chain.

Excised roots from axenically grown sunflower seedlings reduced or oxidized exogenously added 2,6-dichlorophenolindophenol (DCIP), DCIP-sulfonate (DCIP-S), and cytochrome c, and affected simultaneous H+/K+ net fluxes. Experiments were performed with nonpretreated "living" and CN(-)-pretreated "poisoned" roots (control and CN(-)-roots). CN(-)-roots showed no H+/K+ net flux activity but still affected the redox state of the compounds tested. The hydrophobic electron acceptor DCIP decreased the rate of H+ efflux in control roots with extension of the maximum rate and optimal pH ranges, then the total net H+ efflux ([symbol: see text]H+) equalled that of the roots without DCIP. The simultaneously measured K+ influx rate was first inhibited, then inverted into efflux, and finally influx recovered to low rates. This effect could not be due to uptake of the negatively charged DCIP, but due to the lower H+ efflux and the transmembrane electron efflux caused by DCIP, which would depolarize the membrane and open outward K+ channels. The different H+ efflux kinetics characteristics, together with the small but significant DCIP reduction by CN(-)-roots were taken as evidence that an alternative CN(-)-resistant redox chain in the plasma membrane was involved in DCIP reduction. The hydrophilic electron acceptor DCIP-S enhanced both H+ and K+ flux rates by control roots. DCIP-S was not reduced, but slightly oxidized by control roots, after a lag, while CN(-)-roots did not significantly oxidize or reduce DCIP-S. Perhaps the hydrophobic DCIP could have access to and drain electrons from an intermediate carrier deep inside the membrane, to which the hydrophilic DCIP-S could not penetrate. Also cytochrome c enhanced [symbol: see text]H+ and [symbol: see text]K+, consistent with the involvement of the CN(-)-resistant redox chain. Control roots did not reduce but oxidize cytochrome c after a 15 min lag, and CN(-)-roots doubled the rate of cytochrome c oxidation without any lag. NADH in the medium spontaneously reduced cytochrome c, but control or CN(-)-roots oxidized cytochrome c, despite of the presence of NADH. In this case CN(-)-roots were less efficient, while control roots doubled the rate of cytochrome c oxidation by CN(-)-roots, after a 10 min lag in which cytochrome c was reduced at the same rate as the medium plus NADH did. CN(-)-roots seemed to have a fully activated CN(-)-resistant branch. The described effects on K+ flux were consistent with the current hypothesis that redox compounds changed the electric membrane potential (de- or hyperpolarization), which induces the opening of voltage-gated in- or outward K+ channels.

Glucose‐induced activation of rubidium transport and water flux in sunflower root systems

Excised 20‐d‐old sunflower roots (Helianthus annuus L. cv. Sun‐Gro 393) were used to study the effect of different sugars on rubidium and water fluxes. The roots sensed and absorbed glucose from the external medium inducing the activation of rubidium accumulated in the root (Rb+ root), the flux of exuded rubidium (JRb) and, to a lesser degree, the exudation rate (Jv). These effects were also triggered by fructose, but not by 6‐deoxyglucose (6‐dG), a glucose analogue which is not a substrate for hexokinase (HXK). The effect of 2‐deoxyglucose (2‐dG), an analogue that is phosphorylated but not further metabolized, was complex, suggesting an inhibitory effect on solute transport to the xylem. The amounts of glucose required to activate rubidium and water fluxes were similar to those previously reported to regulate different processes in other plants (0.5–10 mM). When sorbitol was used instead of glucose, neither rubidium uptake (Rb+ root plus JRb) nor Jv was activated. It is proposed that glucose present in the root plays an important signalling role in the regulation of Rb+ (K+) and water transport in plant roots.

Glucose‐induced activation of rubidium transport and water flux in sunflower root systems

Excised 20-d-old sunflower roots (Helianthus annuus L. cv. Sun-Gro 393) were used to study the effect of different sugars on rubidium and water fluxes. The roots sensed and absorbed glucose from the external medium inducing the activation of rubidium accumulated in the root (Rb(+) root), the flux of exuded rubidium (J(Rb)) and, to a lesser degree, the exudation rate (J(v)). These effects were also triggered by fructose, but not by 6-deoxyglucose (6-dG), a glucose analogue which is not a substrate for hexokinase (HXK). The effect of 2-deoxyglucose (2-dG), an analogue that is phosphorylated but not further metabolized, was complex, suggesting an inhibitory effect on solute transport to the xylem. The amounts of glucose required to activate rubidium and water fluxes were similar to those previously reported to regulate different processes in other plants (0.5--10 mM). When sorbitol was used instead of glucose, neither rubidium uptake (Rb(+) root plus J(Rb)) nor J(v) was activated. It is proposed that glucose present in the root plays an important signalling role in the regulation of Rb(+) (K(+)) and water transport in plant roots.

Rhizosphere soil aggregation and plant growth promotion of sunflowers by an exopolysaccharide-producing Rhizobium sp. strain isolated from sunflower roots.

Root-adhering soil (RAS) forms the immediate environment where plants take up water and nutrients for their growth. We report the effect of an exopolysaccharide (EPS)-producing rhizobacterium (strain YAS34) on the physical properties of sunflower (Helianthus annuus L.) RAS, associated with plant growth promotion, under both water stress and normal water supply conditions. Strain YAS34 was isolated as a major EPS-producing bacterium from the rhizoplane of sunflowers grown in a French dystric cambisol. Strain YAS34 was assigned to the Rhizobium genus by 16S ribosomal DNA gene sequencing. Inoculation of sunflower seeds and soil with strain YAS34 caused a significant increase in RAS per root dry mass (dm) (up to 100%) and a significant increase in soil macropore volume (12 to 60 microm in diameter). The effect of inoculation on sunflower shoot dm (up to +50%) and root dm (up to +70%) was significant under both normal and water stress conditions. Inoculation with strain YAS34 modified soil structure around the root system, counteracting the negative effect of water deficit on growth. Using [(15)N]nitrate, we showed that inoculation made the use of fertilizer more effective by increasing nitrogen uptake by sunflower plantlets.

Water transport in sunflower root systems: effects of ABA, Ca2+ status and HgCl2

Peterson, 1998). When osmotic gradients are present, the water flow is negligible in the apoplast (Quintero et al., Excised 20-d-old sunflower roots (Helianthus annuus 1998; Steudle and Peterson, 1998) and transport occurs L. cv. Sun-Gro 380) with different Ca2+ status were mainly from cell to cell (Steudle and Frensch, 1989; used to study the effects of root Ca2+ status and Steudle, 1994). In recent years, it has been shown that abscisic acid (ABA) on the exudation rate (Jv), the membrane-integral proteins, or aquaporins, act as waterhydraulic conductivity of the root (Lp r ), the flux of specific channels (Chrispeels and Maurel, 1994; Maggio exuded Ca2+ (J Ca ), and the gradient of osmotic pres- and Joly, 1995). The presence of aquaporins in roots sure between the xylem and the external medium. Jv ( Yamamoto et al., 1991; Niemietz and Tyerman, 1997) and Lp r increased in direct proportion to the Ca2+ suggests that radial flow of water could be mainly transstatus of the root. Addition of ABA (4 mM) at the onset cellular (Chrispeels and Maurel, 1994; Maggio and Joly, of exudation in the external medium made Jv and Lp r 1995) and may be controlled by the opening or closing rise, and this effect also increased with the Ca2+ of these water channels (Chrispeels and Maurel, 1994; status. The effects of HgCl 2 and its interaction with Steudle and Henzler, 1995). ABA on water transport in the root were also studied. The existence of a regulatory mechanism which controls Addition of HgCl 2 (1 mM) 2 h after the onset of exuda- radial transport of water in the root has yet to be tion in the external medium quickly inhibited Jv, inde- determined (Quintero et al., 1998). It is known that roots pendently of the presence of ABA in the root medium. oVer the greatest resistance to water flow ( Weatherley, The results recorded here point to the involvement of 1982), and that hydraulic conductivity of the root (Lp r ) ABA and Ca2+ in the regulation of root water flow, may be aVected by diverse forms of abiotic stress, such as well as the existence of aquaporins in the cell as salinity (Munns and Passioura, 1984; Joly, 1989), membranes of sunflower roots. anaerobiosis (Zhang and Tyerman, 1991), drought (North and Nobel, 1991) or nutritional stress (Radin and

Water transport in sunflower root systems: effects of ABA, Ca2+ status and HgCl2

Water transport in sunflower root systems: effects of ABA, Ca2+ status and HgCl2

Isolation of Soluble Boron Complexes and their Determination Together with Free Boric Acid in Higher Plants

Summary Isolation of soluble boron complexes and their quantitative determination together with free boric acid in higher plants was carried out by gel permeation chromatography in the liquid compartment of plant leaves and roots. The described method,is suitable not only for the isolation of B complexes from plant cell sap, but also allows for their quantitative determination together with free boric acid, which is possible for samples with a B concentration of at least 157#x03BC;molL −1 (Table 2). This gel permeation chromatography method therefore complements methods that allow characterization, but not quantitative determination of B complexes in plant samples. Soluble B complexes have been isolated from sunflower ( Helianthus annuus L.) grown with a marginal to toxic B supply and curly kale ( Brassica oleracea L.). In sunflower root symplasm there was only a small concentration of B complexes that was not dependent on variations in B supply from the marginal to the toxic level. The resulting large variations in total symplasmic B were therefore only reflected in the concentrations of free boric acid. In contrast, the concentration of B complexes in sunflower leaf symplasm was about 50-fold larger than in the root symplasm and made up 31 % of the total symplasmic B. In curly kale leaves even 59 % of the total symplasmic B was present as B complexes. Further experiments are necessary to characterize the B complexes in sunflower and curly kale to investigate their physiological role in plant metabolism.

Characterization of closely related delta-TIP genes encoding aquaporins which are differentially expressed in sunflower roots upon water deprivation through exposure to air.

We isolated five sunflower (Helianthus annuus) cDNAs belonging to the TIP (tonoplast intrinsic protein) family. SunRb7 and Sun gammaTIP (partial sequence) are homologous to tobacco TobRb7 and Arabidopsis gamma-TIP, respectively. SunTIP7, 18 and 20 (SunTIPs) are closely related and homologous to Arabidopsis delta-TIP (SunTIP7 and 20 have already been presented in Sarda et al., Plant J. 12 (1997) 1103-1111). As was previously shown for SunTIP7 and 20, expression of SunTIP18 and SunRb7 in Xenopus oocytes caused an increase in osmotic water permeability demonstrating that they are aquaporins. In roots, in situ hybridization revealed that SunTIP7 and 18 mRNAs accumulate in phloem tissues. The expression of TIP-like genes was studied in roots during 24 h water deprivation through exposure to air. During the course of the treatment, each SunTIP gene displayed an individual response: SunTIP7 transcript abundance increased, SunTIP18 decreased whereas that of SunTIP20 was transitorily enhanced. By contrast, SunRb7 and Sun gammaTIP mRNA levels did not fluctuate. Due to the changes in their transcript levels, it is proposed that SUNTIP aquaporins encoded by delta-TIP-like genes play a role in the sunflower response to drought.

Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem

The radial movement of cis-abscisic acid (ABA) has been investigated in young excised roots of Zea mays L. and Helianthus annuus L. which were grown hydroponically. In addition to the symplastic path, ABA was largely translocated across the root apoplast by solvent drag with the water in the transpiration stream. On the apoplastic path ABA may even cross the endodermis. Depending on the ABA concentration of the medium (range: 5–500 nM) and in the root apoplast, the solvent-drag component of the flow of ABA counteracted the dilution of ABA in the xylem caused by transpirational water flow. Acidification of the rhizosphere and of the root apoplast increased the apoplastic transport component. In sunflower, the apoplastic flow of ABA was significantly weaker than in maize roots. This was also indicated by the larger apparent reflection coefficient (σABA) of sunflower roots for ABA (sunflower: σABA = 0.97 ± 0.02, n = 6 roots; maize: σABA = 0.68 ± 0.06, n = 6 roots; ±SD). For both species, σABA was smaller than unity. Root reflection coefficients were affected by factors such as pH, ABA concentration of the medium, and by the suction force applied to excised root systems. Due to the complex composite structure of the permeation barrier in the root, the reflection coefficient estimated from solvent drag is also complex. Since unstirred layers affected the absolute value of the reflection coefficient, σABA has been termed `apparent'. It is concluded that the pH and ABA concentration of the soil solution as well as the transpiration rate (suction force) modify the intensity of the root-to-shoot signal which is influenced by an apoplastic bypass flow of ABA. The latter may be substantially affected by the existence of Casparian bands in the exodermis, which were lacking in the roots studied in this paper.

The Potential ofFusarium oxysporumf.sp.orthocerasas a Biological Control Agent forOrobanche cumanain Sunflower

Abstract Fusarium oxysporum Schlecht. f.sp. orthoceras (Appel & Wollenw.) Bilay was evaluated for its potential to control the root-parasitic weed Orobanche cumana Wallr. The effectiveness of conidial inoculum of this soil-borne fungus was determined in pots and root chamber trials with the host–parasite association sunflower ( Helianthus annuus L.)/ O. cumana. Conidia applied postemergence infected emerged parasite shoots before they reached their flowering stage and produced seeds. The high level of virulence of the fungus was proven by efficient Orobanche control at low inoculum doses. The preplant application of 10 4 unformulated conidia at a density of 69 conidia/cm 2 , which were incorporated into 1 kg of soil, resulted in significant disease incidence on O. cumana shoots as well as a reduced rate of emergence. The strong suppression of broomrape development after soil incorporation of higher doses of conidia led to 89% increase in dry matter of sunflower, compared to the fungus-free control. Effective control by O. cumana was due to the susceptibility of early underground developmental stages to the pathogen. The mortality of tubercles increased significantly to 85% after inoculation with the fungus. The number of parasitic attachments on sunflower roots was reduced due to mortality of the seed's germ tube. In addition, suppression of seed germination was observed. Hence, each developmental stage of O. cumana represents a potential target for F. oxysporum f.sp. orthoceras attack. Orobanche cernua Loefl. grown on tobacco ( Nicotiana tabacum L . ) was also susceptible to the pathogen. In contrast, biotypes of O. aegyptiaca Pers. on tomato ( Lycopersicon esculentum Mill.) are nonhosts of the fungus.

Effects of salt stress on H +-ATPase activity of plasma membrane-enriched vesicles isolated from sunflower roots

It has previously been shown that mild and severe salt-stress increased both ATP- and PP i-dependent H + transport and induced a Na +/H + exchange activity in tonoplast vesicles from sunflower seedling roots. In this study, the vanadate-sensitive H +-ATPase activity was characterized in plasma membrane-enriched vesicles isolated by discontinuous sucrose gradient centrifugation from sunflower ( Helianthus annuus L.) roots exposed for 3 days to 0, 75 or 150 mM NaCl. Blots immunoassayed with an Arabidopsis thaliana plasma membrane ATPase polyclonal antibody revealed the existence of a band of about 100 kDa, which was highly enriched in the 32/43% sucrose interface. ATP hydrolyzing activity in this fraction was mostly inhibited by vanadate, and scarcely by azide, oligomycin, nitrate and molybdate, indicating that it was essentially enriched in plasma membrane vesicles. Properties of vanadate-sensitive ATPase activity, such as inhibitor sensitivity, pH optimum, substrate specificity, ion effects and kinetic data were consistent with those of other plasma membrane ATPases. Although the pH profile, ion dependence, apparent K m and the amount of antigenic protein were unaffected by salt-stress, the V max of the vanadate-sensitive ATPase activity (measured in the presence of detergent) was reduced as a function of salt treatments. Likewise, while mild and severe salt-stress reduced the basal ATP hydrolyzing activity (measured in the absence of detergent) about a 35 and 55%, respectively, the ATP-dependent H + transport activity sensitive to vanadate remained unchanged. As a consequence, the ratio of H + pumping to the basal phosphohydrolase activity was proportionally increased. These results and those previously reported at the sunflower tonoplast suggest that the activity of H + pumps under salt stress can form part of a tolerance mechanism in sunflower roots, which could regulate the ion fluxes (Na + and Cl −) across the tonoplast and plasma membrane.

Quick Identification of the Members of the Glutamine Synthetase Gene Family from Sunflower by Simultaneous Amplification of cDNA with Degenerate Primers

A single degenerate glutamine synthetase (GS)-specific primer was used to amplify the 3′ end of cDNAs derived from different GS genes that are expressed in leaves and roots of sunflower (Helianthus annuus L. cv. Peredovic). Four types of GS cDNA (I, II, III and IV) were simultaneously amplified from leaves and five types (I, II, V, VI, VII) from roots with a minimum investment of time and experimental work. cDNAs II, III and IV encode chloroplastic isoforms as deduced by the presence of chloroplastic GS-specific features in their sequences. The rest of cDNAs codifies cytosolic isoforms. Using cDNA-specific probes and primers, homologous sequences to all GS cDNAs amplified from cv. Peredovic, except to cDNAs III and IV, were detected in the inbred line R41. This result strongly suggests that the three cDNAs for chloroplastic isoform are allelic sequences from the same locus, and since cDNA type IV contains sequences derived from cDNAs II and III, it indicates a recombinational origin. The results presented are consistent with the existence of a GS gene family in sunflower with at least five members. Four of them, named ggs1.1 to ggs1.4, codify for the cytosolic isoforms (cDNAs I, V, VI and VII). A fifth member, named ggs2, from which three allelic sequences (cDNAs II, III and IV) have been cloned, encodes the chloroplastic isoform.

K+ status and ABA affect both exudation rate and hydraulic conductivity in sunflower roots

Twenty‐day‐old sunflower plants (Helianthus annuus L. cv. Sun‐Gro 380) grown in nutrient solutions with different KCl levels were used to study the effects of K+ status of the root and of abcisic acid (ABA) on the exudation rate (Jv), the hydraulic conductivity of the root (Lp), the fluxes of exuded K+ and Na+ (JK and JNa), and the gradient of osmotic pressure between the xylem and the external medium. Jv and Lp increased in direct proportion to the K+ starvation of the root. Also addition of ABA (4 µM) at the onset of exudation in the external medium made Jv and Lp rise, and this effect also increased with the degree of K+ starvation. Similarly, K+ starvation and ABA promoted both the flux of exuded Na+ and the accumulation of Na+ in the root. We suggest that ABA acts as a regulating signal for the radial transport of water across the root, and that potassium may be an effector of this mechanism.