Ana María Castro

Identification of wheat chromosomes involved with different types of resistance against greenbug (Schizaphis graminum, Rond.) and the Russian wheat aphid (Diuraphis noxia, Mordvilko)

Two sets of intervarietal chromosome substitution lines in the recipient,susceptible cultivar ‘Chinese Spring’ were screened to identify the wheat chromosomes involved with antixenosis, antibiosis and tolerance resistance to greenbug and Russian wheat aphid. The amphiploid ‘Synthetic’ and the cultivar ‘Hope’ were the donor parents. Antixenosis, antibiosis and tolerance were evaluated with conventional tests in controlled environmental conditions using a clone of greenbug biotype C and a clone of RWA collected on wheat. Antixenosis against greenbug was accounted for by several chromosomes in both sets of substitution lines with chromosome 2B contributing the highest level of this type of resistance. The highest levels of antixenosis against RWA were associated with the group of chromosomes 7 of the substitutions CS/Syn set and the chromosome substitutions 2B, 6A and 7D of the CS/Hope set. Antibiosis against both aphids species was accounted for by several different chromosomes. The highest levels of antibiosis for most of RWA resistance traits were recorded from the 1B substitution line of the CS/Hope set. More than one gene appears to determine antibiosis. Tolerance to both greenbug and the RWA was significantly associated with chromosomes 1A,1D, and 6D in the CS/Syn set of substitutions. These lines showed enhanced plant growth under aphid infestation. The highest levels of antixenosis, antibiosis and tolerance against the two aphid species occurred mostly in different substitution lines. Consequently, the different types of resistance for both pests seem to be partially independent. Since different genes seem to be involved in at least several traits of the resistance categories against the two aphid species, such genes could be combined in new cultivars of wheat to broaden their genetic base of resistance against the greenbug and the RWA.

Glyphosate effects on gas exchange and chlorophyll fluorescence responses of two Lolium perenne L. biotypes with differential herbicide sensitivity.

Despite the extensive use of glyphosate, how it alters the physiology and metabolism of plants is still unclear. Photosynthesis is not regarded to be a primary inhibitory target of glyphosate, but it has been reported to be affected by this herbicide. The aim of the current research was to determine the effects of glyphosate on the light and dark reactions of photosynthesis by comparing glyphosate-susceptible and glyphosate-resistant Lolium perenne biotypes. After glyphosate treatment, accumulation of reduced carbohydrates occurred before a decrease in gas exchange. Stomatal conductance and CO(2) assimilation were reduced earlier than chlorophyll fluorescence and the amount of chlorophyll in susceptible plants. In the glyphosate-resistant biotype, stomatal conductance was the only parameter slightly affected only 5 days post-application. In susceptible plants, the initial glyphosate effects on gas exchange could be a response to a feedback regulation of photosynthesis. Since the herbicide affects actively growing tissues regardless of the inhibition of photosynthesis, the demand of assimilates decreased and consequently induced an accumulation of carbohydrates in leaves. We concluded that stomatal conductance could be a very sensitive parameter to assess both the susceptibility/resistance to glyphosate before the phytotoxic symptoms become evident.

Glyphosate Resistance in Perennial Ryegrass (Lolium perenne L.) is Associated with a Fitness Penalty

Abstract The net selection effect of herbicides on herbicide-resistance traits in weeds is conditioned by the fitness benefits and costs associated with resistance alleles. Fitness costs play an important evolutionary role preventing the fixation of adaptive alleles and contributing to the maintenance of genetic polymorphisms within populations. Glyphosate is widely used in world agriculture, which has led to the evolution of widespread glyphosate resistance in many weed species. The fitness of glyphosate-resistant and -susceptible perennial ryegrass plants selected from within a single population were studied in two field experiments conducted during 2011 and 2012 under different soil water availability. Glyphosate-resistant plants showed a reduction in height of 12 and 16%, leaf blade area of 16 and 33%, shoot biomass of 45 and 55%, seed number of 33 and 53%, and total seed mass of 16 and 5% compared to glyphosate-susceptible plants in 2011 and 2012, respectively. The reduction in seed number per plant resulted in a 40% fitness cost associated with the glyphosate-resistance trait in perennial ryegrass. Fitness costs of glyphosate-resistant plants were expressed under both conditions of water availability. These results could be useful for designing management strategies and exploiting the reduced glyphosate-resistant perennial ryegrass fitness in the absence of glyphosate selection. Nomenclature: Glyphosate; perennial ryegrass, Lolium perenne L., LOLPE.

Greenbug damage on the aerial vegetation growth of two barley cultivars

Abstract The aim of this paper was to investigate the degree of greenbug (Schizaphis graminum, Rond.) vegetative damage to tolerant and susceptible barley seedlings. Plants of identical physiological age were selected; half of the plants in the cultivars were infested with 20 adult aphids per plant, the other half remained as uninfested controls. From infestation and then every 2 days, seedlings were randomly sampled to determine: (a) the length and number of sheaths and laminae of expanded and unexpanded leaves, and of the leaf primordia that overtopped apical dome; (b) number of leaf primordia differentiated in the apex; (c) total leaf area; (d) photosynthetic leaf area; (e) chlorotic leaf area; (f) aerial dry weight and (g) aphids per plant. In the susceptible cultivar, aphid infestation resulted in an inhibition in new leaf primordia differentiation on the apex and also caused a shortening in the length of sheaths and laminae, from infestation onwards; as a consequence total leaf area was smaller relative to controls. The infested and the control plants in the resistant cultivar showed similar growth rates. Greenbug primary action on the main shoot apex of susceptible plants is considered to be systemic, disturbing growth-regulating pathways. In the initial days of infestation the action is independent of local leaf tissue damage, which includes destruction of photosynthetic leaf areas and results in chlorotic zones.

Greenbug systemic effect on barley phosphate influx

Abstract Greenbugs ( Schizaphis graminum Rond.) cause considerable yield loss in cereals. Local feeding damage of greenbug-infested leaves includes collapsed mesophyll cells, chlorosis, alterations in photosynthesis and respiration. However, this damage cannot explain rapid changes taking place in plant metabolism (inhibition of new leaf primordia and new root differentiation, within a few hours after attack), or the early death of such plants. This study was aimed at determining whether greenbug feeding induces systemic damage to barley. The phosphate influx by roots of susceptible and tolerant barley ( Hordeum vulgare L.) plants was evaluated as an estimate of aphid systemic damage. Phosphate (P)-influx was determined at two plant growth stages, with two levels of greenbug infestation, at two different greenbug life stages. Plants grown in hydroponics in a glasshouse were infested for 0 (control), 3, 6, 12, 24, 48, and 72 h with the Argentinean biotype C greenbug. The P-influx was not significantly affected in tolerant barley plants by greenbug infestation. In contrast P-influx was significantly reduced 6 h post-infestation in the susceptible cultivar. Plants with one expanded leaf suffered a significantly greater reduction in P-influx than plants with two expanded leaves. By 48 h after infestation, the P-influx of the two-expanded-leaf treatment was similar to that of the controls, whereas P-influx in plants with one expanded leaf remained significantly less than on the controls 72 h after infestation. A larger greenbug population resulted in greater reduction in P-influx. Adult greenbugs, but not third stage nymphs, affected P-influx. In summary, the intensity of greenbug-induced systemic damage was greater when young plant stages were infested by the aphid. Reductions of P-influx may become critical under increasing natural infestation levels.

Efecto del glifosato sobre el crecimiento y acumulación de azúcares libres en dos biotipos de lolium perenne de distinta sensibilidad al herbicida

El movimiento sistemico del glifosato esta determinado por el transporte de fotoasimilados. A su vez, la capacidad de un destino de consumir los asimilados esta condicionada por su actividad metabolica. Pese a su importancia, la relacion entre el glifosato y la sintesis de azucares en hojas fuente ha sido poco abordada. El objetivo del presente trabajo fue evaluar los efectos del glifosato sobre el crecimiento y la acumulacion de azucares libres en dos biotipos de Lolium perenne de baja y alta sensibilidad al herbicida. Se trabajo con clones de ambos tipos de plantas, en macollaje, tratados con 1.440 g e.a. ha-1 de glifosato y sin tratamiento herbicida como controles. Se evaluo periodicamente el efecto del glifosato sobre el rebrote de hojas hasta las 50 horas post-aplicacion y sobre los niveles de azucares libres totales, reductores y no reductores en hojas a 1, 2, 3 y 5 dias post-aplicacion. A partir de las 25 horas post-aplicacion, el glifosato provoco una disminucion del crecimiento del 58% en el biotipo susceptible, con una acumulacion de azucares libres superior al 90% con relacion al control, desde el primer dia post-aplicacion en adelante. La inhibicion del crecimiento, inducida por el glifosato en plantas susceptibles, no depende de la limitacion del traslado de fotoasimilados desde la parte aerea. Por tanto, la acumulacion de azucares libres en hojas podria explicarse por la caida en la tasa de crecimiento. En el biotipo de baja sensibilidad, en el que no se detecto inhibicion del crecimiento, estos efectos fueron limitados.

Genetic resistance to greenbug is expressed with higher contents of proteins and non-structural carbohydrates in wheat substitution lines.

This paper studied the endogenous levels of reduced, non-reduced, total non-structural carbohydrates, soluble proteins and biomass in aerial and rooting structures of bread wheat, Triticum aestivum (2n=6x=42), in response to aphids, as a first step for understanding the cascade of transductional events that may account for antixenosis, antibiosis and tolerance to greenbug. Up to now, few studies have been made on the relationship between aphid resistance and these traits. A set of wheat intervarietal chromosome substitution lines, with ‘‘Chinese Spring’’ (CS, a greenbug susceptible line) as a recipient and a synthetic wheat (Triticum dicoccum x T tauschii, = [Syn]) as the donor, and both parents were used. Plants were cultivated in hydroponic solutions to the fully expanded 3rd leaf stage. Half of the plants of every genotype were infested 72h with greenbugs, and the remaining uninfested plants were used as controls. Carbohydrate and protein contents and dry matter mass were determined for aerial and root tissues Lines 5A and 6A had lower aerial, root and, consequently, total dry weights in both control and infested plants. These lines have been previously reported to be antixenotic against greenbug and Russian Wheat Aphid (RWA), implying these lines carry genes for constitutive defences. Four substitution lines (1A, 1B, 7B and 7D) showed significant increases in protein content when infested, compared to their controls and to the CS susceptible parent. Considering that these substitution lines have been previously reported to reduce greenbug and RWA fertilities and longevities, the antibiotic resistance to greenbugs may be related to gene expression for enhanced protein levels. Most of the D genome substitution lines showed an increase of total root carbohydrates with the greatest increase in total root and aerial carbohydrates under infestation in the 1D and 6D substitution lines. Since these lines have been reported as being tolerant to greenbug, their highest carbohydrate contents probably protect them against biotic stress by enhancing growth. Greenbug resistance genes have been mapped only on the 1A, 6A, 7A and 7D chromosomes. Nonetheless, it was possible to identify other substitution lines that showed effects in the photosynthesis, the C and N metabolisms in the cascade of transductional signals that account for antixenosis, antibiosis and tolerance to greenbug in wheat

Mapping and candidate gene identification of loci induced by phytohormones in barley (Hordeum vulgare L.)

Phytohormones, a group of structurally unrelated small molecules are plant-signalling compounds that trigger induced resistance against certain pathogens and herbivores. The hormones jasmonic acid (JA), ABA, salicilic acid (SA) and ethylene (ET) are known to play major roles in regulating plant defence responses. In order to determine the changes in growth and in the chlorophyll content induced by the exogenous application of these elicitors, a set of DH lines of the Oregon-Wolfe Barley mapping population, previously screened to locate aphid resistant genes, was investigated. The aim of the current research was to map the induced defence genes and to reveal the relationship with aphid resistance. There were highly significant differences between controls and hormone treated plants in the aerial fresh and dry weights (AFW, ADW), the foliar area (FA) and the root dry weight (RDW). More than 15 JA and ET-induced lines exceeded the chlorophyll (Ch) values of their controls. Most of the plant traits were associated with the same genetic windows on chromosomes 3H, 5H and 7H in the controls and hormone treated plants. QTL(s) identified on chromosome 3H and 5H explained most of the variation of AFW, ADW, FA and RDW of controls and treated plants. QTL(s) located on chromosome 5H were associated with the variation of chlorophyll contents on JA-treated plants. The Ch in ET and ABA-treated plants was associated with two different regions on chromosome 7H. One of the latter genetic windows also explained the variation of RDW of ET- and ABA-treated plants. A sequence homology search was performed to derive the putative function of the genes linked to the QTLs. Several QTLs were identified located close to aphid resistance genes previously mapped. This is the first report of genes associated with hormone response in barley that could be involved with insect resistance. Those recombinant lines carrying the appropriate alleles could be useful for breeding barley to enlarge the genetic base of defence against stress.

Mechanism of Resistance to Glyphosate in Lolium perenne from Argentina

In Argentina, glyphosate resistance was reported in a Lolium perenne population after 12 years of successful herbicide use. The aim of the current paper was to put in evidence for the mechanism of glyphosate resistance of this weed. Susceptible leaves treated with different doses of glyphosate and incubated in vitro showed an accumulation of shikimic acid of around three to five times the basal level, while no changes were detected in leaves of glyphosate-resistant plants. The resistance mechanism prevents shikimate accumulation in leaves, even under such tissue-isolation conditions. The activity of the glyphosate target enzyme (EPSPS: 5-enolpyruvylshikimate-3-phosphate synthase) was quantified at different herbicide concentrations. EPSPS from resistant plants showed no difference in glyphosate-sensitivity compared to EPSPS from susceptible plants, and, accordingly, no amino acid substitution causing mutations associated with resistance were found. While the glyphosate target enzymes were equally sensitive, the basal EPSPS activity in glyphosate resistant plants was approximately three-fold higher than the EPSPS activity in susceptible plants. This increased EPSPS activity in glyphosate resistant plants was associated with a 15-fold higher expression of EPSPS compared with susceptible plants. Therefore, the over-expression of EPSPS appears to be the main mechanism responsible for resistance to glyphosate. This mechanism has a constitutive character and has important effects on plant fitness, as recently reported.