Longer epidermal cells underlie a quantitative source of variation in wheat flag leaf size.

The wheat flag leaf is the main contributor of photosynthetic assimilates to developing grains. Understanding how canopy architecture strategies affect source strength and yield will aid improved crop design. We used an eight-founder population to investigate the genetic architecture of flag leaf area, length, width and angle in European wheat. For the strongest genetic locus identified, we subsequently created a near-isogenic line (NIL) pair for more detailed investigation across seven test environments. Genetic control of traits investigated was highly polygenic, with colocalisation of replicated quantitative trait loci (QTL) for one or more traits identifying 24 loci. For QTL QFll.niab-5A.1 (FLL5A), development of a NIL pair found the FLL5A+ allele commonly conferred a c. 7% increase in flag and second leaf length and a more erect leaf angle, resulting in higher flag and/or second leaf area. Increased FLL5A-mediated flag leaf length was associated with: (1) longer pavement cells and (2) larger stomata at lower density, with a trend for decreased maximum stomatal conductance (Gsmax ) per unit leaf area. For FLL5A, cell size rather than number predominantly determined leaf length. The observed trade-offs between leaf size and stomatal morphology highlight the need for future studies to consider these traits at the whole-leaf level.

The impact of growth at elevated [CO2] on stomatal anatomy and behavior differs between wheat species and cultivars.

The ability of plants to respond to changes in the environment is crucial to their survival and reproductive success. The impact of increasing the atmospheric CO2 concentration (a[CO2]), mediated by behavioral and developmental responses of stomata, on crop performance remains a concern under all climate change scenarios, with potential impacts on future food security. To identify possible beneficial traits that could be exploited for future breeding, phenotypic variation in morphological traits including stomatal size and density, as well as physiological responses and, critically, the effect of growth [CO2] on these traits, was assessed in six wheat relative accessions (including Aegilops tauschii, Triticum turgidum ssp. Dicoccoides, and T. turgidum ssp. dicoccon) and five elite bread wheat T. aestivum cultivars. Exploiting a range of different species and ploidy, we identified key differences in photosynthetic capacity between elite hexaploid wheat and wheat relatives. We also report differences in the speed of stomatal responses which were found to be faster in wheat relatives than in elite cultivars, a trait that could be useful for enhanced photosynthetic carbon gain and water use efficiency. Furthermore, these traits do not all appear to be influenced by elevated [CO2], and determining the underlying genetics will be critical for future breeding programmes.

Stomata on the abaxial and adaxial leaf surfaces contribute differently to leaf gas exchange and photosynthesis in wheat.

Although stomata are typically found in greater numbers on the abaxial surface, wheat flag leaves have greater densities on the adaxial surface. We determine the impact of this less common stomatal patterning on gaseous fluxes using a novel chamber that simultaneously measures both leaf surfaces. Using a combination of differential illuminations and CO2 concentrations at each leaf surface, we found that mesophyll cells associated with the adaxial leaf surface have a higher photosynthetic capacity than those associated with the abaxial leaf surface, which is supported by an increased stomatal conductance (driven by differences in stomatal density). When vertical gas flux at the abaxial leaf surface was blocked, no compensation by adaxial stomata was observed, suggesting each surface operates independently. Similar stomatal kinetics suggested some co-ordination between the two surfaces, but factors other than light intensity played a role in these responses. Higher photosynthetic capacity on the adaxial surface facilitates greater carbon assimilation, along with higher adaxial stomatal conductance, which would also support greater evaporative leaf cooling to maintain optimal leaf temperatures for photosynthesis. Furthermore, abaxial gas exchange contributed c. 50% to leaf photosynthesis and therefore represents an important contributor to overall leaf gas exchange.

A single point mutation in the C-terminal extension of wheat Rubisco activase dramatically reduces ADP inhibition via enhanced ATP binding affinity.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase (Rca) is a AAA+ enzyme that uses ATP to remove inhibitors from the active site of Rubisco, the central carboxylation enzyme of photosynthesis. Rca α and ß isoforms exist in most higher plant species, with the α isoform being identical to the ß form but having an additional 25-45 amino acids at the Rca C terminus, known as the C-terminal extension (CTE). Rca is inhibited by ADP, and the extent of ADP sensitivity of the Rca complex can be modulated by the CTE of the α isoform, particularly in relation to a disulfide bond structure that is specifically reduced by the redox-regulatory enzyme thioredoxin-f. Here, we introduced single point mutations of Lys-428 in the CTE of Rca-α from wheat (Triticum aestivum) (TaRca2-α). Substitution of Lys-428 with Arg dramatically altered ADP inhibition, independently of thioredoxin-f regulation. We determined that the reduction in ADP inhibition in the K428R variant is not due to a change in ADP affinity, as the apparent constant for ADP binding was not altered by the K428R substitution. Rather, we observed that the K428R substitution strongly increased ATP substrate affinity and ATP-dependent catalytic velocity. These results suggest that the Lys-428 residue is involved in interacting with the γ-phosphate of ATP. Considering that nucleotide-dependent Rca activity regulates Rubisco and thus photosynthesis during fluctuating irradiance, the K428R substitution could potentially provide a mechanism for boosting the performance of wheat grown in the dynamic light environments of the field.

A Conserved Sequence from Heat-Adapted Species Improves Rubisco Activase Thermostability in Wheat.

The central enzyme of photosynthesis, Rubisco, is regulated by Rubisco activase (Rca). Photosynthesis is impaired during heat stress, and this limitation is often attributed to the heat-labile nature of Rca. We characterized gene expression and protein thermostability for the three Rca isoforms present in wheat (Triticum aestivum), namely TaRca1-ß, TaRca2-α, and TaRca2-ß. Furthermore, we compared wheat Rca with one of the two Rca isoforms from rice (Oryza sativa; OsRca-ß) and Rca from other species adapted to warm environments. The TaRca1 gene was induced, whereas TaRca2 was suppressed by heat stress. The TaRca2 isoforms were sensitive to heat degradation, with thermal midpoints of 35°C ± 0.3°C, the temperature at which Rubisco activation velocity by Rca was halved. By contrast, TaRca1-ß was more thermotolerant, with a thermal midpoint of 42°C, matching that of rice OsRca-ß. Mutations of the TaRca2-ß isoform based on sequence alignment of the thermostable TaRca1-ß from wheat, OsRca-ß from rice, and a consensus sequence representing Rca from warm-adapted species enabled the identification of 11 amino acid substitutions that improved its thermostability by greater than 7°C without a reduction in catalytic velocity at a standard 25°C. Protein structure modeling and mutational analysis suggested that the thermostability of these mutational variants arises from monomeric and not oligomeric thermal stabilization. These results provide a mechanism for improving the heat stress tolerance of photosynthesis in wheat and potentially other species, which is a desirable outcome considering the likelihood that crops will face more frequent heat stress conditions over the coming decades.

Comparative analysis of the susceptibility/tolerance of Spodoptera littoralis to Vip3Aa, Vip3Ae, Vip3Ad and Vip3Af toxins of Bacillus thuringiensis.

The cotton leaf worm Spodoptera littoralis is known for causing serious damages to various crops. In this study, the susceptibility/tolerance of this larvae to four Vip3A (Vip3Aa, Vip3Ae, Vip3Ad and Vip3Af) toxins was investigated. UnlikeVip3Ad which showed no activity to S. littoralis, Vip3Aa, Vip3Ae and Vip3Af exhibited high toxicity to this larva with LC50 of 228.42 ng/cm2, 65.71 ng/cm2, and 388.90 ng/cm2, respectively. Activation of the 90 kDa Vip3A proteins by S. littoralis larvae juice generated four major bands of sizes 62, 45, 33 and 22 kDa. Binding experiments between biotinylated Vip3A toxins and the brush border membrane vesicles (BBMV) revealed two binding proteins of 55 and 100 kDa with Vip3Aa. Vip3Ae and Vip3Af recognized one single putative receptor of 65 kDa, whereas Vip3Ad did not bind to S. littoralis BBMV. In histopathological observations, Vip3Aa, Vip3Ae and Vip3Af toxins showed approximately similar damages on S. littoralis midgut including rupture and disintegration of epithelial layer and cellular vacuolization. These findings showed that Vip3Aa, Vip3Ae and Vip3Af might be useful for controlling S. littoralis.

Heat tolerance in a wild Oryza species is attributed to maintenance of Rubisco activation by a thermally stable Rubisco activase ortholog.

The mechanistic basis of tolerance to heat stress was investigated in Oryza sativa and two wild rice species, Oryza meridionalis and Oryza australiensis. The wild relatives are endemic to the hot, arid Australian savannah. Leaf elongation rates and gas exchange were measured during short periods of supra-optimal heat, revealing species differences. The Rubisco activase (RCA) gene from each species was sequenced. Using expressed recombinant RCA and leaf-extracted RCA, the kinetic properties of the two isoforms were studied under high temperatures. Leaf elongation was undiminished at 45°C in O. australiensis. The net photosynthetic rate was almost 50% slower in O. sativa at 45°C than at 28°C, while in O. australiensis it was unaffected. Oryza meridionalis exhibited intermediate heat tolerance. Based on previous reports that RCA is heat-labile, the Rubisco activation state was measured. It correlated positively with leaf elongation rates across all three species and four periods of exposure to 45°C. Sequence analysis revealed numerous polymorphisms in the RCA amino acid sequence from O. australiensis. The O. australiensis RCA enzyme was thermally stable up to 42°C, contrasting with RCA from O. sativa, which was inhibited at 36°C. We attribute heat tolerance in the wild species to thermal stability of RCA, enabling Rubisco to remain active.

Effect of carbohydrates and night temperature on night respiration in rice.

Global warming causes night temperature (NT) to increase faster than day temperature in the tropics. According to crop growth models, respiration incurs a loss of 40-60% of photosynthate. The thermal sensitivity of night respiration (R(n)) will thus reduce biomass. Instantaneous and acclimated effects of NT on R(n) of leaves and seedlings of two rice cultivars having a variable level of carbohydrates, induced by exposure to different light intensity on the previous day, were investigated. Experiments were conducted in a greenhouse and growth chambers, with R(n) measured on the youngest fully expanded leaves or whole seedlings. Dry weight-based R(n) was 2.6-fold greater for seedlings than for leaves. Leaf R(n) was linearly related to starch (positive intercept) and soluble sugar concentration (zero intercept). Increased NT caused higher R(n) at a given carbohydrate concentration. The change of R(n) at NT increasing from 21 °C to 31 °C was 2.4-fold for the instantaneous response but 1.2- to 1.7-fold after acclimation. The maintenance component of R(n) (R(m)'), estimated by assimilate starvation, averaged 28% in seedlings and 34% in leaves, with no significant thermal effect on this ratio. The acclimated effect of increased NT on R(m)' across experiments was 1.5-fold for a 10 °C increase in NT. No cultivar differences were observed in R(n) or R(m)' responses. The results suggest that the commonly used Q10=2 rule overestimates thermal response of respiration, and R(n) largely depends on assimilate resources.

Different binding sites for Bacillus thuringiensis Cry1Ba and Cry9Ca proteins in the European corn borer, Ostrinia nubilalis (Hübner).

Binding studies using (125)I-Cry9Ca and biotinylated-Cry1Ba proteins showed the occurrence of independent binding sites for these proteins in Ostrinia nubilalis. Our results, along with previously available binding data, indicate that combinations of Cry1A or Cry1Fa proteins with Cry1Ba and/or Cry9Ca could be a good strategy for the resistance management of O. nubilalis.

Specific binding of radiolabeled Cry1Fa insecticidal protein from Bacillus thuringiensis to midgut sites in lepidopteran species.

Cry1Fa insecticidal protein was successfully radiolabeled with (125)I-Na. Specific binding to brush border membrane vesicles was shown for the lepidopteran species Ostrinia nubilalis, Spodoptera frugiperda, Spodoptera exigua, Helicoverpa armigera, Heliothis virescens, and Plutella xylostella. Homologous competition assays were performed to obtain equilibrium binding parameters (K(d) [dissociation constant] and R(t) [concentration of binding sites]) for these six insect species.

Vip3C, a novel class of vegetative insecticidal proteins from Bacillus thuringiensis.

Three vip3 genes were identified in two Bacillus thuringiensis Spanish collections. Sequence analysis revealed a novel Vip3 protein class (Vip3C). Preliminary bioassays of larvae from 10 different lepidopteran species indicated that Vip3Ca3 caused more than 70% mortality in four species after 10 days at 4 µg/cm(2).

Diurnal and Seasonal Variations of Photosynthetic Energy Conversion Efficiency of Field Grown Wheat.

Improving canopy photosynthetic light use efficiency and energy conversion efficiency (ε c ) is a major option to increase crop yield potential. However, so far, the diurnal and seasonal variations of canopy light use efficiency (LUE) and ε c are largely unknown due to the lack of an efficient method to estimate ε c in a high temporal resolution. Here we quantified the dynamic changes of crop canopy LUE and ε c during a day and a growing season with the canopy gas exchange method. A response curve of whole-plant carbon dioxide (CO2) flux to incident photosynthetically active radiation (PAR) was further used to calculate ε c and LUE at a high temporal resolution. Results show that the LUE of two wheat cultivars with different canopy architectures at five stages varies between 0.01 to about 0.05 mol CO2 mol-1 photon, with the LUE being higher under medium PAR. Throughout the growing season, the ε c varies from 0.5 to 3.7% (11-80% of the maximal ε c for C3 plants) with incident PAR identified as a major factor controlling variation of ε c . The estimated average ε c from tillering to grain filling stages was about 2.17%, i.e., 47.2% of the theoretical maximal. The estimated season-averaged radiation use efficiency (RUE) was 1.5-1.7 g MJ-1, which was similar to the estimated RUE based on biomass harvesting. The large variations of LUE and ε c imply a great opportunity to improve canopy photosynthesis for greater wheat biomass and yield potential.

3dCAP-Wheat: An Open-Source Comprehensive Computational Framework Precisely Quantifies Wheat Foliar, Nonfoliar, and Canopy Photosynthesis.

Canopy photosynthesis is the sum of photosynthesis of all above-ground photosynthetic tissues. Quantitative roles of nonfoliar tissues in canopy photosynthesis remain elusive due to methodology limitations. Here, we develop the first complete canopy photosynthesis model incorporating all above-ground photosynthetic tissues and validate this model on wheat with state-of-the-art gas exchange measurement facilities. The new model precisely predicts wheat canopy gas exchange rates at different growth stages, weather conditions, and canopy architectural perturbations. Using the model, we systematically study (1) the contribution of both foliar and nonfoliar tissues to wheat canopy photosynthesis and (2) the responses of wheat canopy photosynthesis to plant physiological and architectural changes. We found that (1) at tillering, heading, and milking stages, nonfoliar tissues can contribute ~4, ~32, and ~50% of daily gross canopy photosynthesis (A cgross; ~2, ~15, and ~-13% of daily net canopy photosynthesis, A cnet) and absorb ~6, ~42, and ~60% of total light, respectively; (2) under favorable condition, increasing spike photosynthetic activity, rather than enlarging spike size or awn size, can enhance canopy photosynthesis; (3) covariation in tissue respiratory rate and photosynthetic rate may be a major factor responsible for less than expected increase in daily A cnet; and (4) in general, erect leaves, lower spike position, shorter plant height, and proper plant densities can benefit daily A cnet. Overall, the model, together with the facilities for quantifying plant architecture and tissue gas exchange, provides an integrated platform to study canopy photosynthesis and support rational design of photosynthetically efficient wheat crops.

Pore-forming properties of the Bacillus thuringiensis toxin Cry9Ca in Manduca sexta brush border membrane vesicles.

The toxicity and pore-forming ability of the Bacillus thuringiensis Cry9Ca insecticidal toxin, its single-site mutants, R164A and R164K, and the 55-kDa fragment resulting from its proteolytic cleavage at residue 164 were investigated using Manduca sexta neonate larvae and fifth-instar larval midgut brush border membrane vesicles, respectively. Neither the mutations nor the proteolytic cleavage altered Cry9Ca toxicity. Compared with Cry1Ac, Cry9Ca and its mutants formed large poorly selective pores in the vesicles. Pore formation was highly dependent on pH, however, especially for wild-type Cry9Ca and both mutants. Increasing pH from 6.5 to 10.5 resulted in an irregular step-wise decrease in membrane permeabilization that was not related to a change in the ionic selectivity of the pores. Pore formation was much slower with Cry9Ca and its derivatives, including the 55-kDa fragment, than with Cry1Ac and its rate was not influenced by the presence of protease inhibitors or a reducing agent.

Effects of mutations within surface-exposed loops in the pore-forming domain of the Cry9Ca insecticidal toxin of Bacillus thuringiensis.

The pore-forming domain of Bacillus thuringiensis insecticidal Cry toxins is formed of seven amphipathic α-helices. Because pore formation is thought to involve conformational changes within this domain, the possible role of its interhelical loops in this crucial step was investigated with Cry9Ca double mutants, which all share the previously characterized R164A mutation, using a combination of homology modeling, bioassays and electrophysiological measurements. The mutations either introduced, neutralized or reversed an electrical charge carried by a single residue of one of the domain I loops. The ability of the 28 Cry9Ca double mutants to depolarize the apical membrane of freshly isolated Manduca sexta larval midguts was tested in the presence of either midgut juice or a cocktail of protease inhibitors because these conditions had been shown earlier to greatly enhance pore formation by Cry9Ca and its R164A single-site mutant. Most mutants retained toxicity toward neonate larvae and a pore-forming ability in the electrophysiological assay, which were comparable to those of their parental toxin. In contrast, mutants F130D, L186D and V189D were very poorly toxic and practically inactive in vitro. On the other hand, mutant E129A depolarized the midgut membrane efficiently despite a considerably reduced toxicity, and mutant Q192E displayed a reduced depolarizing ability while conserving a near wild-type toxicity. These results suggest that the conditions found in the insect midgut, including high ionic strength, contribute to minimizing the influence of surface charges on the ability of Cry9Ca and probably other B. thuringiensis toxins to form pores within their target membrane.

Midgut juice components affect pore formation by the Bacillus thuringiensis insecticidal toxin Cry9Ca.

The pore-forming ability of the Bacillus thuringiensis toxin Cry9Ca, its two single-site mutants R164A and R164K, and the 55-kDa fragment resulting from its proteolytic cleavage at R164 was evaluated under a variety of experimental conditions using an electrophysiological assay. All four toxin preparations depolarized the apical membrane of freshly isolated third-instar Manduca sexta midguts bathing in a solution containing 122 mM KCl at pH 10.5, but the 55-kDa fragment was considerably more active than Cry9Ca and its mutants. The activity of the latter toxins was greatly enhanced, however, when the experiments were conducted in the presence of fifth-instar M. sexta midgut juice. This effect was also observed after midgut juice proteins had been denatured by heating at 95 degrees C or after inorganic ions and small molecules had been removed from the midgut juice by extensive dialysis. A similar stimulation of toxin activity was also observed when the experiments were carried out in the presence of the lipids extracted from an equivalent volume of midgut juice. Depolarization of the cell membrane was also greatly enhanced, in the absence of midgut juice, by the addition of a cocktail of water-soluble protease inhibitors. These results indicate that, depending on the cleavage site and on the experimental conditions used, further proteolysis of the activated Cry9Ca toxin can either stimulate or be detrimental to its activity and that M. sexta midgut juice probably contains protease inhibitors that could play a major role in the activity of B. thuringiensis toxins in the insect midgut.

Genotypic, Developmental and Environmental Effects on the Rapidity of gs in Wheat: Impacts on Carbon Gain and Water-Use Efficiency.

Stomata are the primary gatekeepers for CO2 uptake for photosynthesis and water loss via transpiration and therefore play a central role in crop performance. Although stomatal conductance (gs ) and assimilation rate (A) are often highly correlated, studies have demonstrated an uncoupling between A and gs that can result in sub-optimal physiological processes in dynamic light environments. Wheat (Triticum aestivum L.) is exposed to changes in irradiance due to leaf self-shading, moving clouds and shifting sun angle to which both A and gs respond. However, stomatal responses are generally an order of magnitude slower than photosynthetic responses, leading to non-synchronized A and gs responses that impact CO2 uptake and water use efficiency ( iWUE). Here we phenotyped a panel of eight wheat cultivars (estimated to capture 80% of the single nucleotide polymorphism variation in North-West European bread wheat) for differences in the speed of stomatal responses (to changes in light intensity) and photosynthetic performance at different stages of development. The impact of water stress and elevated [CO2] on stomatal kinetics was also examined in a selected cultivar. Significant genotypic variation was reported for the time constant for stomatal opening (Ki, P = 0.038) and the time to reach 95% steady state A (P = 0.045). Slow gs opening responses limited A by ∼10% and slow closure reduced iWUE, with these impacts found to be greatest in cultivars Soissons, Alchemy and Xi19. A decrease in stomatal rapidity (and thus an increase in the limitation of photosynthesis) (P < 0.001) was found during the post-anthesis stage compared to the early booting stage. Reduced water availability triggered stomatal closure and asymmetric stomatal opening and closing responses, while elevated atmospheric [CO2] conditions reduced the time for stomatal opening during a low to high light transition, thus suggesting a major environmental effect on dynamic stomatal kinetics. We discuss these findings in terms of exploiting various traits to develop ideotypes for specific environments, and suggest that intraspecific variation in the rapidity of stomatal responses could provide a potential unexploited breeding target to optimize the physiological responses of wheat to dynamic field conditions.

Specific binding of Bacillus thuringiensis Cry2A insecticidal proteins to a common site in the midgut of Helicoverpa species.

For a long time, it has been assumed that the mode of action of Cry2A toxins was unique and different from that of other three-domain Cry toxins due to their apparent nonspecific and unsaturable binding to an unlimited number of receptors. However, based on the homology of the tertiary structure among three-domain Cry toxins, similar modes of action for all of them are expected. To confirm this hypothesis, binding assays were carried out with (125)I-labeled Cry2Ab. Saturation assays showed that Cry2Ab binds in a specific and saturable manner to brush border membrane vesicles (BBMVs) of Helicoverpa armigera. Homologous-competition assays with (125)I-Cry2Ab demonstrated that this toxin binds with high affinity to binding sites in H. armigera and Helicoverpa zea midgut. Heterologous-competition assays showed a common binding site for three toxins belonging to the Cry2A family (Cry2Aa, Cry2Ab, and Cry2Ae), which is not shared by Cry1Ac. Estimation of K(d) (dissociation constant) values revealed that Cry2Ab had around 35-fold less affinity than Cry1Ac for BBMV binding sites in both insect species. Only minor differences were found regarding R(t) (concentration of binding sites) values. This study questions previous interpretations from other authors performing binding assays with Cry2A toxins and establishes the basis for the mode of action of Cry2A toxins.

Shared midgut binding sites for Cry1A.105, Cry1Aa, Cry1Ab, Cry1Ac and Cry1Fa proteins from Bacillus thuringiensis in two important corn pests, Ostrinia nubilalis and Spodoptera frugiperda.

First generation of insect-protected transgenic corn (Bt-corn) was based on the expression of Cry1Ab or Cry1Fa proteins. Currently, the trend is the combination of two or more genes expressing proteins that bind to different targets. In addition to broadening the spectrum of action, this strategy helps to delay the evolution of resistance in exposed insect populations. One of such examples is the combination of Cry1A.105 with Cry1Fa and Cry2Ab to control O. nubilalis and S. frugiperda. Cry1A.105 is a chimeric protein with domains I and II and the C-terminal half of the protein from Cry1Ac, and domain III almost identical to Cry1Fa. The aim of the present study was to determine whether the chimeric Cry1A.105 has shared binding sites either with Cry1A proteins, with Cry1Fa, or with both, in O. nubilalis and in S. frugiperda. Brush-border membrane vesicles (BBMV) from last instar larval midguts were used in competition binding assays with (125)I-labeled Cry1A.105, Cry1Ab, and Cry1Fa, and unlabeled Cry1A.105, Cry1Aa, Cry1Ab, Cry1Ac, Cry1Fa, Cry2Ab and Cry2Ae. The results showed that Cry1A.105, Cry1Ab, Cry1Ac and Cry1Fa competed with high affinity for the same binding sites in both insect species. However, Cry2Ab and Cry2Ae did not compete for the binding sites of Cry1 proteins. Therefore, according to our results, the development of cross-resistance among Cry1Ab/Ac, Cry1A.105, and Cry1Fa proteins is possible in these two insect species if the alteration of shared binding sites occurs. Conversely, cross-resistance between these proteins and Cry2A proteins is very unlikely in such case.

Binding site alteration is responsible for field-isolated resistance to Bacillus thuringiensis Cry2A insecticidal proteins in two Helicoverpa species.

BACKGROUND: Evolution of resistance by target pests is the main threat to the long-term efficacy of crops expressing Bacillus thuringiensis (Bt) insecticidal proteins. Cry2 proteins play a pivotal role in current Bt spray formulations and transgenic crops and they complement Cry1A proteins because of their different mode of action. Their presence is critical in the control of those lepidopteran species, such as Helicoverpa spp., which are not highly susceptible to Cry1A proteins. In Australia, a transgenic variety of cotton expressing Cry1Ac and Cry2Ab (Bollgard II) comprises at least 80% of the total cotton area. Prior to the widespread adoption of Bollgard II, the frequency of alleles conferring resistance to Cry2Ab in field populations of Helicoverpa armigera and Helicoverpa punctigera was significantly higher than anticipated. Colonies established from survivors of F(2) screens against Cry2Ab are highly resistant to this toxin, but susceptible to Cry1Ac. METHODOLOGY/PRINCIPAL FINDINGS: Bioassays performed with surface-treated artificial diet on neonates of H. armigera and H. punctigera showed that Cry2Ab resistant insects were cross-resistant to Cry2Ae while susceptible to Cry1Ab. Binding analyses with (125)I-labeled Cry2Ab were performed with brush border membrane vesicles from midguts of Cry2Ab susceptible and resistant insects. The results of the binding analyses correlated with bioassay data and demonstrated that resistant insects exhibited greatly reduced binding of Cry2Ab toxin to midgut receptors, whereas no change in (125)I-labeled-Cry1Ac binding was detected. As previously demonstrated for H. armigera, Cry2Ab binding sites in H. punctigera were shown to be shared by Cry2Ae, which explains why an alteration of the shared binding site would lead to cross-resistance between the two Cry2A toxins. CONCLUSION/SIGNIFICANCE: This is the first time that a mechanism of resistance to the Cry2 class of insecticidal proteins has been reported. Because we found the same mechanism of resistance in multiple strains representing several field populations, we conclude that target site alteration is the most likely means that field populations evolve resistance to Cry2 proteins in Helicoverpa spp. Our work also confirms the presence in the insect midgut of specific binding sites for this class of proteins. Characterizing the Cry2 receptors and their mutations that enable resistance could lead to the development of molecular tools to monitor resistance in the field.