Muthukumar V. Bagavathiannan

Impact of Combined Abiotic and Biotic Stresses on Plant Growth and Avenues for Crop Improvement by Exploiting Physio-morphological Traits

Global warming leads to the concurrence of a number of abiotic and biotic stresses, thus affecting agricultural productivity. Occurrence of abiotic stresses can alter plant–pest interactions by enhancing host plant susceptibility to pathogenic organisms, insects, and by reducing competitive ability with weeds. On the contrary, some pests may alter plant response to abiotic stress factors. Therefore, systematic studies are pivotal to understand the effect of concurrent abiotic and biotic stress conditions on crop productivity. However, to date, a collective database on the occurrence of various stress combinations in agriculturally prominent areas is not available. This review attempts to assemble published information on this topic, with a particular focus on the impact of combined drought and pathogen stresses on crop productivity. In doing so, this review highlights some agriculturally important morpho-physiological traits that can be utilized to identify genotypes with combined stress tolerance. In addition, this review outlines potential role of recent genomic tools in deciphering combined stress tolerance in plants. This review will, therefore, be helpful for agronomists and field pathologists in assessing the impact of the interactions between drought and plant-pathogens on crop performance. Further, the review will be helpful for physiologists and molecular biologists to design agronomically relevant strategies for the development of broad spectrum stress tolerant crops.

Agricultural Weed Research: A Critique and Two Proposals

Abstract Two broad aims drive weed science research: improved management and improved understanding of weed biology and ecology. In recent years, agricultural weed research addressing these two aims has effectively split into separate subdisciplines despite repeated calls for greater integration. Although some excellent work is being done, agricultural weed research has developed a very high level of repetitiveness, a preponderance of purely descriptive studies, and has failed to clearly articulate novel hypotheses linked to established bodies of ecological and evolutionary theory. In contrast, invasive plant research attracts a diverse cadre of nonweed scientists using invasions to explore broader and more integrated biological questions grounded in theory. We propose that although studies focused on weed management remain vitally important, agricultural weed research would benefit from deeper theoretical justification, a broader vision, and increased collaboration across diverse disciplines. To initiate change in this direction, we call for more emphasis on interdisciplinary training for weed scientists, and for focused workshops and working groups to develop specific areas of research and promote interactions among weed scientists and with the wider scientific community.

Modeling the Evolution of Glyphosate Resistance in Barnyardgrass (Echinochloa crus-galli) in Cotton-Based Production Systems of the Midsouthern United States

Abstract Glyphosate-resistant (GR) weeds have been a prime challenge to the sustainability of GR cotton-based production systems of the midsouthern United States. Barnyardgrass is known to be a high-risk species for evolving herbicide resistance, and a simulation model was developed for understanding the likelihood of glyphosate resistance evolution in this species in cotton-based systems. Under a worst-case scenario of five glyphosate applications in monoculture GR cotton, the model predicts resistance evolution in about 9 yr of continuous glyphosate use, with about 47% risk by year 15. A unique insight from this model is that management in response to GR Palmer amaranth in this system (a reactive response) provided a proactive means to greatly reduce the risks of glyphosate resistance evolution in barnyardgrass. Subsequent model analysis revealed that the risk of resistance is high in fields characterized by high barnyardgrass seedbank levels, seedling emergence, and seed production per square meter, whereas the risk is low in fields with high levels of postdispersal seed loss and annual seedbank loss. The initial frequency of resistance alleles was a high determinant of resistance evolution (e.g., 47% risk at year 15 at an initial frequency of 5e−8 vs. 4% risk at 5e−10). Monte Carlo simulations were performed to understand the influence of various glyphosate use patterns and production practices in reducing the rate and risk of glyphosate resistance evolution in barnyardgrass. Early planting and interrow cultivation are useful tools. Crop rotation is effective, but the diversity of weed management options practiced in the rotational crop is more important. Diversifying weed management options is the key, yet application timing and the choice of management option is critical. Model analyses illustrate the relative effectiveness of a number of diversified glyphosate use strategies in preventing resistance evolution and preserving the long-term utility of glyphosate in midsouthern U.S. cotton-based production systems. Nomenclature: Glyphosate; barnyardgrass; Echinochloa crus-galli (L.) Beauv. ECHCG; cotton; Gossypium hirsutum L. Resumen Las malezas resistentes a glyphosate (GR) han sido un reto primordial a la sostenibilidad de los sistemas de producción basados en algodón GR en el sur-medio de los Estados Unidos. Echinochloa crus-galli es reconocida como una maleza de alto riesgo de evolución de resistencia a herbicidas por lo que se desarrolló un modelo de simulación para entender la probabilidad de la evolución de resistencia a glyphosate en esta especie en sistemas basados en algodón. En el caso del peor escenario con cinco aplicaciones de glyphosate en monocultivo de algodón GR, el modelo predice la evolución de resistencia en aproximadamente 9 años de uso continuo de glyphosate, con cerca de 47% de riesgo en el año 15. Un detalle único de este modelo es que el manejo en respuesta a Amaranthus palmeri GR en este sistema (una respuesta reactiva) brindó los medios proactivos para reducir ampliamente el riesgo de la evolución de resistencia a glyphosate en E. crus-galli. El análisis siguiente del modelo reveló que el riesgo de resistencia es alto en campos caracterizados por tener niveles altos de bancos de semillas, emergencia de plántulas, y producción de semilla de E. crus-galli por metro cuadrado, mientras que el riesgo es bajo en campos con altos niveles de pérdida de semilla post-dispersión y pérdidas anuales del banco de semillas. La frecuencia inicial de alelos de resistencia fue un determinante importante en la evolución de resistencia (e.g., 47% de riesgo en el año 15 a una frecuencia inicial de 5e−8 vs. 4% de riesgo a 5e−10). Se realizaron simulaciones Monte Carlo para entender la influencia de varios patrones de uso de glyphosate y prácticas de producción en la reducción del riesgo y la tasa de evolución de resistencia a glyphosate en E. crus-galli. La siembra temprana y el cultivo entre hileras son herramientas útiles. La rotación de cultivos es efectiva, pero la diversidad en opciones de manejo de malezas en el cultivo de rotación es más importante. El diversificar las opciones de manejo de malezas es la clave, aunque el momento de aplicación y la escogencia de la opción de manejo son críticos. Análisis de modelos ilustran la efectividad relativa de utilizar un número variado de estrategias de uso de glyphosate en la prevención de la evolución de resistencia y la preservación de la utilidad de glyphosate en el largo plazo en los sistemas de producción basados en algodón en el sur-medio de los Estados Unidos.

Does Resistance to Propanil or Clomazone Alter the Growth and Competitive Abilities of Barnyardgrass (Echinochloa crus-galli )?

Abstract Barnyardgrass biotypes resistant (R) to propanil (PR) or clomazone (CR) have been confirmed in rice production systems in Arkansas. However, it is not clear whether resistance to these herbicides impose any fitness cost on the R biotypes compared to susceptible barnyardgrass (S ). The overall objective of this experiment was to determine if the growth and competitiveness of barnyardgrass is altered by resistance to propanil or clomazone and to establish a competitive hierarchy among the S, PR, and CR biotypes. A replacement series study was conducted in a greenhouse using five proportions of S and R biotypes (0 ∶ 100, 25 ∶ 75, 50 ∶ 50, 75 ∶ 25, and 100 ∶ 0). The study was carried out in a completely randomized design (CRD) with four replications. The variables, including plant height, number of tillers, number of leaves, and shoot dry weight, were used for quantifying the differences in competitive abilities. Replacement series indices were calculated to explore the competitiveness. Expected (He) and observed (Ho) values for relative yield (RY) and relative yield total (RYT) were compared for number of tillers, number of leaves, and shoot dry weight for each biotype comparison. Other replacement series indices including competitive ratio (CR), relative crowding coefficient (RCC), and aggressiveness index (AI) also were calculated for these variables. The results showed that there were no major differences among the S and R biotypes for these variables, indicating that in the absence of selection pressure, resistance to propanil or clomazone does not influence the growth and competitiveness of barnyardgrass. The findings will be useful for predicting the dynamics of resistant populations in the absence of herbicide selection and for designing suitable management strategies. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv.

Multiple-Herbicide Resistance Is Widespread in Roadside Palmer Amaranth Populations.

Herbicide-resistant Palmer amaranth is a widespread issue in row-crop production in the Midsouthern US. Palmer amaranth is commonly found on roadside habitats in this region, but little is known on the degree of herbicide resistance in these populations. Herbicide resistance in roadside Palmer amaranth populations can represent the spread of an adaptive trait across a selective landscape. A large-scale survey was carried out in the Mississippi Delta region of eastern Arkansas to document the level of resistance in roadside Palmer amaranth populations to pyrithiobac and glyphosate, two important herbicides with broad history of use in the region. A total of 215 Palmer amaranth populations collected across 500 random survey sites were used in the evaluations. About 89 and 73% of the surveyed populations showed >90% survival to pyrithiobac and glyphosate, respectively. Further, only 3% of the populations were completely susceptible to glyphosate, while none of the populations was completely controlled by pyrithiobac. Among the 215 populations evaluated, 209 populations showed multiple resistance to both pyrithiobac and glyphosate at varying degrees. Dose-response assays confirmed the presence of high levels of herbicide resistance in the five selected populations (≥ 25-fold compared to a susceptible standard). Results demonstrate the prevalence of multiple-herbicide resistance in roadside Palmer amaranth populations in this region. Growers should be vigilant of Palmer amaranth infestation in roadsides adjacent to their fields and implement appropriate control measures to prevent likely spread of herbicide resistance into their fields.

Selective fertilization with phosphite allows unhindered growth of cotton plants expressing the ptxD gene while suppressing weeds

Significance An increasing number of herbicide-resistant weeds are being reported in the United States, Argentina, and Brazil. This is becoming a global challenge for the production of several major crops, such as cotton, maize, and soybean. New strategies for weed control are required to sustain agricultural production while reducing our dependence on herbicides. Here, we report that selective fertilization of transgenic cotton, expressing a bacterial phosphite dehydrogenase (PTXD), with phosphite provides an effective way to suppress weed growth. Importantly, we show that the ptxD-transgenic cotton plants successfully outcompete a highly aggressive glyphosate-resistant weed. The ptxD/phosphite system represents one of the most promising technologies of recent times with potential to solve many of the agricultural and environmental problems that we encounter currently. Weeds, which have been the bane of agriculture since the beginning of civilization, are managed manually, mechanically, and, more recently, by chemicals. However, chemical control options are rapidly shrinking due to the recent rise in the number of herbicide-resistant weeds in crop fields, with few alternatives on the horizon. Therefore, there is an urgent need for alternative weed suppression systems to sustain crop productivity while reducing our dependence on herbicides and tillage. Such a development will also allay some of the negative perceptions associated with the use of herbicide-resistance genes and heavy dependence on herbicides. Transgenic plants expressing the bacterial phosphite dehydrogenase (ptxD) gene gain an ability to convert phosphite (Phi) into orthophosphate [Pi, the metabolizable form of phosphorus (P)]. Such plants allow for a selective fertilization scheme, based on Phi as the sole source of P for the crop, while offering an effective alternative for suppressing weed growth. Here, we show that, when P is supplied in the form of Phi, ptxD-expressing cotton (Gossypium hirsutum L.) plants outcompete, in both artificial substrates and natural soils from agricultural fields, three different monocot and dicot weed species intentionally introduced in the experiments, as well as weeds naturally present in the tested soils. Importantly, the ptxD/Phi system proved highly efficacious in inhibiting the growth of glyphosate-resistant Palmer amaranth. With over 250 weed species resistant to currently available herbicides, ptxD-transgenic plants fertilized with Phi could provide an effective alternative to suppressing the growth of these weeds while providing adequate nutrition to the crop.

Opportunities and challenges for harvest weed seed control in global cropping systems

The opportunity to target weed seeds during grain harvest was established many decades ago following the introduction of mechanical harvesting and the recognition of high weed-seed retention levels at crop maturity; however, this opportunity remained largely neglected until more recently. The introduction and adoption of harvest weed seed control (HWSC) systems in Australia has been in response to widespread occurrence of herbicide-resistant weed populations. With diminishing herbicide resources and the need to maintain highly productive reduced tillage and stubble-retention practices, growers began to develop systems that targeted weed seeds during crop harvest. Research and development efforts over the past two decades have established the efficacy of HWSC systems in Australian cropping systems, where widespread adoption is now occurring. With similarly dramatic herbicide resistance issues now present across many of the worlds cropping regions, it is timely for HWSC systems to be considered for inclusion in weed-management programs in these areas. This review describes HWSC systems and establishing the potential for this approach to weed control in several cropping regions. As observed in Australia, the inclusion of HWSC systems can reduce weed populations substantially reducing the potential for weed adaptation and resistance evolution.

PAM: Decision Support for Long-Term Palmer Amaranth (Amaranthus palmeri) Control

Palmer amaranth is the most troublesome weed problem in mid-southern US crop production. Herbicides continue to be the most commonly employed method for managing Palmer amaranth, despite the weeds widespread resistance to them. Therefore, farmers need research and extension efforts that promote the adoption of integrated weed management (IWM) techniques. Producers, crop consultants, educators, and researchers would be more likely to deploy diversified chemical and nonchemical weed management options if they are more informed about long-term biological and economic implications via user-friendly decision-support software. Described within is a recently developed software that demonstrates the effects of Palmer amaranth management practices on soil seedbank, risk of resistance evolution, and economics over a 10-year planning horizon. Aiding this objective is a point-and-click interface that provides feedback on resistance risk, yield potential, profitability, soil seedbank dynamics, and error checking of management options. Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats.

Recurrent Sublethal-Dose Selection for Reduced Susceptibility of Palmer Amaranth (Amaranthus palmeri) to Dicamba

The management of glyphosate-resistant Palmer amaranth has been a challenge in southern United States cropping systems. Registration of dicamba-resistant crops will provide an alternative management option to control herbicide-resistant Palmer amaranth populations, particularly those having resistance to herbicide Groups 2, 3, 5, 9, 14, and 27. However, repeated use of sublethal doses of dicamba may lead to rapid evolution of herbicide resistance, especially in Palmer amaranth—a species with a strong tendency to evolve resistance. Therefore, selection experiments with dicamba were conducted on Palmer amaranth using sublethal doses. In the greenhouse, a known susceptible Palmer amaranth population was subjected to sublethal dicamba doses for three generations (P1–P3). Susceptibility of the individuals to dicamba was evaluated, and its susceptibility to 2,4-D was characterized. Based on the greenhouse study, following three generations of dicamba selection, the dose required to cause 50% mortality increased from 111 g ae ha−1 for parental individuals (P0) to 309 g ae ha−1 for the P3. Furthermore, reduced susceptibility of the P3 to 2,4-D was also evident. This research presents the first evidence that recurrent use of sublethal dicamba doses can lead to reduced susceptibility of Palmer amaranth to dicamba as well as 2,4-D. Here, we show that selection from sublethal dicamba doses has an important role in rapid evolution of Palmer amaranth with reduced susceptibility to auxin-type herbicides. Nomenclature: 2,4-D, dicamba, Palmer amaranth, Amaranthus palmeri S. Wats.

Interaction of femtosecond laser pulses with plants: towards distinguishing weeds and crops using plasma temperature

Abstract The ability to distinguish between crops and weeds using sensors from a distance will greatly benefit the farming community through improved and efficient scouting for weeds, reduced herbicide input costs and improved profitability. In the present study, we examined the utility of femtosecond laser-induced breakdown spectroscopy (LIBS) for plant species differentiation. Greenhouse-grown plants of dallisgrass, wheat, soybean and bell pepper were evaluated using LIBS under an ambient environment. LIBS experiments were performed on the leaf samples of different plant species using a femtosecond laser system with an inexpensive lightweight detector. Temperatures of laser-induced plasma in plants depend on many parameters and were determined for each of the study species by the constituent elements interacting with femtosecond laser pulses. Using elemental calcium transitions in plant tissue samples to measure plasma temperatures, we report consistent differences among the four study species, with average values ranging from 5090 ± 168 K (soybean) to 5647 ± 223 K (dallisgrass).