Studies on the limitations to photosynthesis in leaves of the atrazine-resistant mutant ofSenecio vulgaris L

Deep haplotype analyses of target-site resistance locus ACCase in blackgrass enabled by pool-based amplicon sequencing

Rapid adaptation of weeds to herbicide applications in agriculture through resistance development is a widespread phenomenon. In particular, the grass Alopecurus myosuroides is an extremely problematic weed in cereal crops with the potential to manifest resistance in only a few generations. Target-site resistances (TSRs), with their strong phenotypic response, play an important role in this rapid adaptive response. Recently, using PacBio's long-read amplicon sequencing technology in hundreds of individuals, we were able to decipher the genomic context in which TSR mutations occur. However, sequencing individual amplicons are costly and time-consuming, thus impractical to implement for other resistance loci or applications. Alternatively, pool-based approaches overcome these limitations and provide reliable allele frequencies, although at the expense of not preserving haplotype information. In this proof-of-concept study, we sequenced with PacBio High Fidelity (HiFi) reads long-range amplicons (13.2 kb), encompassing the entire ACCase gene in pools of over 100 individuals, and resolved them into haplotypes using the clustering algorithm PacBio amplicon analysis (pbaa), a new application for pools in plants and other organisms. From these amplicon pools, we were able to recover most haplotypes from previously sequenced individuals of the same population. In addition, we analysed new pools from a Germany-wide collection of A. myosuroides populations and found that TSR mutations originating from soft sweeps of independent origin were common. Forward-in-time simulations indicate that TSR haplotypes will persist for decades even at relatively low frequencies and without selection, highlighting the importance of accurate measurement of TSR haplotype prevalence for weed management.

Triose phosphate utilization and beyond: from photosynthesis to end product synthesis

During photosynthesis, plants fix CO2 from the atmosphere onto ribulose-bisphosphate, producing 3-phosphoglycerate, which is reduced to triose phosphates (TPs). The TPs are then converted into the end products of photosynthesis. When a plant is photosynthesizing very quickly, it may not be possible to commit photosynthate to end products as fast as it is produced, causing a decrease in available phosphate and limiting the rate of photosynthesis to the rate of triose phosphate utilization (TPU). The occurrence of an observable TPU limitation is highly variable based on species and especially growth conditions, with TPU capacity seemingly regulated to be in slight excess of typical photosynthetic rates the plant might experience. The physiological effects of TPU limitation are discussed with an emphasis on interactions between the Calvin-Benson cycle and the light reactions. Methods for detecting TPU-limited data from gas exchange data are detailed and the impact on modeling of some physiological effects are shown. Special consideration is given to common misconceptions about TPU.

Effects of Environmental Conditions on the Fitness Penalty in Herbicide Resistant Brachypodium hybridum

Herbicide-resistance mutations may impose a fitness penalty in herbicide-free environments. Moreover, the fitness penalty associated with herbicide resistance is not a stable parameter and can be influenced by ecological factors. Here, we used two Brachypodium hybridum accessions collected from the same planted forest, sensitive (S) and target-site resistance (TSR) to photosystem II (PSII) inhibitors, to study the effect of agro-ecological parameters on fitness penalty. Both accessions were collected in the same habitat, thus, we can assume that the genetic variance between them is relatively low. This allow us to focus on the effect of PSII TSR on plant fitness. S plants grains were significantly larger than those of the TSR plants and this was associated with a higher rate of germination. Under low radiation, the TSR plants showed a significant fitness penalty relative to S plants. S plants exhibiting dominance when both types of plants were grown together in a low-light environment. In contrast to previous documented studies, under high-light environment our TSR accession didn't show any significant difference in fitness compared to the S accession. Nitrogen deficiency had significant effect on the R compared to the S accession and was demonstrated in significant yield reduction. TSR plants also expressed a high fitness penalty, relative to the S plants, when grown in competition with wheat plants. Two evolutionary scenarios can be suggested to explain the coexistence of both TSR and S plants in the same habitat. The application of PSII inhibitors may have created selective pressure toward TSR dominancy; termination of herbicide application gave an ecological advantage to S plants, creating changes in the composition of the seed bank. Alternatively, the high radiation intensities found in the Mediterranean-like climate may reduce the fitness penalty associated with TSR. Our results may suggest that by integrating non-herbicidal approaches into weed-management programs, we can reduce the agricultural costs associated with herbicide resistance.

A review of physiological and biochemical aspects of resistance to atrazine and paraquat in Hungarian weeds

The most important results in the field of atrazine and paraquat resistance research by Hungarian researchers are reviewed. Pleiotropic effects accompanying atrazine resistance were investigated in atrazine-resistant (AR) and susceptible (S) biotypes of horseweed (Conyza canadensis (L) Cronq). No significant difference in carbon dioxide assimilation rate was found between the AR and S plants. The rates of the Hill reaction of the AR and S chloroplasts exhibited different temperature dependence. The thylakoid membrane lipids contained a lower amount of polar lipid and the fatty acid content exhibited a higher degree of unsaturation in the AR biotype. Photosynthetic apparatus of the AR biotype had better adaptive ability at low temperature and showed enhanced susceptibility to high-temperature stress. AR horseweed plants had reduced activity of xanthophyll cycle, limited capacity of light-induced non-photochemical and photochemical quenching, higher photosensitivity and susceptibility to photo-inhibition. In the case of paraquat resistance, horseweed found in Hungary exhibited a resistance factor of 450; the resistance is not based on an elevated level and activity of the antioxidant enzyme system. The suggested role of polyamines in the resistance mechanisms can be excluded. The higher putrescine and total polyamine content of paraquat-treated R leaves can be regarded as a general stress response rather than as a symptom of paraquat resistance. A paraquat-inducible, nuclear-coded protein, which presumably functions by carrying paraquat to the vacuole, is supposed to play a role in resistance.

Measurements of the quantum yield of carbon assimilation and chlorophyll fluorescence for assessment of photosynthetic performance of crops in the field

An apparatus is described for the rapid measurement in the field of the quantum yield of CO 2 assimilation, ø, and chlorophyll fluorescence kinetics from attached leaves exposed to ambient CO 2 concentrations. This apparatus was used to measure ø and the ratio of variable to maximal fluorescence, F v /F M , of fully expanded leaves of a maize crop in northeast Essex at monthly intervals throughout the growing season. The quantum yield of CO 2 assimilation and F v /F M increased from May to August and then decreased in September. A linear correlation between ø and F v /F M was observed for the leaves. The relations between light, temperature, the quantum yield of CO 2 assimilation and fluorescence emission kinetics of leaves of a maize crop during the early growing season were also examined. Decreases in ø associated with chilling temperatures and high light were observed and identified from analyses of fluorescence kinetics as being attributable to photoinhibitory damage of the photosynthetic apparatus. The possibility of using measurements of ø and fluorescence kinetics for screening the photosynthetic performance of crops is considered. Studies with winter rape demonstrated that changes in ø during the growing season were correlated with changes in the efficiency of light-energy conversion to dry matter by the crop.

Analysis of spatial variation in CO 2 uptake within the intact leaf and its significance in interpreting the effects of environmental stress on photosynthesis

The responses of CO 2 uptake to environmental variables and to stress effects in vivo have been largely limited to measurements of whole leaves or ‘representative’ portions of the leaf. These responses therefore reflect an average for the photosynthetic cells within a leaf, which may differ in environmental pre-history, stage of development and ontogeny. These differences may also result in variation in responses to stress. Improvements in the resolution and accuracy with which gaseous exchanges can be measured in open and closed systems allow the use of small chambers that facilitate a separation of the contributions made by different parts of the leaf to total CO 2 uptake. In amphistomatous leaves with large internal resistances simultaneous measurements of CO 2 , N 2 O and H 2 O vapour allow separation of the contributions made by the upper and lower mesophyll. These techniques are applied to Zea mays leaves to examine (1) heterogeneity in the responses of CO 2 uptake to light and to internal air-space CO 2 pressure fraction, and (2) heterogeneity in the susceptibility to photoinhibition during chilling.

Photoacoustic characteristics of leaves of atrazine-resistant weed mutants

The photosynthetic characteristics of leaves of atrazine-resistant and-susceptible biotypes of several weed species (Solanum nigrum, Senecio vulgaris, Epilobium ciliatum and Chenopodium album) were compared using the photoacoustic method. Analysis of the dependence of the photoacoustic signal of the modulation frequency indicated that, in Solanum, Epilobium and Senecio, the relative quantum yield of O2 evolution ϕ (estimated by the ratio of the amplitude of the O2 signal, AOX, to that of the photothermal signal, APT) was substantially reduced in the atrazine-resistant mutant, without any changes in the O2 diffusion characteristics of the leaves. In contrast, in Chenopodium, atrazine-resistance was associated with a concomitant change in ϕ and in the leaf diffusion parameters. This latter change suggests that the leaf internal anatomy was modified in the resistant Chenopodium. Measurements of the Emerson enhancement indicated that the reduction of ϕ observed in the atrazine-resistant mutants was caused by a marked decrease in the photochemical potential of PS II (β). The study of the light intensity dependence of the AOX/APT ratio showed that saturation of O2 evolution occurred at the same light level (around 2000 μmol m(-2) s(-1)) in both types of plants. However, the relative maximal rate of O2 evolution was slightly lower (-10%) in the atrazine-resistant biotype as compared to the wild type. Reduced ϕ and light-saturated rate of O2 evolution were also measured in atrazine-resistant weed biotypes using a conventional Clark-type O2 electrode.