Design, Synthesis and Biological Evaluation of Strigolactone and Strigolactam Derivatives for Potential Crop Enhancement Applications in Modern Agriculture.

Strigolactones have been known as signaling molecules in the rhizosphere of plants since more than 50 years. However, their roles as phytohormone have been only recognized since 2008. We describe here a very efficient synthetic access to representative canonical strigolactones displaying the A-B-C-D tetracyclic structure and to non-canonical strigolactones as carlactonoic acid and methyl carlactonoate. In addition, we report the design and the synthesis of strigolactams as promising derivatives of strigolactones for potential use in modern agriculture. Among the synthetic methods developed for this project, the intramolecular [2+2] cycloaddition of keteneiminium salts to C=C bond has been particularly useful to the synthesis of natural strigolactones and their potentially improved analogues.

Strigolactones: Plant Hormones with Promising Features.

Almost 80 years after the discovery of the first plant hormone, auxin, a few years ago a new class of plant hormones, the strigolactones, was discovered. These molecules have unprecedented biological activity in a number of highly important biological processes in plants but also outside the plant in the rhizosphere, the layer of soil surrounding the roots of plants and teeming with life. The exploitation of this amazing biological activity is not without challenges: the synthesis of strigolactones is complicated and designing the desired activity a difficult task. This minireview describes the current state of knowledge about the strigolactones and how synthetic analogs can be developed that can potentially contribute to the development of a sustainable agriculture.

Strigolactone derivatives for potential crop enhancement applications.

New technologies able to mitigate the main abiotic stresses (i.e., drought, salinity, cold and heat) represent a substantial opportunity to contribute to a sustainable increase of agricultural production. In this context, the recently discovered phytohormone strigolactone is an important area of study which can underpin the quest for new anti-stress technologies. The pleiotropic roles played by strigolactones in plant growth/development and in plant adaptation to environmental changes can pave the way for new innovative crop enhancement applications. Although a significant scientific effort has been dedicated to the strigolactone subject, an updated review with emphasis on the crop protection perspective was missing. This paper aims to analyze the advancement in different areas of the strigolactone domain and the implications for agronomical applications.

Simplified strigolactams as potent analogues of strigolactones for the seed germination induction of Orobanche cumana Wallr.

BACKGROUND: Strigolactones play an important role in the rhizosphere as signalling molecules stimulating the seed germination of parasitic weed seeds and hyphal branching of arbuscular micorrhiza, and also act as hormones in plant roots and shoots. Strigolactone derivatives, e.g. strigolactams, could be used as suicidal germination inducers in the absence of a host crop for the decontamination of land infested with parasitic weed seeds. RESULTS: We report the stereoselective synthesis of novel strigolactams, together with some of their critical physicochemical properties, such as water solubility, hydrolytic stability, as well as their short soil persistence. In addition, we show that such strigolactams are potent germination stimulants of O. cumana parasitic weed seeds and do not affect the seed germination and the root growth of sunflower. CONCLUSIONS: The novel strigolactam derivatives described here compare favourably with the corresponding GR-28 strigolactones in terms of biological activity and physicochemical properties. However, we believe strigolactone and strigolactam derivatives require further structural optimisation to improve their soil persistence to demonstrate a potential for agronomical applications. © 2016 Society of Chemical Industry.

Chemicals inducing seed germination and early seedling development.

Seed germination and early seedling development are essential events in the plant life cycle that are controlled largely by the interplay and cross-talk between several plant hormones. Recently, major progress has been achieved in the elucidation at the molecular level of the signalling of these phytohormones. In this review, we summarise the data for the most promising classes of compounds, which could find potential agronomic applications for promoting seed germination and early seedling development even under abiotic stress conditions. Structural modifications of plant hormones are required to improve their biological performance and their specificity to allow commercial application.

Mutagenesis and functional studies with succinate dehydrogenase inhibitors in the wheat pathogen Mycosphaerella graminicola.

A range of novel carboxamide fungicides, inhibitors of the succinate dehydrogenase enzyme (SDH, EC 1.3.5.1) is currently being introduced to the crop protection market. The aim of this study was to explore the impact of structurally distinct carboxamides on target site resistance development and to assess possible impact on fitness. We used a UV mutagenesis approach in Mycosphaerella graminicola, a key pathogen of wheat to compare the nature, frequencies and impact of target mutations towards five subclasses of carboxamides. From this screen we identified 27 amino acid substitutions occurring at 18 different positions on the 3 subunits constituting the ubiquinone binding (Qp) site of the enzyme. The nature of substitutions and cross resistance profiles indicated significant differences in the binding interaction to the enzyme across the different inhibitors. Pharmacophore elucidation followed by docking studies in a tridimensional SDH model allowed us to propose rational hypotheses explaining some of the differential behaviors for the first time. Interestingly all the characterized substitutions had a negative impact on enzyme efficiency, however very low levels of enzyme activity appeared to be sufficient for cell survival. In order to explore the impact of mutations on pathogen fitness in vivo and in planta, homologous recombinants were generated for a selection of mutation types. In vivo, in contrast to previous studies performed in yeast and other organisms, SDH mutations did not result in a major increase of reactive oxygen species levels and did not display any significant fitness penalty. However, a number of Qp site mutations affecting enzyme efficiency were shown to have a biological impact in planta.Using the combined approaches described here, we have significantly improved our understanding of possible resistance mechanisms to carboxamides and performed preliminary fitness penalty assessment in an economically important plant pathogen years ahead of possible resistance development in the field.

Mandipropamid targets the cellulose synthase-like PiCesA3 to inhibit cell wall biosynthesis in the oomycete plant pathogen, Phytophthora infestans.

Oomycete plant pathogens cause a wide variety of economically and environmentally important plant diseases. Mandipropamid (MPD) is a carboxylic acid amide (CAA) effective against downy mildews, such as Plasmopara viticola on grapes and potato late blight caused by Phytophthora infestans. Historically, the identification of the mode of action of oomycete-specific control agents has been problematic. Here, we describe how a combination of biochemical and genetic techniques has been utilized to identify the molecular target of MPD in P. infestans. Phytophthora infestans germinating cysts treated with MPD produced swelling symptoms typical of cell wall synthesis inhibitors, and these effects were reversible after washing with H(2)O. Uptake studies with (14)C-labelled MPD showed that this oomycete control agent acts on the cell wall and does not enter the cell. Furthermore, (14)C glucose incorporation into cellulose was perturbed in the presence of MPD which, taken together, suggests that the inhibition of cellulose synthesis is the primary effect of MPD. Laboratory mutants, insensitive to MPD, were raised by ethyl methane sulphonate (EMS) mutagenesis, and gene sequence analysis of cellulose synthase genes in these mutants revealed two point mutations in the PiCesA3 gene, known to be involved in cellulose synthesis. Both mutations in the PiCesA3 gene result in a change to the same amino acid (glycine-1105) in the protein. The transformation and expression of a mutated PiCesA3 allele was carried out in a sensitive wild-type isolate to demonstrate that the mutations in PiCesA3 were responsible for the MPD insensitivity phenotype.

Recognition of RNA by amide modified backbone nucleic acids: molecular dynamics simulations of DNA-RNA hybrids in aqueous solution.

Thermodynamic and structural properties of a chemically modified DNA-RNA hybrid in which a phosphodiester linkage is replaced by a neutral amide-3 linkage (3'-CH(2)-CONH-5') were investigated using UV melting experiments, molecular dynamics simulations in explicit water, and continuum solvent models. van't Hoff analysis of the experimental UV melting curves suggests that the significant increase of the thermodynamic stability of a 15-mer DNA-RNA with seven alternated amide-3 modifications (+11 degrees C) is mainly due to an increased binding enthalpy. To further evaluate the origin in the observed affinities differences, the electrostatic contribution to the binding free energy was calculated by solving the Poisson-Boltzmann equation numerically. The nonelectrostatic contribution was estimated as the product of a hydrophobic surface tension coefficient and the surface area that is buried upon double strand formation. Structures were taken from 10 ns molecular dynamics simulations computed in a consistent fashion using explicit solvent, counterions, and the particle-mesh Ewald procedure. The present preliminary thermodynamic study suggests that the favorable binding free energy of the amide-3 DNA single strand to the complementary RNA is equally driven by electrostatic and nonpolar contributions to the binding compared to their natural analogues. In addition, molecular dynamics simulations in explicit water were performed on an amide-3 DNA single strand and the corresponding natural DNA. Results from the conformations cluster analysis of the simulated amide-3 DNA single strand ensembles suggest that the 25% of the population sampled within 10 ns has a pre-organized conformation where the sugar C3' endo pucker is favored at the 3'-flanking nucleotides. These structural and thermodynamic features contribute to the understanding of the observed increased affinities of the amide-3 DNA-RNA hybrids at the microscopic level.

Correct identification of the chloroplastic protoporphyrinogen IX oxidase N-terminus places the biochemical data in frame.

Maize (Zea mays) protoporphyrinogen IX oxidase (PPO: EC 1.3.3.4) possesses a chloroplast transit peptide (CTP) that delivers the enzyme into the chloroplast. The cleavage site yielding the mature protein was predicted by using the ChloroP software and by comparing conserved regions of the available plant PPO sequences. In parallel, the processed NH(2)-terminus of native PPO was identified experimentally by microsequencing the immunoprecipitated plant PPO from maize etioplasts. The cleavage sites identified using the bioinformatic approaches did not match the experimental result. The three sequences have been cloned and expressed in bacteria and their kinetics were compared in order to understand if the generated proteins had biochemically relevant differences. Recombinant PPO corresponding to the native PPO accumulated at higher level and was more active than the two homologues. A cysteine present in the CTP seems to be able to modify the redox state of the enzyme and to be responsible for the alteration of the kinetic features. In contrast, the sensitivity to different herbicides was unaffected by modifications at the NH(2)-terminus, suggesting that the mode of action is non-competitive and that the NH(2)-terminus is involved in the recognition of the natural substrate.