Sydnone imines as a new class of promising plant growth modulators and potential drought-protective phytoeffectors – a first experimental structure-activity-analysis overview

Due to the oncoming climate changes, drought is the factor dramatically affecting crop yields and quality of produced foods. Therefore, to meet the growing food demand of the human population, improvement of drought tolerance of the currently cultured crops is required. The knowledge of the molecular mechanisms behind, e.g., drought tolerance provides a versatile instrument to correct the plant metabolism by chemical tools and to increase, thereby, their adaptive potential. This will preserve crop productivity and quality under water deficit conditions. Endogenously produced nitric oxide (NO) is one of the key signaling factors activating drought tolerance mechanisms in plants. Thus, application of synthetic NO donors as drought-protective phytoeffectors might support maintaining plant growth and productivity under dehydration conditions. Sydnone imines (Sydnonimines) are a class of clinically established mesoionic heterocyclic NO donors which represent a promising candidate group for such phytoeffectors. Therefore, here we provide an overview of the current progress in application of sydnone imines as exogenous NO donors in plants with a special emphasis on their potential as herbicides as well as herbicide antidotes, growth stimulants and suggest a deeper look into them as drought protectors, applicable to different plant species. We specifically address the structure-activity relationships in the context of the growth modulating activity of sydnone imines. According to the actual state of the art, growth-stimulating or antidote effects sre typical for 4-(α-hydroxybenzyl) derivatives of sydnone imines containing an alkyl substituent in position N-3. The nature of the substituent of the N-6 atom has a significant influence on the activity profile and the intensity of the effect. Nevertheless, further investigations are necessary to establish reliable structure-activity relationships (SAR). Consequently, sydnone imines might be considered as promising phytoeffector candidates, which are expected to exert either protective effects on plants growing under unfavorable conditions, or herbicidal ones, depending on the exact structure