We investigated the modification of cold-induced mechanisms of photosynthetic apparatus adjustment and phytohormone response by brassinosteroid 24-epibrassinolide (EBR) and its consequences for frost tolerance of perennial ryegrass (Lolium perenne L.). We recorded the responses of two cultivars with contrasting frost tolerances to foliar hormone application, both in non-acclimated plants and plants cold acclimated for 3 and 6 weeks at 4 °C. In non-cold-acclimated plants of both cultivars, EBR induced increases in carbon fixation and lowered sucrose levels. Temporary suppression in quantum efficiency of PSII of photosystem II and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase and sucrose phosphate synthase, a consequence of energy dissipation in non-photochemical quenching, was observed in the leaves of the highly frost-tolerant cultivar after 3 weeks of cold acclimation. After 6 weeks of cold acclimation, EBR accelerated recovery of photosynthesis, reflecting adjustment to cold conditions, and increased frost tolerance. As carbohydrate export from leaves is favored during cold acclimation, EBR application did not increase frost tolerance of the moderately tolerant cultivar, reflecting the downregulation of photosynthesis due to high leaf sucrose concentrations. It is also likely that EBR participated in the enhancement of frost tolerance by regulation of stress-related signaling compounds such as JA and ethylene but not SA, in winter ryegrass undergoing cold acclimation. Taken together, our results demonstrate that EBR-induced changes are temperature dependent. The beneficial effect of EBR is not universal under cold conditions, as genetically determined mechanisms are apparently dominant relative to EBR action.

Heterodera schachtii is a common parasite of many important crops such as beets and Brassicaceae (oilseed rape, cabbage or mustard). Arabidopsis thaliana is a model plant also used for studying defence responses to pathogens or pest infections. Defence responses of plants are often regulated and fine-tuned by stress phytohormones: salicylic acid (SA), jasmonic acid (JA), ethylene (Et) and abscisic acid (ABA), of which the role of ABA in these responses is the least examined. The aim of this study was to show, if and which genes related to ABA turnover can be modulated during the development of nematode-induced feeding sites in A. thaliana roots. To answer the question, we performed infection tests on wild type and ABA mutant roots and analysed the expression levels of selected ABA-related genes (ABI1, ABI2, ABI5, PYL5, PYL6, CYP707A1 and CYP707A4) at the early stage of root infection. Our results show that the expression of ABI2, ABI5 (ABA signalling) and CYP707A4 (ABA metabolism) genes was upregulated in feeding sites at 4 dpi, whereas the level of expression of PYL5 and PYL6 (ABA receptors) genes was decreased. Mutations in ABI1, ABI2, ABI5, CYP707A1 or CYP707A4 genes led to a decrease of A. thaliana susceptibility verbalised as the number of fully developed females, whereas mutations in PYL5 or PYL6 genes did not influence the number of females of the nematode. Based on the results, it can be concluded that the modifications of analysed ABA-related gene expression are required for the proper development of nematodes; however, further in-depth analyses are required.