During plant embryogenesis, regardless of whether it begins with a fertilized egg cell (zygotic embryogenesis) or an induced somatic cell (somatic embryogenesis), significant epigenetic reprogramming occurs with the purpose of parental or vegetative transcript silencing and establishment of a next-generation epigenetic patterning. To ensure genome stability of a developing embryo, large-scale transposon silencing occurs by an RNA-directed DNA methylation (RdDM) pathway, which introduces methylation patterns de novo and as such potentially serves as a global mechanism of transcription control during developmental transitions. RdDM is controlled by a two-armed mechanism based around the activity of two RNA polymerases. While PolIV produces siRNAs accompanied by protein complexes comprising the methylation machinery, PolV produces lncRNA which guides the methylation machinery toward specific genomic locations. Recently, RdDM has been proposed as a dominant methylation mechanism during gamete formation and early embryo development in Arabidopsis thaliana, overshadowing all other methylation mechanisms. Here, we bring an overview of current knowledge about different roles of DNA methylation with emphasis on RdDM during plant zygotic and somatic embryogenesis. Based on published chromatin immunoprecipitation data on PolV binding sites within the A. thaliana genome, we uncover groups of auxin metabolism, reproductive development and embryogenesis-related genes, and discuss possible roles of RdDM at the onset of early embryonic development via targeted methylation at sites involved in different embryogenesis-related developmental mechanisms.
Rising temperatures and pronounced drought are significantly affecting biodiversity worldwide and reducing yields and quality of Brassica crops. To elucidate the mechanisms of tolerance, 33 kale accessions (B. oleracea var. acephala) were evaluated for individual (osmotic and elevated temperature stress) and combined stress (osmotic + temperature). Using root growth, biomass and proline content as reliable markers, accessions were evaluated for stress responses. Four representatives were selected for further investigation (photosynthetic performance, biochemical markers, sugar content, specialized metabolites, transcription level of transcription factors NAC, HSF, DREB and expression of heat shock proteins HSP70 and HSP90): very sensitive (392), moderately sensitive (395), tolerant (404) and most tolerant (411). Accessions more tolerant to stress conditions were characterized by higher basal content of proline, total sugars, glucosinolates and higher transcription of NAC and DREB. Under all stress conditions, 392 was characterized by a significant decrease in biomass, root growth, photosynthesis performance, fructan content, especially under osmotic and combined stress, a significant increase in HSF transcription and HSP accumulation under temperature stress and a significant decrease in NAC transcription under all stresses. The most tolerant accession under all applied stresses, 411 showed the least changes in all analyzed parameters compared with the other accessions.
Tomatoes are one of the most important vegetables for human consumption. In the Mediterranean’s semi-arid and arid regions, where tomatoes are grown in the field, global average surface temperatures are predicted to increase. We investigated tomato seed germination at elevated temperatures and the impact of two different heat regimes on seedlings and adult plants. Selected exposures to 37 °C and heat waves at 45 °C mirrored frequent summer conditions in areas with a continental climate. Exposure to 37 °C or 45 °C differently affected seedlings’ root development. Both heat stresses inhibited primary root length, while lateral root number was significantly suppressed only after exposure to 37 °C. Heat stress treatments induced significant accumulation of indole-3-acetic acid (IAA) and reduced abscisic acid (ABA) levels in seedlings. As opposed to the heat wave treatment, exposure to 37 °C increased the accumulation of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), which may have been involved in the root architecture modification of seedlings. Generally, more drastic phenotypic changes (chlorosis and wilting of leaves and bending of stems) were found in both seedlings and adult plants after the heat wave-like treatment. This was also reflected by proline, malondialdehyde and heat shock protein HSP90 accumulation. The gene expression of heat stress-related transcription factors was perturbed and DREB1 was shown to be the most consistent heat stress marker.
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