Evolutionary diversification of cytokinin-specific glucosyltransferases in angiosperms and enigma of missing cis-zeatin O-glucosyltransferase gene in Brassicaceae

Spatially distributed cytokinins: Metabolism, signaling, and transport

Cytokinins are mobile phytohormones that regulate plant growth, development, and environmental adaptability. The major cytokinin species include isopentenyl adenine (iP), trans-zeatin (tZ), cis-zeatin (cZ), and dihydrozeatin (DZ). The spatial distributions of different cytokinin species in different organelles, cells, tissues, and organs are primarily shaped by biosynthesis via isopentenyltransferases (IPT), cytochrome P450 monooxygenase, and 5'-ribonucleotide phosphohydrolase and by conjugation or catabolism via glycosyltransferase or cytokinin oxidase/dehydrogenase. Cytokinins bind to histidine receptor kinases in the endoplasmic reticulum or plasma membrane and relay signals to response regulators in the nucleus via shuttle proteins known as histidine phosphotransfer proteins. The movements of cytokinins from sites of biosynthesis to sites of signal perception usually require long-distance, intercellular, and intracellular transport. In the past decade, ATP-binding cassette (ABC) transporters, purine permeases (PUP), AZA-GUANINE RESISTANT (AZG) transporters, equilibrative nucleoside transporters (ENT), and Sugars Will Eventually Be Exported transporters (SWEET) have been characterized as involved in cytokinin transport processes. This review begins by introducing the spatial distributions of various cytokinins and the subcellular localizations of the proteins involved in their metabolism and signaling. Highlights focus on an inventory of the characterized transporters involved in cytokinin compartmentalization, including long-distance, intercellular, and intracellular transport, and the regulation of the spatial distributions of cytokinins by environmental cues. Future directions for cytokinin research are also discussed.

Whole-genome sequencing of tetraploid potato varieties reveals different strategies for drought tolerance

Climate changes leading to increasingly longer seasonal drought periods in large parts of the world increase the necessity for breeding drought-tolerant crops. Cultivated potato (Solanum tuberosum), the third most important vegetable crop worldwide, is regarded as drought-sensitive due to its shallow root architecture. Two German tetraploid potato cultivars differing in drought tolerance and their F1-progeny were evaluated under various drought scenarios. Bulked segregant analyses were combined with whole-genome sequencing (BSA-Seq) using contrasting bulks of drought-tolerant and drought-sensitive F1-clones. Applying QTLseqr, 15 QTLs comprising 588,983 single nucleotide polymorphisms (SNPs) in 2325 genes associated with drought stress tolerance were identified. SeqSNP analyses in an association panel of 34 mostly starch potato varieties using 1-8 SNPs for each of 188 selected genes narrowed the number of candidate genes down to 10. In addition, ent-kaurene synthase B was the only gene present under QTL 10. Eight of the identified genes (StABP1, StBRI1, StKS, StLEA, StPKSP1, StPKSP2, StYAB5, and StZOG1) address plant development, the other three genes (StFATA, StHGD and StSYP) contribute to plant protection under drought stress. Allelic variation in these genes might be explored in future breeding for drought-tolerant potato varieties.

Identification of a Comprehensive Gene Co-Expression Network Associated with Autotetraploid Potato (Solanum tuberosum L.) Development Using WGCNA Analysis

The formation and development of potato tissues and organs is a complex process regulated by a variety of genes and environmental factors. The regulatory mechanisms underlying the growth and development are still unclear. In this work, we aimed to explore the changes in gene expression patterns and genetic characteristics of potato tissues throughout different developmental stages. To achieve this, we used autotetraploid potato JC14 as an experimental subject to analyze the transcriptome of the root, stem, and leaf at the seedling, tuber formation, and tuber expansion stages. The results revealed thousands of differentially expressed genes, predominantly involved in defense response and carbohydrate metabolism according to KEGG pathway enrichment analysis. Weighted gene co-expression network analysis (WGCNA) revealed a total of 12 co-expressed gene modules, with 4 modules showing the highest correlation with potato stem development. By calculating the connectivity of genes within the module, hub genes were identified, and functional annotations were subsequently performed. A total of 40 hub genes from the four modules were identified, and their functions were found to be related to carbohydrate metabolism, defense response, and transcription factors. These findings provide important insights for further understanding of the molecular regulation and genetic mechanisms involved in potato tissue development.

Molecular Regulation and Evolution of Cytokinin Signaling in Plant Abiotic Stresses

The sustainable production of crops faces increasing challenges from global climate change and human activities, which leads to increasing instances of many abiotic stressors to plants. Among the abiotic stressors, drought, salinity and excessive levels of toxic metals cause reductions in global agricultural productivity and serious health risks for humans. Cytokinins (CKs) are key phytohormones functioning in both normal development and stress responses in plants. Here, we summarize the molecular mechanisms on the biosynthesis, metabolism, transport and signaling transduction pathways of CKs. CKs act as negative regulators of both root system architecture plasticity and root sodium exclusion in response to salt stress. The functions of CKs in mineral-toxicity tolerance and their detoxification in plants are reviewed. Comparative genomic analyses were performed to trace the origin, evolution and diversification of the critical regulatory networks linking CK signaling and abiotic stress. We found that the production of CKs and their derivatives, pathways of signal transduction and drought-response root growth regulation are evolutionarily conserved in land plants. In addition, the mechanisms of CK-mediated sodium exclusion under salt stress are suggested for further investigations. In summary, we propose that the manipulation of CK levels and their signaling pathways is important for plant abiotic stress and is, therefore, a potential strategy for meeting the increasing demand for global food production under changing climatic conditions.

Desiccation as a Post-maturation Treatment Helps Complete Maturation of Norway Spruce Somatic Embryos: Carbohydrates, Phytohormones and Proteomic Status

Exposure of Norway spruce (Picea abies) somatic embryos and those of many other conifers to post-maturation desiccation treatment significantly improves their germination. An integration analysis was conducted to understand the underlying processes induced during the desiccation phase at the molecular level. Carbohydrate, protein and phytohormone assays associated with histological and proteomic studies were performed for the evaluation of markers and actors in this phase. Multivariate comparison of mature somatic embryos with mature desiccated somatic embryos and/or zygotic embryos provided new insights into the processes involved during the desiccation step of somatic embryogenesis. Desiccated embryos were characterized by reduced levels of starch and soluble carbohydrates but elevated levels of raffinose family oligosaccharides. Desiccation treatment decreased the content of abscisic acid and its derivatives but increased total auxins and cytokinins. The content of phytohormones in dry zygotic embryos was lower than in somatic embryos, but their profile was mostly analogous, apart from differences in cytokinin profiles. The biological processes "Acquisition of desiccation tolerance", "Response to stimulus", "Response to stress" and "Stored energy" were activated in both the desiccated somatic embryos and zygotic embryos when compared to the proteome of mature somatic embryos before desiccation. Based on the specific biochemical changes of important constituents (abscisic acid, raffinose, stachyose, LEA proteins and cruciferins) induced by the desiccation treatment and observed similarities between somatic and zygotic P. abies embryos, we concluded that the somatic embryos approximated to a state of desiccation tolerance. This physiological change could be responsible for the reorientation of Norway spruce somatic embryos toward a stage suitable for germination.