Effects of exogenous calcium on the drought response of the tea plant (Camellia sinensis (L.) Kuntze)

Background

Drought is one of the major factors reducing the yield of many crops worldwide, including the tea crop (Camellia sinensis (L.) Kuntze). Calcium participates in most of cellular signaling processes, and its important role in stress detection and triggering a response has been shown in many crops. The aim of this study was to evaluate possible effects of calcium on the tea plant response to drought.

Methods

Experiments were conducted using 3-year-old potted tea plants of the best local cultivar Kolkhida. Application of ammonium nitrate (control treatment) or calcium nitrate (Ca treatment) to the soil was performed before drought induction. Next, a 7-day drought was induced in both groups of plants. The following physiological parameters were measured: relative electrical conductivity, pH of cell sap, and concentrations of cations, sugars, and amino acids. In addition, relative expression levels of 40 stress-related and crop quality-related genes were analyzed.

Results

Under drought stress, leaf electrolyte leakage differed significantly, indicating greater damage to cell membranes in control plants than in Ca-treated plants. Calcium application resulted in greater pH of cell sap; higher accumulation of tyrosine, methionine, and valine; and a greater Mg²⁺ content as compared to control plants. Drought stress downregulated most of the quality-related genes in both groups of tea plants. By contrast, significant upregulation of some genes was observed, namely CRK45, NAC26, TPS11, LOX1, LOX6, Hydrolase22, DREB26, SWEET2, GS, ADC, DHN2, GOLS1, GOLS3, and RHL41. Among them, three genes (LOX1, RHL41, and GOLS1) showed 2-3 times greater expression in Ca-treated plants than in control plants. Based on these results, it can be speculated that calcium affects galactinol biosynthesis and participates in the regulation of stomatal aperture not only through activation of abscisic-acid signaling but also through jasmonic-acid pathway activation. These findings clarify calcium-mediated mechanisms of drought defense in tree crops. Thus, calcium improves the drought response in the tea tree.

Genetic Diversity and Genome Size Variability in the Russian Genebank Collection of Tea Plant [Camellia sinensis (L). O. Kuntze]

The tea collection of the FRC SSC RAS (Sochi, Maykop in Russia) represents one of the northernmost germplasm comprising a number of locally derived cultivars and ɣ-irradiation mutants. The latter are often characterized by larger genome size, which may lead to better adaptation to biotic and abiotic stress. Such genotypes may be a valuable genetic resource for better adaptability to extreme environmental conditions, which could enable tea cultivation outside global growing regions. Microsatellite markers are often the best choice for genetic diversity analysis in genebank collections. However, their use in polyploid species is questionable because simple sequence repeat (SSR) allele dosage cannot be readily determined. Therefore, the efficiency of SSR and start codon targeted (SCoT) markers was investigated using 43 selected cultivars from the Russian genebank collection derived from mutant breeding and clonal selection. Previously, the increase in genome size was confirmed in 18 mutants within this collection. Despite the presence of polyploid tea genotypes, our study revealed higher efficiency of SSR markers than SCoT markers. Subsequent SSR analysis of the 106 genotypes in the Russian genebank collection revealed three distinct genetic clusters after STRUCTURE analysis. Greater genetic variation was observed within genetic clusters than between clusters, indicating low genetic variation between collections. Nevertheless, the northernmost tea collection exhibited a greater genetic distance from the other two clusters than they did from each other. Close genetic relationships were found between many cultivars with particularly large leaves and mutant forms. Pearson's correlation analysis revealed a significant, moderate correlation between genome size and leaf area size. Our study shows that microsatellite fingerprinting is useful to estimate the genetic diversity and genetic background of tea germplasm in Russia despite polyploid tea accessions. Thus, the results of our study contribute to the development of future tea germplasm conservation strategies and modern tea breeding programs.

[Comparative analysis of gene expression in tea plant (Camellia sinensis (L.) Kuntze) under low-temperature stress]

Низкотемпературный стресс – один из главных факторов, ограничивающих распространение и снижающих урожайность многих субтропических культур, в том числе и чая. Для эффективной селекции чая на устойчивость к морозу необходимо выявить генетические особенности ответа на холод у устойчивых генотипов и найти маркеры для определения доноров устойчивости в коллекциях. В настоящей работе про- веден сравнительный анализ экспрессии 18 генов (ICE1, CBF1, DHN1, DHN2, DHN3, NAC17, NAC26, NAC30, bHLH7, bHLH43, P5CS, WRKY2, LOX1, LOX6, LOX7, SnRK1.1, SnRK1.2, SnRK1.3), вовлеченных в абиотический стрессовый ответ у двух контрастных по устойчивости генотипов чая в условиях холода и мороза. Низкотемператур- ный стресс индуцировали путем помещения растений в холодильные камеры и снижением температуры до 0…+2 °С на семь дней (холодовой стресс) с последующим снижением температуры до –4…–6 °С на пять дней (промораживание). Кондуктометрическим методом измеряли электропроводность тканей листа, в ре- зультате чего были подтверждены различия по признаку устойчивости у двух исследуемых генотипов чая: холодовое воздействие не приводило к изменению электропроводности тканей листа, но после промора- живания этот показатель возрастал в большей степени у неустойчивого генотипа. Методом qRT-PCR анализи- ровали относительный уровень экспрессии генов на фоне референсного гена актина. При индукции стресса показана повышенная экспрессия всех исследуемых генов. У устойчивого генотипа чая выявлен ряд генов, более активно экспрессирующихся по сравнению с неустойчивым генотипом: ICE1, CBF1, DHN2, NAC17, NAC26, bHLH43, WRKY2, P5CS, LOX6, SnRK1.1, SnRK1.3. Эти гены могут быть маркерами устойчивости для поиска доно- ров в коллекциях геноресурсов. Показано, что у устойчивого генотипа чая экспрессия генов холодового от- вета начинается уже на стадии акклиматизации. Для дальнейших исследований комплексной устойчивости растений к низкотемпературному стрессу актуальным является изучение экспрессии этих генов в других органах чайного растения (побегах, корнях) при разной силе низкотемпературного воздействия.

Comparative Expression Analysis of Stress-Inducible Candidate Genes in Response to Cold and Drought in Tea Plant [Camellia sinensis (L.) Kuntze]

Cold and drought are two of the most severe threats affecting the growth and productivity of the tea plant, limiting its global spread. Both stresses cause osmotic changes in the cells of the tea plant by decreasing their water potential. To develop cultivars that are tolerant to both stresses, it is essential to understand the genetic responses of tea plant to these two stresses, particularly in terms of the genes involved. In this study, we combined literature data with interspecific transcriptomic analyses (using Arabidopsis thaliana and Solanum lycopersicum) to choose genes related to cold tolerance. We identified 45 stress-inducible candidate genes associated with cold and drought responses in tea plants based on a comprehensive homologous detection method. Of these, nine were newly characterized by us, and 36 had previously been reported. The gene network analysis revealed upregulated expression in ICE1-related cluster of bHLH factors, HSP70/BAM5 connected genes (hexokinases, galactinol synthases, SnRK complex, etc.) indicating their possible co-expression. Using qRT-PCR we revealed that 10 genes were significantly upregulated in response to both cold and drought in tea plant: HSP70, GST, SUS1, DHN1, BMY5, bHLH102, GR-RBP3, ICE1, GOLS1, and GOLS3. SnRK1.2, HXK1/2, bHLH7/43/79/93 were specifically upregulated in cold, while RHL41, CAU1, Hydrolase22 were specifically upregulated in drought. Interestingly, the expression of CIP was higher in the recovery stage of both stresses, indicating its potentially important role in plant recovery after stress. In addition, some genes, such as DHN3, bHLH79, PEI54, SnRK1.2, SnRK1.3, and Hydrolase22, were significantly positively correlated between the cold and drought responses. CBF1, GOLS1, HXK2, and HXK3, by contrast, showed significantly negative correlations between the cold and drought responses. Our results provide valuable information and robust candidate genes for future functional analyses intended to improve the stress tolerance of the tea plant and other species.

Physiological, biochemical and genetic responses of Caucasian tea (Camellia sinensis (L.) Kuntze) genotypes under cold and frost stress

Background

Cold and frost are two serious factors limiting the yield of many crops worldwide, including the tea plant (Camellia sinensis (L.) Kuntze). The acclimatization of tea plant from tropical to temperate climate regions resulted in unique germplasm in the North–Western Caucasus with extremely frost-tolerant genotypes.

Methods

The aim of the current research was to evaluate the physiological, biochemical and genetic responses of tolerant and sensitive tea cultivars exposed to cold (0 to +2 °C for 7 days) and frost (−6 to −8 °C for 5 days). Relative water content, cell membranes integrity, pH of the cell sap, water soluble protein, cations, sugars, amino acids were measured under cold and frost. Comparative expression of the following genes ICE1, CBF1, WRKY2, DHN1, DHN2, DHN3, NAC17, NAC26, NAC30, SnRK1.1, SnRK1.2, SnRK1.3, bHLH7, bHLH43, P5CS, LOX1, LOX6, LOX7 were analyzed.

Results

We found elevated protein (by 3–4 times) and cations (potassium, calcium and magnesium) contents in the leaves of both cultivars under cold and frost treatments. Meanwhile, Leu, Met, Val, Thr, Ser were increased under cold and frost, however tolerant cv. Gruzinskii7 showed earlier accumulation of these amino acids. Out of 18 studied genes, 11 were expressed at greater level in the frost- tolerant cultivar comparing with frost-sensitive one: ICE1, CBF1, WRKY2, DHN2, NAC17, NAC26, SnRK1.1, SnRK1.3, bHLH43, P5CS and LOX6. Positive correlations between certain amino acids namely, Met, Thr, Leu and Ser and studied genes were found. Taken together, the revealed cold responses in Caucasian tea cultivars help better understanding of tea tolerance to low temperature stress and role of revealed metabolites need to be further evaluated in different tea genotypes.