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Magnesium (Mg) plays important roles in photosynthesis, sucrose partitioning, and biomass allocation in plants. However, the specific mechanisms of tea plant response to Mg deficiency remain unclear. In this study, we investigated the effects of Mg deficiency on the quality constituents of tea leaves. Our results showed that the short-term (7 days) Mg deficiency partially elevated the concentrations of polyphenols, free amino acids, and caffeine but decreased the contents of chlorophyll and Mg. However, long-term (30 days) Mg-deficient tea displayed decreased contents of these constituents. Particularly, Mg deficiency increased the index of catechins' bitter taste and the ratio of total polyphenols to total free amino acids. Moreover, the transcription of key genes involved in the biosynthesis of flavonoid, caffeine, and theanine was differentially affected by Mg deficiency. Additionally, short-term Mg deficiency induced global transcriptome change in tea leaves, in which a total of 2522 differentially expressed genes were identified involved in secondary metabolism, amino acid metabolism, and chlorophyll metabolism. These results may help to elucidate why short-term Mg deficiency partially improves the quality constituents of tea, while long-term Mg-deficient tea may taste more bitter, more astringent, and less umami.

Nitrogen (N) is an important contributor in regulating plant growth and development as well as secondary metabolites synthesis, so as to promote the formation of tea quality and flavor. Theanine, polyphenols, and caffeine are important secondary metabolites in tea plant. In this study, the responses of

Cation diffusion facilitators (CDFs) play central roles in metal homeostasis and tolerance in plants, but the specific functions of Camellia sinensis CDF-encoding genes and the underlying mechanisms remain unknown. Previously, transcriptome sequencing results in our lab indicated that the expression of CsMTP8.2 in tea plant shoots was down-regulated exposed to excessive amount of Mn2+ conditions. To elucidate the possible mechanisms involved, we systematically identified 13 C. sinensis CsMTP genes from three subfamilies and characterized their phylogeny, structures, and the features of the encoded proteins. The transcription of CsMTP genes was differentially regulated in C. sinensis shoots and roots in responses to high concentrations of Mn, Zn, Fe, and Al. Differences in the cis-acting regulatory elements in the CsMTP8.1 and CsMTP8.2 promoters suggested the expression of these two genes may be differentially regulated. Transient expression analysis indicated that CsMTP8.2 was localized to the plasma membrane in tobacco and onion epidermal cells. Moreover, when heterologously expressed in yeast, CsMTP8.2 conferred tolerance to Ni and Mn but not to Zn. Additionally, heterologous expression of CsMTP8.2 in Arabidopsis thaliana revealed that CsMTP8.2 positively regulated the response to manganese toxicity by decreasing the accumulation of Mn in plants. However, there was no difference in the accumulation of other metals, including Cu, Fe, and Zn. These results suggest that CsMTP8.2 is a Mn-specific transporter that contributes to the efflux of excess Mn2+ from plant cells.

Additional file 3: Table S2. List of predicted miRNA target sites of CsHsf genes.

Additional file 9: Figure S2. Subcellular localization of CsHsfA5a and CsHsfA8 in onion epidermal cells. CsHsfA5a-GFP and CsHsfA8-GFP indicate CsHsfA5a and CsHsfA8 fused with GFP at the C-terminus, and GFP-CsHsfA5a and GFP-CsHsfA8 indicate CsHsfA5a and CsHsfA8 fused with GFP at the N-terminus. The GFP signal was detected using a Zeiss LSM700 confocal laser-scanning microscope (Carl Zeiss Inc., USA) at 488 nm. Bars = 50 μm.

Additional file 1: Figure S1. The distribution of conserved motifs and their corresponding sequence logos of CsHsf proteins. a 25 conserved motifs of CsHsf proteins were analyzed by MEME (http://meme-suite.org/tools/meme). b Sequence logos of the 25 conserved motifs.

Additional file 9: Figure S2. Subcellular localization of CsHsfA5a and CsHsfA8 in onion epidermal cells. CsHsfA5a-GFP and CsHsfA8-GFP indicate CsHsfA5a and CsHsfA8 fused with GFP at the C-terminus, and GFP-CsHsfA5a and GFP-CsHsfA8 indicate CsHsfA5a and CsHsfA8 fused with GFP at the N-terminus. The GFP signal was detected using a Zeiss LSM700 confocal laser-scanning microscope (Carl Zeiss Inc., USA) at 488 nm. Bars = 50 μm.