Tea cultivars with yellow- or white-leaf variations have a high economic value due to their high amino acid (especially theanine) concentration. However, the dynamic changes of amino acid components (especially theanine) and related gene expression during new shoot development in these cultivars are still unclear. In this study, 264 tea samples from four representative varieties picked during the harvest period in spring were analyzed for their amino acid profiles. The dynamic change rules of ethylamine and 19 amino acids were summarized in normal green and yellow cultivars during new shoot development. Interestingly, the theanine concentration in the yellow cultivar was significantly higher than that in the green cultivar, and increased gradually as the leaves matured until they reached a maximum in the one bud and three leaves stage. The amino acid concentration in the leaves of the yellow cultivar increased significantly with leaf position, which was generally in contrast to the normal green cultivar. Transcriptome and correlation analyses revealed that CsGS1, CsPDX2, CsGGP5, CsHEMA3 and CsCLH4 might be the key genes potentially responsible for the differential accumulation of theanine in green and yellow tea cultivars. These results provide further information for the utilization and improvement of tea plants.

Rhododendron species provide excellent ornamental use worldwide, yet heat stress (HS) is one of the major threats to their cultivation. However, the intricate mechanisms underlying the photochemical and transcriptional regulations associated with the heat stress response in Rhododendron remain relatively unexplored. In this study, the analyses of morphological characteristics and chlorophyll fluorescence (ChlF) kinetics showed that HS (40 °C/35 °C) had a notable impact on both the donor’s and acceptor’s sides of photosystem II (PSII), resulting in reduced PSII activity and electron transfer capacity. The gradual recovery of plants observed following a 5-day period of culture under normal conditions indicates the reversible nature of the HS impact on Rhododendron × pulchrum. Analysis of transcriptome data unveiled noteworthy trends: four genes associated with photosynthesis-antenna protein synthesis (LHCb1, LHCb2 and LHCb3) and the antioxidant system (glutamate-cysteine ligase) experienced significant down-regulation in the leaves of R. × pulchrum during HS. Conversely, aseorbate peroxidase and glutathione S-transferase TAU 8 demonstrated an up-regulated pattern. Furthermore, six down-regulated genes (phos-phoenolpyruvate carboxylase 4, sedoheptulose-bisphosphatase, ribose-5-phosphate isomerase 2, high cyclic electron flow 1, beta glucosidase 32 and starch synthase 2) and two up-regulated genes (beta glucosidase 2 and UDP-glucose pyrophosphorylase 2) implicated in photosynthetic carbon fixation and starch/sucrose metabolism were identified during the recovery process. To augment these insights, a weighted gene co-expression network analysis yielded a co-expression network, pinpointing the hub genes correlated with ChlF dynamics’ variation trends. The cumulative results showed that HS inhibited the synthesis of photosynthesis-antenna proteins in R. × pulchrum leaves. This disruption subsequently led to diminished photochemical activities in both PSII and PSI, albeit with PSI exhibiting heightened thermostability. Depending on the regulation of the reactive oxygen species scavenging system and heat dissipation, photoprotection sustained the recoverability of R. × pulchrum to HS.

Rhododendron simsii (R. simsii), a significant ornamental plant species, is adversely affected by the severe soil heavy metal pollution resulting from rapid industrialization, particularly in terms of its growth environment. Cadmium (Cd), a representative heavy metal pollutant, poses a significant threat to plant growth and photosynthetic physiology. Despite the importance of understanding Cd stress resistance in rhododendrons, research in this area is limited. This study focused on the role of exogenous melatonin (MT) in mitigating Cd-induced stress, emphasizing its impact on photosynthetic physiology. Gas exchange parameters, prompt and delayed fluorescence (DF), 820 nm modulated reflectance (Mr₈₂₀), and antioxidant enzyme activity, were measured. The findings revealed that under Cd stress, MT-free treatment imposed a more severe limitation on both stomatal and non-stomatal processes in R. simsii leaves, significantly reducing the net photosynthetic rate. In contrast, exogenous MT improved photosynthetic efficiency by increasing the maximum photochemical efficiency of photosystem II, the quantum yield of electron transport, and the photosynthetic performance index. DF and Mr₈₂₀ analysis demonstrated that MT provided robust protection to both the donor and receptor sides of photosystems I and II. Furthermore, MT significantly decreased malondialdehyde (MDA) content, a marker of oxidative stress, and enhanced the activity of antioxidant enzymes, including superoxide dismutase (SOD) and guaiacol peroxidase (POD). In conclusion, exogenous MT plays a critical role in alleviating Cd-induced stress by enhancing antioxidant defense mechanisms and safeguarding the photosynthetic apparatus, thereby improving the Cd tolerance of R. simsii.

(1) Rhododendron is one of the top ten traditional flowers in China, with both high ornamental and economic values. However, with the change of the environment, Rhododendron suffers from various biological stresses. The WRKY transcription factor is a member of the most crucial transcription factor families, which plays an essential regulatory role in a variety of physiological processes and developmental stresses. (2) In this study, 57 RsWRKYs were identified using genome data and found to be randomly distributed on 13 chromosomes. Based on gene structure and phylogenetic relationships, 57 proteins were divided into three groups: I, II, and III. Multiple alignments of RsWRKYs with Arabidopsis thaliana homologous genes revealed that WRKY domains in different groups had different conserved sites. RsWRKYs have a highly conserved domain, WRKYGQK, with three variants, WRKYGKK, WRKYGEK, and WRKYGRK. Furthermore, cis-acting elements analysis revealed that all of the RsWRKYs had stress and plant hormone cis-elements, with figures varying by group. Finally, the expression patterns of nine WRKY genes treated with gibberellin acid (GA), methyl jasmonate (MeJA), heat, and drought in Rhododendron were also measured using quantitative real-time PCR (qRT-PCR). The results showed that the expression levels of the majority of RsWRKY genes changed in response to multiple phytohormones and abiotic stressors. (3) This current study establishes a theoretical basis for future studies on the response of RsWRKY transcription factors to various hormone and abiotic stresses as well as a significant foundation for the breeding of new stress-tolerant Rhododendron varieties.

Rhododendron (Ericaceae) not only has ornamental value, but also has great medicinal and edible values. Many Rhododendron species are native to acid soils where aluminum (Al) toxicity limits plant productivity and species distribution. However, it remains unknown how Rhododendron adapts to acid soils. Here, we investigated the physiological and molecular mechanisms of Al tolerance in Rhododendron yunnanense Franch. We found that the shoots of R. yunnanense Franch did not accumulate Al after exposure of seedlings to 50 μM Al for 7 days but predominantly accumulated in roots, suggesting that root Al immobilization contributes to its high Al tolerance. Whole-genome de novo transcriptome analysis was carried out for R. yunnanense Franch root apex in response to 6 h of 50 μM Al stress. A total of 443,639 unigenes were identified, among which 1,354 and 3,413 were up- and down-regulated, respectively, by 6 h of 50 μM Al treatment. Both Gene Ontology (GO) enrichment and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses revealed that genes involved in “ribosome” and “cytoskeleton” are overrepresented. Additionally, we identified Al-tolerance homologous genes including a tonoplast-localized ABC transporter RyALS3; 1. Overexpression of RyALS3; 1 in tobacco plants confers transgenic plants higher Al tolerance. However, root Al content was not different between wild-type plants and transgenic plants, suggesting that RyALS3; 1 is responsible for Al compartmentalization within vacuoles. Taken together, integrative transcriptome, physiological, and molecular analyses revealed that high Al tolerance in R. yunnanense Franch is associated with ALS3; 1-mediated Al immobilization in roots.

Heat shock transcription factors are key players in a number of transcriptional regulatory pathways that function during plant growth and development. However, their mode of action in Rhododendron simsii is still unclear. In this study, 22 RsHsf genes were identified from genomic data of R. simsii. The 22 genes were randomly distributed on 12 chromosomes, and were divided into three major groups according to their phylogenetic relationships. The structures and conserved motifs were predicted for the 22 genes. Analysis of cis-acting elements revealed stress-responsive and phytohormone-responsive elements in the gene promoter regions, but the types and number varied among the different groups of genes. Transcriptional profile analyses revealed that RsHsfs were expressed in a tissue-specific manner, with particularly high transcript levels in the roots. The transcriptional profiles under abiotic stress were detected by qRT-PCR, and the results further validated the critical function of RsHsfs. This study provides basic information about RsHsf family in R. simsii, and paves the way for further research to clarify their precise roles and to breed new stress-tolerant varieties.