The phytohormone gibberellic acids (GAs) play a crucial role in the processes of growth, organ development, and secondary metabolism. However, the mechanism of exogenous GA₃ regulating the growth and flavonoid synthesis in Phellodendron chinense Schneid (P. chinense Schneid) seedlings remains unclear. In this study, the physicochemical properties, gene expression level, and secondary metabolite of P. chinense Schneid seedlings under GA₃ treatment were investigated. The results showed that GA₃ significantly improved the plant height, ground diameter, fresh weight, chlorophyll content, soluble substance content, superoxide dismutase, and peroxidase activities. This was accompanied by elevated relative expression levels of Pc(S)-GA2ox, Pc(S)-DELLA, Pc(S)-SAUR50, Pc(S)-PsaD, Pc(S)-Psb 27, Pc(S)-PGK, Pc(S)-CER3, and Pc(S)-FBA unigenes. Conversely, a notable reduction was observed in the carotenoid content, catalase activity and the relative expression abundances of Pc(S)-KAO, Pc(S)-GID1/2, and Pc(S)-GH 3.6 unigenes in leaves of P. chinense Schneid seedlings (p < 0.05). Furthermore, GA₃ evidently decreased the contents of pinocembrin, pinobanksin, isosakuranetin, naringin, naringenin, (−)-epicatechin, tricetin, luteolin, and vitexin belonged to flavonoid in stem bark of P. chinense Schneid seedlings (p < 0.05). These results indicated that exogenous GA₃ promoted growth through improving chlorophyll content and gene expression in photosynthesis and phytohormone signal pathway and inhibited flavonoid synthesis in P. chinense Schneid seedlings.

Phoebe bournei is a rare and endangered plant endemic to China with higher-value uses in essential oil and structural wood production. Its seedlings are prone to death because of its undeveloped system. Paclobutrazol (PBZ) can improve root growth and development in certain plants, but its concentration effect and molecular mechanism remain unclear. Here, we studied the physiological and molecular mechanisms by which PBZ regulates root growth under different treatments. We found that, with moderate concentration treatment (MT), PBZ significantly increased the total root length (69.90%), root surface area (56.35%), and lateral root number (47.17%). IAA content was the highest at MT and was 3.83, 1.86, and 2.47 times greater than the control, low, and high-concentration treatments. In comparison, ABA content was the lowest and reduced by 63.89%, 30.84%, and 44.79%, respectively. The number of upregulated differentially expressed genes (DEGs) induced at MT was more than that of down-regulated DEGs, which enriched 8022 DEGs in response to PBZ treatments. WGCNA showed that PBZ-responsive genes were significantly correlated with plant hormone content and involved in plant hormone signal transduction and MAPK signal pathway-plant pathways, which controls root growth. The hub genes are observably associated with auxin, abscisic acid syntheses, and signaling pathways, such as PINs, ABCBs, TARs, ARFs, LBDs, and PYLs. We constructed a model which showed PBZ treatments mediated the antagonism interaction of IAA and ABA to regulate the root growth in P. bournei. Our result provides new insights and molecular strategies for solving rare plants’ root growth problems.

Fertilization is widely acknowledged as being an essential practice to improve forest productivity in forest ecosystems. However, too little consideration has been given to the taxonomic and functional compositions of rhizosphere soil microbes and their interactions with root exudates under different fertilizer regimes in forest plantations. Here, we investigated the effects of four typical fertilizer regimes (CK, no fertilizer; CF, compound fertilizer; OF, organic fertilizer; CMF, compound microbial fertilizer) on soil microbial communities and their potential functional groups in Phoebe bournei young plantations, as well as their associations with soil physicochemical properties and root exudates. These results showed that fertilizer regimes strikingly affected the rhizosphere soil microbial community compositions and alpha diversity indices. The pathotroph was the dominant fungal guild. With the applications of three fertilizations, the relative abundances of the plant pathogen and arbuscular mycorrhiza increased. The alpha diversity of soil bacteria was highest under the OF regime, and soil fungal diversity was more powerfully affected by the amendment of CMF. Additionally, while the fungal community was simultaneously influenced by soil physiochemical factors and root exudates, the bacterial community in the rhizosphere was mostly impacted by root exudates. More importantly, the application of OF and CF induced dramatic growths of Fusarium, while CMF treatment including Bacillus suppressed the development of Fusarium via adjusting bacterial species. Overall, our findings exhibit the divergent responses of rhizosphere bacteria and fungi to fertilizer regimes in P. bournei young plantations. The application of organic fertilizer provides benefits for rhizosphere bacteria, and microbial fertilizer can help alleviate inhibition through changing pathogens.

Phoebe bournei is a potential medicinal plant. Its essential oils (Eos) are mainly composed of sesquiterpenes that has potential activities of anti-bacteria and anti-tumors. In this study, we evaluated the effects of compost and compound fertilizer on the total amount and main components of Eos in P. bournei, we also studied the molecular mechanism undergoing this process by deep sequencing the genes involved in the biosynthesis of sesquiterpenes. Fertilization enhanced the total amount of main components in Eos from both leaves and twigs. Bicyclogermacrene, the primary sesquiterpene in the leaf EO, was significantly increased under compost treatment, while bicyclogermacrene and δ-cadinene (the second most abundant sesquiterpene) were decreased under compound fertilizer treatment. The two fertilizers had no significant effect on the abundance of the primary (+) - δ-cadinene in the twig EO, but had a positive effect on the second most abundant sesquiterpene copaene. Significant differences were observed in the number of differentially expressed genes (DEGs) with the leaves showing greater number of DEGs as compared to the twigs after compost treatment. Terpenoid backbone biosynthesis (TBB) is a key pathway of sesquiterpenes synthesis. The expression of genes regulating several important enzymes in TBB was altered after fertilization. After the compost treatment, the expression of the leaf DXS gene (ACQ66107.1), being closely related to the sesquiterpene biosynthesis in P. bournei leaves, was decreased. Compost and compound fertilizer altered the expression of the two important branch-point enzymes (FPPS and GGPPS) genes (ART33314.1 and ATT59265.1), which contributed to the changes of the total amount and components of P. bournei sesquiterpenes. This study provides a new insight into the future use of P. bournei for Eos.