Colonization by the endophytic fungus Phyllosticta fallopiae combined with the element Si promotes the growth of Dendrobium nobile

Endophytic fungi can promote plant growth and development, particularly of Orchidaceae species. Previously, we found that the endophytic fungus Phyllosticta fallopiae DN14, collected from Dendrobium nobile growing on rocks in a wild habitat, significantly promoted growth of its host plant D. nobile, an important herb in Chinese traditional medicine that contains the bioactive component dendrobine. Phyllosticta was positively correlated with FW and dendrobine content of D. nobile and with Si content of the epiphytic matrix. Si is also highly beneficial for the growth and productivity of many plants. Here, we co-cultured D. nobile with P. fallopiae DN14 in half-strength Murashige and Skoog medium with and without various concentrations of Si to investigate the effects of DN14 and Si on plant fresh weight and dendrobine content. We also explored the effects of DN14 infection and colonization on host plant growth, Si accumulation and transport, and expression of key genes, as well as the interaction between DN14 and Si. The combination of DN14 and Si promoted the lignification of D. nobile roots, stems, and leaves and markedly increased the thickening of xylem cell walls. Co-culture with DN14 increased transport of Si from roots to stems and from stems to leaves. Transcriptome sequencing and qRT-PCR analyses showed that enhancement of D. nobile growth by DN14 and Si may involve upregulation of plant hormone-related genes (AUX/IAA and MYC) and lignin biosynthesis genes (HCT, PAL1, and PAL2). Insoluble Si promoted the growth of DN14, perhaps through downregulation of genes (e.g., FBP, MPI, RPIAD) related to carbohydrate metabolism, and DN14 in turn promoted the transformation of insoluble Si into soluble Si for plant uptake. These findings demonstrate that endophytic fungi and Si can improve the growth of D. nobile and therefore show promise as organic amendments for commercial cultivation.