Piriformospora/Serendipita indica has been frequently proved to play a crucial role in enhancing plant adaptation to environmental stresses. However, its influence on blueberry (Vaccinium corymbosum) drought tolerance has not yet been studied. Here, we reported that P. indica colonization can significantly enhance the drought tolerance of blueberry. Physio-biochemical parameter determination results showed that, compared to non-colonized controls (CK), P. indica-colonized (PI) plants exhibited higher leaf chlorophyll and carotenoids contents, photosynthetic capacity, biomass and root antioxidant enzyme activities (superoxide dismutase and catalase), while also exhibiting lower root malondialdehyde content under drought stress (DS). To explore the underlying mechanism, comparative root transcriptome analysis of well-watered (WW) and DS-treated CK and PI blueberry plants was conducted. In total, we identified 14,587 differentially expressed genes (DEGs) across CK-WW vs. CK-DS, PI-WW vs. PI-DS, CK-WW vs. PI-WW and CK-DS vs. PI-DS comparisons. Under DS, stress-, metabolism- and regulation-related DEGs were overwhelmingly upregulated in PI, while being downregulated in CK. Weighted gene co-expression network analysis categorized DEGs into four modules. Of them, the MEblack module was significantly correlated with the PI-DS group, with DEGs enriched in the cell wall macromolecule catabolic process, carbohydrate metabolic process, phenylpropanoid biosynthesis, and so on. Several defense-related genes, including four thaumatin family proteins, were identified as hub genes of this module. DEGs in the MEblue module were expressed at the highest level in CK-DS, followed by in PI-DS. Hub genes of the MEblue module included DEG-encoding lipid transfer protein, abscisic stress ripening protein, and so on. This study demonstrates that P. indica enhances blueberry drought tolerance by enhancing antioxidant ability and mediating the expression of genes related to stress, carbohydrate and secondary metabolism, and cell wall metabolism.

Dihydroflavonol 4-reductase (DFR) genes contribute greatly to anthocyanin biosynthesis in plants. Up to now, however, research on the DFR gene family and the key anthocyanin-related DFR members in blueberries (Vaccinium corymbosum) has been limited. In this study, we performed a genome-wide identification of the blueberry DFR gene family, identifying 36 VcDFR genes categorized into five subfamilies. Gene expression analysis showed that three Subfamily III members (VcDFR11/29/34) and four Subfamily V members (VcDFR4/7/30/33) are highly expressed in blueberry fruits, particularly at late ripening stages. Transient overexpression analysis in apple fruits verified the contributions of VcDFR11 and VcDFR30 to anthocyanin biosynthesis, with VcDFR11 showing better promoting effects. Blueberry fruit-based transient overexpression further confirmed the promoting effects of VcDFR11 on anthocyanin accumulation and the expression of anthocyanin-related structural genes (especially its downstream anthocyanindin synthase (ANS) and UDP-glucose: flavonoid 3-O-glycosyltransferase (UFGT) genes). The VcDFR11 promoter contains binding sites for both bHLH and MYB transcription factors (TFs). Consistently, yeast one-hybrid and dual-luciferase assays confirmed that anthocyanin-related VcMYB-1 and VcbHLHs can bind to and activate the VcDFR11 promoter. Furthermore, co-overexpressing VcMYB-1/VcbHLHs with VcDFR11 led to much higher anthocyanin accumulation than overexpressing VcDFR11 alone, indicating that these TFs positively regulate anthocyanin biosynthesis by upregulating VcDFR11. In summary, our study characterized the blueberry DFR gene family and demonstrated the role of VcDFR11 in anthocyanin biosynthesis.

Banana plants require substantial nutrients, and their growth is significantly hindered by nutrient deficiency. This study investigated the influences of Piriformospora indica colonization on the growth of clean river sand-cultivated banana plants under varying phosphorus (P) and potassium (K) levels. Banana plants without (CK) and with P. indica colonization (PI) were watered using Hoagland solutions with four gradients of P or K (100%, 50%, 25%, and 0%). Results showed that P. indica colonization promoted the growth of banana plants under different concentrations of P and K treatments. Further analysis revealed that the pseudo-stem P and K contents were very significantly (p < 0.01) positively and positively correlated with biomass-related parameters (aboveground part fresh weight, root fresh weight, and total plant biomass), respectively. Root starch, sucrose, proline (PRO), and anthocyanins contents, as well as leaf malondialdehyde (MDA) and PRO contents, were positively correlated with most growth-related parameters. However, root and leaf flavonoid contents, total antioxidant capacity (T-AOC), and leaf anthocyanin content showed negative correlations with growth-related parameters. Moreover, a very significant negative correlation (p < 0.01) was identified between root T-AOC and root starch content. Additionally, P. indica altered the P and K reduction-caused starch content change patterns in both leaves and roots. Our study demonstrated that P. indica colonization promoted the growth of banana under different concentrations of P and K treatments by mediating the accumulation of carbohydrates, secondary metabolites, osmoprotectants, and so on.