Inonotus obliquus (Fr.) Pilat is a white rot fungus belonging to the family Hymenochaetaceae in the Basidiomycota. In nature, this fungus rarely forms a fruiting body but usually an irregular shape of sclerotial conk called 'Chaga'. Characteristically, I. obliquus produces massive melanins released to the surface of Chaga. As early as in the sixteenth century, Chaga was used as an effective folk medicine in Russia and Northern Europe to treat several human malicious tumors and other diseases in the absence of any unacceptable toxic side effects. Chemical investigations show that I. obliquus produces a diverse range of secondary metabolites including phenolic compounds, melanins, and lanostane-type triterpenoids. Among these are the active components for antioxidant, antitumoral, and antiviral activities and for improving human immunity against infection of pathogenic microbes. Geographically, however, this fungus is restricted to very cold habitats and grows very slowly, suggesting that Chaga is not a reliable source of these bioactive compounds. Attempts for culturing this fungus axenically all resulted in a reduced production of bioactive metabolites. This review examines the current progress in the discovery of chemical diversity of Chaga and their biological activities and the strategies to modulate the expression of desired pathways to diversify and up-regulate the production of bioactive metabolites by the fungus grown in submerged cultures for possible drug discovery.

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While Inonotus obliquus produces a diverse range of bioactive metabolites in its natural habitats, it accumulates less in its submerged cultures. We show here that coculture of I. obliquus with Phellinus punctatus resulted in less production of mycelial biomass but an increased accumulation of phenolic compounds, melanins, and lanostane-type triterpenoids. Metabolites increased in production by coculture include phelligridin C, phelligridin H, methyl inoscavin A, inoscavin C, inoscavin B, davallialactone, methyl davallialactone, foscoparianol D, 21,24-cyclopentalanosta-3β,21,25-triol-8-en, lanosta-7,9(11),23-triene-3β,22,25-triol, and inotodisaccharide and melanins. Metabolites from coculture also showed an increased potential for scavenging free radicals and inhibiting the proliferation of HeLa 229 cells. Davallialactone, methyl davallialactone, and minor phenolic components are the major contributors for scavenging DPPH and hydroxyl radical in monoculture, and phelligridin C, phelligridin H, methyl inoscavin A, inoscavin C, methyl davallialactone, foscoparianol D, and inotodisaccharide are those for scavenging the tested radicals in coculture. Lanostane-type triterpenoids indicated limited roles in scavenging free radicals. Nearly all the detected metabolites correlate positively with inhibiting proliferation of HeLa 229 cells. Thus, coculture of I. obliquus with other fungi seems to be a cost-effective strategy for upregulating biosynthesis of bioactive metabolites.

Salt stress is one of the major devastating factors affecting the growth and yield of almost all crops, including the crucial staple food crop sweet potato. To understand their molecular responses to salt stress, comparative transcriptome and proteome analysis of salt-tolerant cultivar Xushu 22 and salt-sensitive cultivar Xushu 32 were investigated. The results showed the two genotypes had distinct differences at the transcription level and translation level even without salt stress, while inconsistent expression between the transcriptome and proteome data was observed. A total of 16,396 differentially expressed genes (DEGs) and 727 differentially expressed proteins (DEPs) were identified. Wherein, 1,764 DEGs and 93 DEPs were specifically expressed in the tolerant genotype. Furthermore, the results revealed that the significantly upregulated genes were mainly related to the regulation of ion accumulation, stress signaling, transcriptional regulation, redox reactions, plant hormone signal transduction, and secondary metabolite accumulation, which may be involved in the response of sweet potato to salt stress and/or may determine the salt tolerance difference between the two genotypes. In addition, 1,618 differentially expressed regulatory genes were identified, including bZIP, bHLH, ERF, MYB, NAC, and WRKY. Strikingly, transgenic Arabidopsis overexpressing IbNAC7 displayed enhanced salt tolerance compared to WT plants, and higher catalase (CAT) activity, chlorophyll and proline contents, and lower malondialdehyde (MDA) content and reactive oxygen species (ROS) accumulation were detected in transgenic plants compared with that of WT under salt stress. Furthermore, RNA-seq and qRT-PCR analysis displayed that the expression of many stress-related genes was upregulated in transgenic plants. Collectively, these findings provide revealing insights into sweet potato molecular response to salt stress and underlie the complex salt tolerance mechanisms between genotypes, and IbNAC7 was shown as a promising candidate gene to enhance salt tolerance of sweet potato.

NAC (NAM, ATAF, and CUC) transcription factors function as the nodes of regulatory networks in response to various biotic and abiotic stresses. Although they have been widely studied in many species, no knowledge concerning salt‐stress‐responsive NAC genes is available in sweetpotato (Ipomoea batatas L.). In the present study, 12 novel putative NAC genes, designated as IbNAC1L and IbNAC3 through IbNAC13, were isolated on the basis of the salt‐stress‐treated RNA sequencing data of two sweetpotato cultivars (‘XuShu 22’ and ‘XuShu 32’) with different levels of salt tolerance. Motif compositions showed that all the IbNAC proteins except IbNAC9 contain five conserved subdomains in the NAC domain region that characterizes the NAC gene family. Furthermore, systematic expression analysis and function evaluation of these IbNAC genes were conducted using quantitative real‐time polymerase chain reaction. The results showed that the expression of most IbNAC genes was exclusively induced by multiple abiotic stresses, including NaCl, dehydration, cold, and heat. Varying degrees of induction were also observed in the transcription levels of these IbNACs, except IbNAC10, when treated with various hormones, including abscisic acid, 1‐aminocyclopropane‐1‐carboxylate, gibberellic acid, and jasmonic acid. Cumulatively, the data of stress‐responsive IbNAC genes in the present study will provide valuable information for further exploring the crucial and diverse roles of IbNAC genes in sweetpotato stress tolerance.

A fungal elicitor prepared from the cell debris of the plant-pathogenic ascomycete Alternaria alternata induces multiple responses by Inonotus obliquus cells, including an increase in generation of nitric oxide (NO), activity of phenylalanine ammonia lyase (PAL) and accumulation of total mycelial phenolic compounds (TMP), but does not trigger production of oxylipins or jasmonic acid (JA). The role of NO in TMP production was investigated via the effects of the NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPITO) and the nitric oxide synthase (NOS) inhibitor aminoguanidine (AG). TMP profiles were assayed using 1 H NMR spectroscopy combining multivariate pattern recognition strategies. Pretreatment of I. obliquus mycelia with cPITO or AG suppressed not only elicitor-enhanced NO generation and PAL activity, but also the elicitor-induced increase in TMP production. This TMP reduction by either a NO scavenger or a NOS inhibitor was reversed by exogenous addition of either a NO donor, sodium nitroprusside, or JA separately. NMR-based metabonomic analysis of TMP profiles showed that the induced TMP were hispidin analogues including inoscavins, phelligridins, davallialactone and methyldavallialactone, which possess high antioxidant activities. Thus, NO mediates an elicitor-induced increase in production of antioxidant polyphenols in I. obliquus via a signalling pathway independent of oxylipins or JA, a mechanism which differs from those in some higher plants.

A field experiment was established to study sweet potato growth, starch dynamic accumulation, key enzymes and gene transcription in the sucrose-to-starch conversion and their relationships under six K2O rates using Ningzishu 1 (sensitive to low-K) and Xushu 32 (tolerant to low-K). The results indicated that K application significantly improved the biomass accumulation of plant and storage root, although treatments at high levels of K, i.e., 300–375 kg K2O ha−1, significantly decreased plant biomass and storage root yield. Compared with the no-K treatment, K application enhanced the biomass accumulation of plant and storage root by 3–47% and 13–45%, respectively, through promoting the biomass accumulation rate. Additionally, K application also enhanced the photosynthetic capacity of sweet potato. In this study, low stomatal conductance and net photosynthetic rate (Pn) accompanied with decreased intercellular CO2 concentration were observed in the no-K treatment at 35 DAT, indicating that Pn was reduced mainly due to stomatal limitation; at 55 DAT, reduced Pn in the no-K treatment was caused by non-stomatal factors. Compared with the no-K treatment, the content of sucrose, amylose and amylopectin decreased by 9–34%, 9–23% and 6–19%, respectively, but starch accumulation increased by 11–21% under K supply. The activities of sucrose synthetase (SuSy), adenosine-diphosphate-glucose pyrophosphorylase (AGPase), starch synthase (SSS) and the transcription of Susy, AGP, SSS34 and SSS67 were enhanced by K application and had positive relationships with starch accumulation. Therefore, K application promoted starch accumulation and storage root yield through regulating the activities and genes transcription of SuSy, AGPase and SSS in the sucrose-to-starch conversion.