Araliaceae species produce various classes of triterpene and triterpenoid saponins, such as the oleanane-type triterpenoids in Aralia species and dammarane-type saponins in Panax, valued for their medicinal properties. The lack of genome sequences of Panax relatives has hindered mechanistic insight into the divergence of triterpene saponins in Araliaceae. Here, we report a chromosome-level genome of Aralia elata with a total length of 1.05 Gb. The loss of 12 exons in the dammarenediol synthase (DDS)-encoding gene in A. elata after divergence from Panax might have caused the lack of dammarane-type saponin production, and a complementation assay shows that overexpression of the PgDDS gene from Panax ginseng in callus of A. elata recovers the accumulation of dammarane-type saponins. Tandem duplication events of triterpene biosynthetic genes are common in the A. elata genome, especially for AeCYP72As, AeCSLMs, and AeUGT73s, which function as tailoring enzymes of oleanane-type saponins and aralosides. More than 13 aralosides are de novo synthesized in Saccharomyces cerevisiae by overexpression of these genes in combination. This study sheds light on the diversity of saponins biosynthetic pathway in Araliaceae and will facilitate heterologous bioproduction of aralosides.

Sugar will eventually be exported transporter (SWEET) is a family of sugar transporters that plays a critical role during host and pathogen interaction (Bezrutczyk et al., 2018). In rice, SWEET11/Xa13, SWEET13/Xa25, and SWEET14 were identified as targets of Xanthomonas oryzae pv. oryzae effectors (Antony et al., 2010; Hutin et al., 2015; Yang et al., 2006). However, the role of SWEET genes in rice and Rhizoctonia solani, the causative agent of sheath blight disease (ShB), is largely unknown. Our previous transcriptome data showed that SWEET14 expression is sensitive to R. solani infection (De Peng Yuan, 2020). qPCR results verified that R. solani infection dramatically induces SWEET14 expression along with a pathogen‐related protein PBZ1 (Figure 1a).

Anthocyanins are common secondary metabolites in plants that confer red, blue, and purple colorations in plants and are highly desired by consumers for their visual appearance and nutritional quality. In the last two decades, the anthocyanin biosynthetic pathway and transcriptional regulation of anthocyanin biosynthetic genes (ABGs) have been well characterized in many plants. From numerous studies on model plants and horticultural crops, many signaling regulators have been found to control anthocyanin accumulation via regulation of anthocyanin-promoting R2R3-MYB transcription factors (so-called R2R3-MYB activators). The regulatory mechanism of R2R3-MYB activators is mediated by multiple environmental factors (e.g., light, temperature) and internal signals (e.g., sugar, ethylene, and JA) in complicated interactions at multiple levels. Here, we summarize the transcriptional control of R2R3-MYB activators as a result of natural variations in the promoter of their encoding genes, upstream transcription factors and epigenetics, and posttranslational modifications of R2R3-MYB that determine color variations of horticultural plants. In addition, we focus on progress in elucidating the integrated regulatory network of anthocyanin biosynthesis mediated by R2R3-MYB activators in response to multiple signals. We also highlight a few gene cascade modules involved in the regulation of anthocyanin-related R2R3-MYB to provide insights into anthocyanin production in horticultural plants.

The effects of irradiating blue, UV-A, UV-B and a combination of the lights on anthocyanin accumulation at different hypocotyl positions were investigated in seedlings of the purple top turnip ‘Tsuda’. The location of anthocyanin accumulation varied depending on different light spectra. Stronger accumulation of anthocyanin was induced (i) at the upper hypocotyl positions by blue light; (ii) mainly at the upper position, but also at the middle position by UV-B light; and (iii) at the upper to lower position by UV-A light. There were synergistic effects between blue and UV-B, while such effects were not observed for the other light combinations. Among the six chalcone synthase (CHS) genes identified in the ‘Tsuda’ turnip, BrCHS1, 4 and 5 exhibited light-dependent expression patterns, while the other three showed no apparent light responses. The expression of BrCHS1, 4 and 5 was increased particularly by UV-A and blue + UV-B irradiation at the middle to lower hypocotyl positions, in accordance with anthocyanin accumulation patterns. The highest induction of gene expression was observed for BrCHS4 upon blue + UV-B co-irradiation. In contrast, CHS expression was induced only slightly at higher hypocotyl positions by blue light. The R2R3-type MYB transcription factor genes PAP1, MYB4, MYB12 and MYB111 exhibited differential expression patterns at different hypocotyl positions; these patterns were unique for different light spectra. These unique anthocyanin accumulation patterns and gene expression profiles depending on hypocotyl positions and light sources demonstrate that there is a distinct UV-A response, blue + UV-B synergistic response and blue/UV-A light response for anthocyanin biosynthesis in turnip. UV-A light-dependent anthocyanin biosynthesis appeared to be regulated in a manner that is distinct from that mediated by cryptochromes and UV-B photoreceptors.