Unable to move on their own, plants have acquired the ability to produce a wide variety of low molecular weight compounds to survive against various stresses. It is estimated that there are as many as one million different types of low-molecular-weight compounds. These low-molecular-weight compounds are derived from primary metabolites such as lipids, carbohydrates, and amino acids, and are therefore generally referred to as secondary metabolites. However, recent research has increased the importance of these compounds as essential for plant survival strategies, and researchers of higher plants are now using the term “specialized metabolites” instead. In this special issue, we would like to provide reviews of this research field by asking leading researchers in Japan to write about the forefront of research aimed at understanding specialized metabolism in plants and furthermore, synthetic biological methods.

Triterpenoids and their glycosides, referred to as saponins, are a class of structurally diverse plant specialized metabolites. Although the biological role of triterpenoids in plants remains largely unclear, these compounds have been shown to possess various medicinal properties. For instance, they may be anti-inflammatory, anti-carcinogenic, and antiviral. Several compounds like glycyrrhizin in licorice and ginsenosides in ginseng form major components in traditional herbal medicines published in Japanese pharmacopoeia. Triterpenoids often accumulate at low levels in plants and their chemical synthesis can be challenging due to their complex structures. Recently, remarkable progress has been made in engineering the production of various plant triterpenoids by utilizing microorganisms, particularly Saccharomyces cerevisiae, as alternative hosts. This review describes representative examples of de novo production of plant-derived triterpenoids in S. cerevisiae and strategies to improve productivity of target compounds, including strategies to enhance metabolic flux, pathway enzyme engineering to improve enzyme activity, engineering of the cytochrome P450 monooxygenase (CYP) - NADPH cytochrome P450 reductase (CPR) interaction to enhance electron transfer efficiency in order to optimize the microenvironment of CYP, and expansion of endoplasmic reticulum (ER) to increase the abundance of ER-associated enzymes and consequential improvement in metabolic capacity. Additionally, perspectives on the use of pathway compartmentalization and metabolite transport strategies to improve productivity of target compound are also discussed.

Steroid is a general term for compounds with the sterane skeleton. In plants, sterols are biosynthesized from cycloartenol as a precursor, and various steroid compounds with various biological activities are biosynthesized from phytosterols as precursors. Phytosterols and brassinosteroids are essential for all seed plants, and therefore, almost all of their biosynthetic genes have been identified. On the other hand, many steroid compounds accumulate in a wide variety of plant species, and their chemical structures and biological activities are also diverse, and therefore, their biosynthesis is different in each plant species, which has delayed the biosynthetic research of steroid compounds. Recent advances in biotechnology have accumulated knowledge on the biosynthesis genes of steroid compounds, and this review summarizes the recent findings on biosynthetic enzymes and discusses the molecular evolution of steroid biosynthetic systems in plants.

Plants produce a large diversity of bioactive molecules of low molecular weight, while the utilization of those chemicals has been still limited due to the low content and complex mixture in plant tissues. To overcome these limitations, microbial production of phytochemicals has been developed in recent years using synthetic biology-based approaches. Meroterpenes form a composite-type metabolite group composed of phenolics and terpenoids, and thus far approximately 1,000 chemical structures have been elucidated. A variety of bioactivities such as anti-inflammatory and anti-obesity have been reported with prenyl side chains often crucial to functions of meroterpenes, and moreover, anti-SARS-CoV-2 activities are also expected. The prenyl chains are transferred to phenolic core structures by the UbiA-type prenyltransferase (PT) family. This review summarizes recent advances in the key enzyme family and meroterpene production in microorganisms. In synthetic biology, breakthroughs have been consecutively reported on microbial production of clinically used alkaloids, such as opioids, tropane alkaloids, and monoterpene indole alkaloids. These molecules are produced via highly complicated biosynthetic pathways spanning multiple organelles and cell types in plants. We also explain cutting-edge techniques leading to the functional reconstruction of biosynthetic pathways of these metabolites.

Isoprenoids are the most structurally diverse natural organic compounds, with more than 50,000 primary and secondary/specialized metabolites. The common precursor of isoprenoids is 5-carbon isopentenyl diphosphate (IPP) or its isomer dimethylallyl diphosphate. In addition, isoprenoids with a wide range of carbon numbers are biosynthesized from linear oligoprenyl diphosphates or polyprenyl diphosphates, formed by the sequential condensation of IPP, as hub intermediates. Natural rubber (NR), is the most industrially important polyisoprenoid because its unique physical properties are suitable for manufacturing tire. Currently, most of NR used in rubber industry is obtained from latex of the Para rubber tree (Hevea brasiliensis), cultivated in tropical and subtropical countries. To meet the ever-increasing demands for NR, metabolic engineering of H. brasiliensis to enhance the NR productivity and synthetic biology to establish NR bioproduction system in alternative species are desired. Here, I review recent progress in elucidation of the biosynthetic mechanisms of NR and related polyisoprenoids in plants and future perspectives on metabolic engineering for polyisoprenoid bioproduction.

The decarboxylation of a basic aliphatic amino acid, lysine, is the first step of biosynthesis of large family of plant alkaloids, such as quinolizidine and lycopodium alkaloids. Produced cadaverine is incorporated into alkaloid molecules like as building blocks to form diverse molecular structures. In plants, lysine decarboxylation is catalyzed by a bifunctional lysine/ornithine decarboxylase (L/ODC) which had been evolutionally derived from omnipresent ornithine decarboxylase (ODC). An amino-acid substitutions from His residue to Phe or Tyr were found between ODC and L/ODC near the active center. These substitutions to aromatic residues determine the cavity size of L/ODC, which arise the acceptability of both ornithine and lysine as substrates and bifunctionality of enzymes. L/ODC genes were found specifically in the species producing lysine-derived alkaloids and soybean producing cadaverine. From these results, the presence of L/ODC should be the trigger of alkaloid production. Furthermore, metabolic engineering using lupin L/ODC was performed to create new metabolic flow in model plants.

In angiosperms, the transition from vegetative to reproductive stage is a critical event for reproductive success and is tightly regulated by both environmental and endogenous signals. In the model plant Arabidopsis thaliana, flowering is strongly induced in response to long photoperiods (LDs), a characteristic seasonal cue for spring and summer. This photoperiodic floral induction is mediated by the function of FLOWERING LOCUS T (FT), a “florigen”, which is synthesized in the leaf vasculature and transported to the shoot apical meristem (SAM). On the other hand, the onset of flowering under short-day conditions (SDs) is severely delayed and occurs through a default pathway that absolutely requires the phytohormone gibberellin (GA). Although the exogenous application of GA accelerates flowering, it is still unclear how GA biosynthesis, catabolism and signaling contribute to flowering in many plant species. In this review, I summarize the recent progress in our understanding of GA signaling and discuss its possible link to the floral transition.

Transmembrane transport of bioactive molecules is a pivotal mechanism that regulates their activities/functions in coordination with the biosynthesis and degradation (inactivation) of the compounds. NITRATE TRANSPORTER 1/PEPTIDE TRANSPORT FAMILY (NPF) proteins have been regarded for a long time as nitrate transporters involved in nitrogen acquisition and allocation in plants. However, recent genetic and biochemical studies have revealed that the family members transport various types of small compounds including plant hormones. This review summarizes the physiological functions of NPFs by mainly focusing on their roles as transporters of plant hormones and plant-specialized metabolites.

This research note reviews the physiological and morphological characteristics of winter bud (both vegetative and floral buds) differentiation, development and dormancy during tree dormancy season in Rosaceae fruit trees, to which many important fruit tree crops such as apple (Malus × domestica), peach (Prunus persica), and Japanese apricot (P. mume) belong. Bud dormancy and bud break in woody perennials is thought to be regulated by a number of molecular and metabolic pathways, among which abscisic acid (ABA), SHORT VEGETATIVE PHASE/AGAMOUS LIKE 24 (SVP/AGL24) -clade ［called DORMANCY-ASSOCIATED MADS-box (DAM)］ and FLOWERING LOCUS C (FLC)-clade MADS-box transcription factors have been recently implicated in the regulation of dormancy. ABA-DAM regulon and FLC-like were mainly associated with chilling and heat requirement, respectively, of winter buds for dormancy release and bud break in Rosaceae fruit trees. DAMs and FLCs were responsive to ambient temperature changes where histone modifications mediate. In poplar (Populus spp.), model woody plant, a member of SVP/AGL24-clade gene, called SHORT VEGETATIVE PHASE-LIKE (SVL), mediates short photoperiod-induced ABA-dependent vegetative bud dormancy induction and establishment and acts as a bud break repressor. We will present our own and reported research results on the discovery and physiological roles of PmDAM6, FLC-like, and SVL, in the regulation of bud dormancy and bud break in woody perennials.

Introduction of foreign DNA into plant genomes to create genetically engineered plants not only requires a lengthy testing period and high developmental costs but also is not well-accepted by the public due to safety concerns about its effects on human and animal health and the environment. We have developed methods to modify plant by using functional peptides, and here, we present a high-throughput nucleic acids delivery platform for plants using peptide nanocarriers applied to the leaf surface by spraying. The translocation of sub-micrometer-scale nucleic acid/peptide complexes upon spraying varied depending on the physicochemical characteristics of the peptides and was controlled by a stomata-dependent-uptake mechanism in plant cells. We observed efficient delivery of DNA molecules into plants using cell-penetrating peptide (CPP)-based foliar spraying. Moreover, using foliar spraying, we successfully performed gene silencing by introducing small interfering RNA molecules in plant nuclei via siRNA-CPP complexes and, more importantly, in chloroplasts via our CPP/chloroplast-targeting peptide-mediated delivery system. This technology enables effective nontransgenic engineering of economically important plant traits in agricultural systems.

Prohexadione-calcium is used to control growth of turfgrasses and contributes to labor saving of mowing. It also has additional effects on turfgrass growth and development, increasing shoot number and root biomass and improving drought tolerance. Poa annua L, the most troublesome weed in the turfgrass management, affects the quality of turf by seed heading in putting greens. Bispyribac-sodium inhibits the seed-heading of P. annua and keeps putting quality favorable on the greens. Benzylaminopurine (BA) also suppresses the seed-heading of P. annua and its effect is synergistically enhanced by tank-mixing with ethephon. BA also has a protective effect against Halo blight in creeping bentgrass.