Research on strigolactones （SLs）, originating from isolation and identification of germination stimulants for seeds of root parasitic weeds, has led to scientific findings of immense basic and applied importance. The term, strigolactones, was first coined in 1995 by the late Larry Butler to define chemicals analogous to strigol in structure and active as Striga germination stimulants. Since the discovery of the novel physiological roles of SLs in hyphal branching in arbuscular mycorrhizal fungi in 2005 and in shoot branching in planta in 2008, an avalanche of articles, reflecting the outstanding importance of these molecules, appeared in eminent scientific journals. With accumulation of our knowledge on SLs, the original definition of the term, based on the structure and the biological activity, is becoming difficult to encompass this class of natural products due to emerging structural diversity and physiological functions. The 1st International Congress on Strigolactones was timely held in March, 2014, to exchange ideas and knowledge on SLs and to increase the solidarity and collaboration within the SL community. On the occasion, this special issue was undertaken to review recent progresses on SL research with emphasis on biosynthesis, perception and signaling. The articles contributed by scientists, who have led the progresses of SLs research, will, for sure, be of help to readers to understand a wide range of aspects of SLs, including chemistry, biochemistry and physiology, and enable design and conductance of attractive research to further our understandings and knowledge on these fascinating apocarotenoids.
Strigolactones （SLs） act as rhizosphere signaling molecules which induce seed germination of root-parasitic witchweeds and broomrapes, and stimulate the symbiotic interaction with arbuscular mycorrhizal（AM） fungi. SLs are important regulators of plant growth and development, including shoot and root architecture, germination, secondary growth and leaf senescence. Scientists are working on the function of SLs in plant development, the molecular biology of their biosynthesis, transport, perception and downstream signaling, the chemistry of naturally occurring compounds and synthetic derivatives, and the role in interaction with AM fungi and parasitic weeds. The 1st International Congress on Strigolactones was held in Wageningen, the Netherlands on 1st-6th March 2015, to exchange ideas and knowledge on SLs between different disciplines and to increase the solidarity and collaboration within the strigolactone community.
Strigolactones （SLs）, which were initially characterized as seed germination stimulants for root parasitic plants about 50 years ago, are now known as symbiotic signals for arbuscular mycorrhizal fungi as well as plant hormones that regulate shoot branching and so on. The discovery of SLs as a new class of plant hormones in 2008 was also significant as an initial characterization of SL biosynthetic mutants, which greatly facilitated its biosynthetic studies. In 2012, an SL-like compound called carlactone （CL） was identified from in vitro biochemical studies using three recombinant proteins of biosynthetic enzymes. The discovery of CL further moved ahead with this research field, and so far the nearly entire picture of the SL biosynthetic pathway has been unveiled. In this review, we will introduce the latest knowledge on the SL biosynthetic pathway as well as current efforts toward the characterization of the bioactive forms of SLs as plant hormones.
Analyses of strigolactone-insensitive mutants have enabled the identification of important factors for strigolactone perception and signaling; D14, an α/β hydrolase, D3/MAX2, an F-box protein and D53/SMXLs. It is postulated that strigolactone is received by D14 and then D14 interacts with D3/MAX2. This interaction stimulates the degradation of repressors of strigolactone signaling, including D53/SMXL proteins. D14 cleaves strigolactone and form a covalent bond between D14 and a cleaved D-ring fragment of strigolactone. This evokes dramatic change in the lid domain of D14 and the exposed surfaces in the changed lid domain interact with D3/MAX2. Karrikins are another class of butenolide molecules found in smoke. Strigolactones and karrikins are recognized through highly similar signaling mechanisms. Karrikin receptor, KAI2/HTL, is a paralog of D14 and involved in karrikin-regulated seed germination and photomorphogenesis. Parasitic plants have divergent KAI2 homologs, KAI2c, KAI2i and KAI2d. Recent papers have demonstrated that strigolactones are received by ShKAI2d to induce seed germination in Striga hermonthica. Crystal structural studies of ShKAI2d revealed that ShKAI2d has larger binding pocket than that of Arabidopsis KAI2 and some amino acid residues in its binding pocket are changed to increase an affinity to strigolactones. These data will help us to develop novel chemical regulators of strigolactone functions for agricultural applications.
Strigolactones （SLs） are carotenoid-derived plant metabolites that can be classified into two groups based on their chemical structures. Strigol and related compounds that contain a tricyclic ring system （ABC ring） connected to a methylbutenolide group （D ring） via an enol-ether bridge are called canonical SLs. These SLs are synthesized from carotenoids via an intermediate, carlactone （CL）. CL serves as a basis for the production of novel germination stimulants, such as avenaol （from black oat）, heliolactone （from sunflower）, and methyl zealactonoate （zealactone, from maize） which are structurally distinct from canonical SLs. CL itself shows a weak but distinct Striga and Orobanche germination activity. Therefore, it is reasonable to include these germination stimulants in the SL family. Here, we introduce current definition of “strigolactone” and new members of the SL family.
In plants, long-distance signaling acts as one of the mechanisms for adaptation to environmental change and growth regulation to maintain a balance between distanced organs. Many studies suggest that gibberellins （GAs） behave as long-distance transmitters. GA precursors can move through vascular system. This review focuses on the functions of GA signaling between shoot and root. Root-expressed genes which responding to shoot-derived GAs were identified by microarray analysis in Arabidopsis. Two GA-responding genes （1） IRT1 （Iron Regulated-Transporter 1） and （2） CLE6 （CLAVATA/ESR-RELATED 6） have been identified. IRT1 plays a key role in Fe absorption into root cells from soil. GA promotes the gene expression of IRT1 and Fe absorption in root. GAs would be signal transmitters for the acquirement of essential nutrients such as Fe. CLE6 belongs to peptide hormones. Expression of CLE6 in root is found in vascular tissue and endodermis and promoted by shoot-derived GAs. Phenotype of GA deficient mutant shows partial recovery recovered by grafting with CLE6 overexpression transgenic line. CLE6 is up-regulated by GAs and acts as long-distance signal for organ growth. In conclusion, mentioned GA functions are considered essential for information exchange between shoot and root.
Iron is essential for the survival and proliferation of all plants. Higher plants have developed two distinct strategies to acquire iron, which is only slightly soluble, from the rhizosphere: the reduction strategy of nongraminaceous plants and the chelation strategy of graminaceous plants. Graminaceous plants secrete mugineic acid family phytosiderophores, which solubilise iron in the soil, and then take up the resulting iron-phytosiderophore complexes. Although phytosiderophore secretion is crucial for plant growth, its molecular mechanism remained unknown. We showed that the efflux of deoxymugineic acid, the primary phytosiderophore from rice, barley and maize, involves the TOM1,HvTOM1 and ZmTOM1 genes, respectively. Our identification of phytosiderophore efflux transporters has revealed the final piece in the molecular machinery of iron acquisition in graminaceous plants. Furthermore, we showed that the TOM2, one of the rice homologues of TOM1, is involved in the internal transport of deoxymugineic acid, which is required for normal plant growth. We have also identified ENA1 and ENA2, genes encoding efflux transporters of nicotianamine, a chelater of metals and an essential intermediate in the production of deoxymugineic acid.
New Plant Breeding Techniques （NPBT） are used for genome modification in various plant and animal species. Here I introduce the outline about 6 technologies which are genome editing, oligonucleotide directed mutagenesis （ODM）, cisgenesis/intragenesis, grafting, reduced generation time and recurrent breeding by transgenic male sterility in plant. And I also listed application of genome editing in animal. Regulatory policy of NPBT is already fixed in United State, Canada, Argentina and New Zealand. Several crops derived from NPBT were approved as out of regulation in United State. Moreover United State, Canada, Germany and other 5 European countries decided that Cibus herbicide canola developed by ODM was non-GMO. However, the European Commission is still discussing NBTs’ regulatory status and the decision of the ODM-canola may change the status in European countries.
Chemical biology is an interdisciplinary scientific field covering chemistry and biology. It uses chemical-based molecules to reveal or control biological processes. Due to the nature and potential of chemicals, they provide an alternative way to identify novel genetic elements associated with redundancy or lethality. In addition, bioactive chemicals can be applied practically as pharmaceutical drugs or agrochemicals. In plant science, many biologically active small molecules were isolated so far from high-throughput screenings using libraries of diverse compounds and their successes demonstrated the usefulness and advantage of this approach. The use, experimental design, tips and future perspective of chemical screening procedures are described herein by sharing experiences based on the previous screening campaigns.
Members of the genus Colletotrichum infect many commercially important crops and cause anthracnose disease. Colletotrichum species infect diverse hosts and/or adopt a variety of infection lifestyles. Even though most members are identified as hemibiotrophic plant pathogens, some species have been categorized as endophytes. In the past several years, it has been made possible to elucidate the molecular mechanisms underlying the host specificity or the diversity of different lifestyles by sequencing the genomes of different Colletotrichum species. This genome information enables us to relate genomic features to different Colletotrichum species phenotypes. In this review, we discuss recent comparative genomic analyses of the Colletotrichum genus.