SUMMARYBrassinosteroids (BRs) are growth-promoting steroidal hormones. Despite the importance of BRs in plant biology, the signal that initiates BR biosynthesis remains unknown. Among the enzymes involved in BR biosynthesis in Arabidopsis (Arabidopsis thaliana), DWARF4 catalyzes the rate-determining step. Through both the histochemical analysis of DWF4pro:GUS plants and the direct measurement of endogenous BR content, we discovered that BR biosynthesis is stimulated by auxin. When DWF4pro:GUS was subjected to auxin dose-response tests and a time-course analysis, GUS activity started to increase at an auxin concentration of 10 nM, rising noticeably after 1 h of auxin treatment. In addition, the analysis of the DWF4pro:GUS line in BR-and auxin-mutant backgrounds revealed that the induction by auxin requires auxinsignaling pathways but not BRs, which implies that auxin signaling directly controls BR biosynthesis. Furthermore, chromatin immunoprecipitation assays confirmed that auxin inhibits the binding of the transcriptional repressor, BZR1, to the DWF4 promoter. A microarray analysis that was designed to examine the transcriptomes after treatment with auxin alone or auxin plus brassinazole (a BR biosynthetic inhibitor) revealed that genes previously characterized as being auxin responsive are not properly regulated when BR biosynthesis is disrupted by brassinazole. Therefore, our results support the idea that auxin regulates BR biosynthesis, and that auxin thus relies on synthesized BRs for some of its growth-promoting effects in Arabidopsis.

The prevalence of the coronavirus disease 2019 (COVID-19) pandemic in its second year has led to massive global human and economic losses. The high transmission rate and the emergence of diverse SARS-CoV-2 variants demand rapid and effective approaches to preventing the spread, diagnosing on time, and treating affected people. Several COVID-19 vaccines are being developed using different production systems, including plants, which promises the production of cheap, safe, stable, and effective vaccines. The potential of a plant-based system for rapid production at a commercial scale and for a quick response to an infectious disease outbreak has been demonstrated by the marketing of carrot-cell-produced taliglucerase alfa (Elelyso) for Gaucher disease and tobacco-produced monoclonal antibodies (ZMapp) for the 2014 Ebola outbreak. Currently, two plant-based COVID-19 vaccine candidates, coronavirus virus-like particle (CoVLP) and Kentucky Bioprocessing (KBP)-201, are in clinical trials, and many more are in the preclinical stage. Interim phase 2 clinical trial results have revealed the high safety and efficacy of the CoVLP vaccine, with 10 times more neutralizing antibody responses compared to those present in a convalescent patient’s plasma. The clinical trial of the CoVLP vaccine could be concluded by the end of 2021, and the vaccine could be available for public immunization thereafter. This review encapsulates the efforts made in plant-based COVID-19 vaccine development, the strategies and technologies implemented, and the progress accomplished in clinical trials and preclinical studies so far.

Vaccination is one of the most successful strategies to prevent diseases caused by pathogens. Although various expression systems including Escherichia coli , yeast, insect, and mammalian cells are currently used for producing many of vaccines, these conventional platforms have the limitation of post-translational modification, high cost, and expensive scalability. In this respect, the plant-based expression system has been considered as an attractive platform to produce recombinant vaccines due to fast, cost-effective and scalable production as well as safety. This review discusses the development of plant-derived vaccines and the current stage of plant-based expression system.

SUMMARYPhytohormone homeostasis is essential for proper growth and development of plants. To understand the growth mechanisms mediated by hormonal levels, we isolated a gulliver1 (gul1) mutant that had tall stature in the presence of both brassinazole and the light. The gul1 phenotype depended on functional BR biosynthesis; the genetic introduction of dwarf4, a BR biosynthetic mutation, masked the long hypocotyl phenotype of gul1. Furthermore, BR biosynthesis was dramatically enhanced, such that the level of 22-hydroxy campesterol was 5.8-fold greater in gul1. Molecular cloning revealed that gul1 was a missense mutation, resulting in a glycine to arginine change at amino acid 116 in SUPERROOT2 (CYP83B1), which converts indole acetaldoxime to an S-alkyl thiohydroximate adduct in the indole glucosinolate pathway. Auxin metabolite profiling coupled with quantitative reverse transcription polymerase chain reaction (RT-PCR) analysis of auxin biosynthetic genes revealed that gul1/sur2-7 activated multiple alternative branches of tryptophan-dependent auxin biosynthetic pathways. Furthermore, exogenous treatment of gul1/sur2-7 with BRs caused adventitious roots from hypocotyls, indicative of an increased response to BRs relative to wildtype. Different from severe alleles of sur2, gul1/sur2-7 lacked 'high-auxin' phenotypes that include stunted growth and callus-like disintegration of hypocotyl tissues. The auxin level in gul1/sur2-7 was only 1.6-fold greater than in the wild-type, whereas it was 4.2-fold in a severe allele like sur2-8. Differences in auxin content may account for the range of phenotypes observed among the sur2 alleles. This unusual allele provides long-sought evidence for a synergistic interaction between auxin and BRs in promoting growth in Arabidopsis at the level of their biosynthetic enzymes.

Plant growth-stimulating hormones brassinosteroids (BRs) function via interactions with other hormones. However, the mechanism of these interactions remains to be elucidated. The unique phenotypes of brassinosteroid insensitive2/dwarf12-D (bin2/dwf12-D) mutants, such as twisted inflorescences and leaves, suggested that BIN2, a negative regulator of BR signaling, may be involved in auxin signaling. Furthermore, previously, we showed that auxin stimulates DWF4 expression. To determine the possible role of BIN2/DWF12 in Auxin signaling, we measured DWARF4pro:GUS activity through both GUS histochemical staining and in vivo GUS assay. We found that the GUS activity in the bin2/dwarf12-1D background dramatically increased relative to control. In addition, the number of lateral roots (LR) in bin2/dwf12-1D was greater than wild type, and the optimal concentration for auxin-mediated lateral root induction was lower in bin2/ dwf12-1D; these findings suggest that BIN2 plays a positive role in auxin signaling. In contrast, ABA repressed both DWF4pro:GUS expression and lateral root development. However, the degree of repression was lower in bin2/ dwf12-1D background, suggesting that BIN2 plays a role in ABA-mediated DWF4pro:GUS expression and subsequently in lateral root development, too. Therefore, it is likely that BIN2 plays a role of signal integrator for multiple hormones, such as BRs, auxin, and ABA.