Brassinosteroids are inherently biosynthesized in the primary roots of maize, Zea mays L

Novel insights into maize (Zea mays) development and organogenesis for agricultural optimization

In maize, intrinsic hormone activities and sap fluxes facilitate organogenesis patterning and plant holistic development; these hormone movements should be a primary focus of developmental biology and agricultural optimization strategies. Maize (Zea mays) is an important crop plant with distinctive life history characteristics and structural features. Genetic studies have extended our knowledge of maize developmental processes, genetics, and molecular ecophysiology. In this review, the classical life cycle and life history strategies of maize are analyzed to identify spatiotemporal organogenesis properties and develop a definitive understanding of maize development. The actions of genes and hormones involved in maize organogenesis and sex determination, along with potential molecular mechanisms, are investigated, with findings suggesting central roles of auxin and cytokinins in regulating maize holistic development. Furthermore, investigation of morphological and structural characteristics of maize, particularly node ubiquity and the alternate attachment pattern of lateral organs, yields a novel regulatory model suggesting that maize organ initiation and subsequent development are derived from the stimulation and interaction of auxin and cytokinin fluxes. Propositions that hormone activities and sap flow pathways control organogenesis are thoroughly explored, and initiation and development processes of distinctive maize organs are discussed. Analysis of physiological factors driving hormone and sap movement implicates cues of whole-plant activity for hormone and sap fluxes to stimulate maize inflorescence initiation and organ identity determination. The physiological origins and biogenetic mechanisms underlying maize floral sex determination occurring at the tassel and ear spikelet are thoroughly investigated. The comprehensive outline of maize development and morphogenetic physiology developed in this review will enable farmers to optimize field management and will provide a reference for de novo crop domestication and germplasm improvement using genome editing biotechnologies, promoting agricultural optimization.

Rapid and Automatic Annotation of Multiple On-Tissue Chemical Modifications in Mass Spectrometry Imaging with Metaspace

On-tissue chemical derivatization is a valuable tool for expanding compound coverage in untargeted metabolomic studies with matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Applying multiple derivatization agents in parallel increases metabolite coverage even further but results in large and more complex datasets that can be challenging to analyze. In this work, we present a pipeline to provide rigorous annotations for on-tissue derivatized MSI data using Metaspace. To test and validate the pipeline, maize roots were used as a model system to obtain MSI datasets after chemical derivatization with four different reagents, Girard's T and P for carbonyl groups, coniferyl aldehyde for primary amines, and 2-picolylamine for carboxylic acids. Using this pipeline helped us annotate 631 unique metabolites from the CornCyc/BraChem database compared to 256 in the underivatized dataset, yet, at the same time, shortening the processing time compared to manual processing and providing robust and systematic scoring and annotation. We have also developed a method to remove false derivatized annotations, which can clean 5-25% of false derivatized annotations from the derivatized data, depending on the reagent. Taken together, our pipeline facilitates the use of broadly targeted spatial metabolomics using multiple derivatization reagents.

Excavation of Genes Responsive to Brassinosteroids by Transcriptome Sequencing in Adiantum flabellulatum Gametophytes

Brassinosteroids (BRs) are a class of polyhydroxysteroid plant hormones; they play important roles in the development and stress resistance of plants. The research on BRs has mainly been carried out in angiosperms, but in ferns-research is still limited to the physiological level and is not in-depth. In this study, Adiantum flabellulatum gametophytes were used as materials and treated with 10^-6 M brassinolide (BL). The differentially expressed genes (DEGs) responsive to BRs were identified by transcriptome sequencing, GO, KEGG analysis, as well as a quantitative real-time polymerase chain reaction. From this, a total of 8394 DEGs were screened. We found that the expressions of photosynthetic genes were widely inhibited by high concentrations of BL in A. flabellulatum gametophytes. Moreover, we detected many BR synthase genes, except BR6ox2, which may be why castasterone (CS) rather than BL was detected in ferns. Additionally, we identified (for the first time) that the expressions of BR synthase genes (CYP90B1, CYP90C1, CYP90D1, CPD, and BR6ox1) were negatively regulated by BL in fern gametophytes, which indicated that ferns, including gametophytes, also needed the regulatory mechanism for maintaining BR homeostasis. Based on transcriptome sequencing, this study can provide a large number of gene expression data for BRs regulating the development of fern gametophytes.

Occurrence of brassinosteroids and influence of 24-epibrassinolide with brassinazole on their content in the leaves and roots of Hordeum vulgare L. cv. Golden Promise

24-epibrassinolide overcame the inhibitory effect of brassinazole on the barley growth and the content of brassinosteroids. The present work demonstrates the occurrence of mainly castasterone, brassinolide and cathasterone and lower amounts of 24-epibrassinolide, 24-epicastasterone, 28-homobrassinolide, typhasterol, 6-deoxocastasterone and 6-deoxotyphasterol in 14-day-old de-etiolated barley (Hordeum vulgare L. cv. Golden Promise). We also investigated the endogenous level of brassinosteroids (BRs) in barley seedlings treated with 24-epibrassinolide (EBL) and/or brassinazole (Brz). To our knowledge, this is the first report related to the occurrence of BRs and application of EBL and Brz in terms of the endogenous content of BRs in barley. Brz as a specific inhibitor of BR biosynthetic reactions decreased the level of BRs in the leaves. Application of EBL showed a weak promotive effect on the BR content in Brz-treated seedlings. Brz also inhibited growth of the seedlings; however, addition of EBL overcame the inhibition. The EBL applied alone at 0.01-1 µM increased the BR level in the leaves but at 10 µM lowered the BR content. In opposition to leaves, the Brz in the concentration range from 0.1 to 1 µM did not significantly affect the content of BRs in the roots. However, application of 10 µM Brz caused BRs to decrease, but treatment of EBL concentrations overcame the inhibitory effect of Brz.

Changes in endogenous phytohormones regulated by microRNA-target mRNAs contribute to the development of Dwarf Autotetraploid Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Polyploidization is considered as the major force that drives plant species evolution and biodiversity. The leaves of Chinese cabbage, an important vegetable crop valued for its nutritional quality, constitute the main edible organ. In this study, we found that autotetraploid Chinese cabbage (Brassica rapa ssp. pekinensis) generated from a doubled haploid (DH) line via isolated microspore culture exhibits a dwarf phenotype, along with thick leaves and delayed flowering. Abscisic acid (ABA) and brassinosteroid (BR) levels were significantly lower in autotetraploids compared to DHs. Comparative transcriptome analysis was performed to examine the gene regulatory network. A total of 13,225 differentially expressed genes (DEGs) were detected. Further microRNA (miRNA) analysis identified 102 DEGs that correspond to 35 differentially expressed miRNAs (DEMs). Subsequent screening of these 102 genes identified 13 key genes with 12 corresponding differentially expressed miRNAs that are related to leaf development and dwarfism. These 13 genes are involved in the regulation of various processes, including BR synthesis (dwarfing), plant growth, flowering time delay, ABA pathway-related growth and metabolism, leaf morphology and development, and cell extension. Two dwarfing-related genes (BraA01000252 and BraA05004386) regulated by two miRNAs (novel_15 and novel_54) were determined to be downregulated, indicating their possible role in leaf thickness and dwarfism in autotetraploid plants. We also propose two possible miRNA-dependent regulatory pathways that contribute to trait formation in autotetraploid Chinese cabbage. These results provide a theoretical basis for further work involving Chinese cabbage varieties by inducing polyploidy.

Drought-tolerant and drought-sensitive genotypes of maize (Zea mays L.) differ in contents of endogenous brassinosteroids and their drought-induced changes

The contents of endogenous brassinosteroids (BRs) together with various aspects of plant morphology, water management, photosynthesis and protection against cell damage were assessed in two maize genotypes that differed in their drought sensitivity. The presence of 28-norbrassinolide in rather high quantities (1-2 pg mg-1 fresh mass) in the leaves of monocot plants is reported for the first time. The intraspecific variability in the presence/content of the individual BRs in drought-stressed plants is also described for the first time. The drought-resistant genotype was characterised by a significantly higher content of total endogenous BRs (particularly typhasterol and 28-norbrassinolide) compared with the drought-sensitive genotype. On the other hand, the drought-sensitive genotype showed higher levels of 28-norcastasterone. Both genotypes also differed in the drought-induced reduction/elevation of the levels of 28-norbrassinolide, 28-norcastasterone, 28-homocastasterone and 28-homodolichosterone. The differences observed between both genotypes in the endogenous BR content are probably correlated with their different degrees of drought sensitivity, which was demonstrated at various levels of plant morphology, physiology and biochemistry.

Function and molecular regulation of DWARF1 as a C-24 reductase in brassinosteroid biosynthesis in Arabidopsis

DWARF1 (DWF1) is a sterol C-24 reductase that catalyses the conversion of 24-methylenecholesterol (24-MCHR) to campesterol (CR) in Arabidopsis. A loss-of-function mutant, dwf1, showed similar phenotypic abnormalities to brassinosteroid (BR)-deficient mutants. These abnormalities were reversed in the wild-type phenotype by exogenous application of castasterone (CS) and brassinolide (BL), but not dolichosterone (DS). Accumulation of DS and decreased CS were found in quantitative analysis of endogenous BRs in dwf1. The enzyme solution prepared from dwf1 was unable to convert 6-deoxoDS to 6-deoxoCS and DS to CS, as seen in either wild-type or 35S:DWF1 transgenic plants. This suggests that DWF1 has enzyme activity not only for a sterol C-24 reductase, but also for a BR C-24 reductase that catalyses C-24 reduction of 6-deoxoDS to 6-deoxoCS and of DS to CS in Arabidopsis. Overexpression of DWF1 in a BR-deficient mutant (det2 35S:DWF1) clearly rescued abnormalities found in det2, indicating that DWF1 functions in biosynthesis of active BRs in Arabidopsis. Expression of DWF1 is down-regulated by application of CS and BL and in a BR-dominant mutant, bes1-D. E-boxes in the putative promoter region of DWF1 directly bind to a BR transcription factor, BES1, implying that DWF1 expression is feedback-regulated by BR signaling via BES1. Overall, biosynthesis of 24-methylene BR is an alternative route for generating CS, which is mediated and regulated by DWF1 in Arabidopsis.

Brassinosteroids Regulate Root Growth, Development, and Symbiosis

Brassinosteroids (BRs) are natural plant hormones critical for growth and development. BR deficient or signaling mutants show significantly shortened root phenotypes. However, for a long time, it was thought that these phenotypes were solely caused by reduced cell elongation in the mutant roots. Functions of BRs in regulating root development have been largely neglected. Nonetheless, recent detailed analyses, revealed that BRs are not only involved in root cell elongation but are also involved in many aspects of root development, such as maintenance of meristem size, root hair formation, lateral root initiation, gravitropic response, mycorrhiza formation, and nodulation in legume species. In this review, current findings on the functions of BRs in mediating root growth, development, and symbiosis are discussed.

Growth-limiting proteins in maize coleoptiles and the auxin-brassinosteroid hypothesis of mesocotyl elongation

The shoot of grass coleoptiles consists of the mesocotyl, the node, and the coleoptile (with enclosed primary leaf). Since the 1930s, it is known that auxin (indole-3-acetic acid, IAA), produced in the tip of the coleoptile, is the central regulator of turgor-driven organ growth. Fifty years ago, it was discovered that antibiotics that suppress protein biosynthesis, such as cycloheximide, inhibit auxin (IAA)-induced cell elongation in excised sections of coleoptiles and stems. Based on such inhibitor studies, the concept of "growth-limiting proteins (GLPs)" emerged that was subsequently elaborated and modified. Here, we summarize the history of this idea with reference to IAA-mediated shoot elongation in maize (Zea mays) seedlings and recent studies on the molecular mechanism underlying auxin action in Arabidopsis thaliana. In addition, the analysis of light-induced inhibition of shoot elongation in intact corn seedlings is discussed. We propose a concept to account for the GLP-mediated epidermal wall-loosening process in coleoptile segments and present a more general model of growth regulation in intact maize seedlings. Quantitative proteomic and genomic studies led to a refinement of the classic "GLP concept" to explain phytohormone-mediated cell elongation at the molecular level (i.e., the recently proposed theory of a "central growth regulation network," CGRN). Novel data show that mesocotyl elongation not only depends on auxin but also on brassinosteroids (BRs). However, the biochemical key processes that regulate the IAA/BR-mediated loosening of the expansion-limiting epidermal wall(s) have not yet been elucidated.

Propiconazole is a specific and accessible brassinosteroid (BR) biosynthesis inhibitor for Arabidopsis and maize

Brassinosteroids (BRs) are steroidal hormones that play pivotal roles during plant development. In addition to the characterization of BR deficient mutants, specific BR biosynthesis inhibitors played an essential role in the elucidation of BR function in plants. However, high costs and limited availability of common BR biosynthetic inhibitors constrain their key advantage as a species-independent tool to investigate BR function. We studied propiconazole (Pcz) as an alternative to the BR inhibitor brassinazole (Brz). Arabidopsis seedlings treated with Pcz phenocopied BR biosynthetic mutants. The steady state mRNA levels of BR, but not gibberellic acid (GA), regulated genes increased proportional to the concentrations of Pcz. Moreover, root inhibition and Pcz-induced expression of BR biosynthetic genes were rescued by 24epi-brassinolide, but not by GA(3) co-applications. Maize seedlings treated with Pcz showed impaired mesocotyl, coleoptile, and true leaf elongation. Interestingly, the genetic background strongly impacted the tissue specific sensitivity towards Pcz. Based on these findings we conclude that Pcz is a potent and specific inhibitor of BR biosynthesis and an alternative to Brz. The reduced cost and increased availability of Pcz, compared to Brz, opens new possibilities to study BR function in larger crop species.

Brd1 gene in maize encodes a brassinosteroid C-6 oxidase

The role of brassinosteroids in plant growth and development has been well-characterized in a number of plant species. However, very little is known about the role of brassinosteroids in maize. Map-based cloning of a severe dwarf mutant in maize revealed a nonsense mutation in an ortholog of a brassinosteroid C-6 oxidase, termed brd1, the gene encoding the enzyme that catalyzes the final steps of brassinosteroid synthesis. Homozygous brd1–m1 maize plants have essentially no internode elongation and exhibit no etiolation response when germinated in the dark. These phenotypes could be rescued by exogenous application of brassinolide, confirming the molecular defect in the maize brd1-m1 mutant. The brd1-m1 mutant plants also display alterations in leaf and floral morphology. The meristem is not altered in size but there is evidence for differences in the cellular structure of several tissues. The isolation of a maize mutant defective in brassinosteroid synthesis will provide opportunities for the analysis of the role of brassinosteroids in this important crop system.

[Progress in the analysis of brassinosteroids]

Brassinosteroids have been considered as a class of plant hormones with high activity. However, the complex matrix of the plant samples and the ultra-trace level of naturally occurring brassinosteroids make their separation and determination very difficult. This review summarizes the progress in the development of sample pretreatment and determination of brassinosteroids.

Withanolide A is inherently de novo biosynthesized in roots of the medicinal plant Ashwagandha (Withania somnifera)

Ashwagandha (Withania somnifera Dunal., Solanaceae) is one of the most reputed medicinal plants of Ayurveda, the traditional medical system. Several of its traditionally proclaimed medicinal properties have been corroborated by recent molecular pharmacological investigations and have been shown to be associated with its specific secondary metabolites known as withanolides, the novel group of ergostane skeletal phytosteroids named after the plant. Withanolides are structurally distinct from tropane/nortropane alkaloids (usually found in Solanaceae plants) and are produced only by a few genera within Solanaceae. W. somnifera contains many structurally diverse withanolides in its leaves as well as roots. To date, there has been little biosynthetic or metabolism-related research on withanolides. It is thought that withanolides are synthesized in leaves and transported to roots like the tropane alkaloids, a group of bioactive secondary metabolites in Solanaceae members known to be synthesized in roots and transported to leaves for storage. To examine this, we have studied incorporation of (14)C from [2-(14)C]-acetate and [U-(14)C]-glucose into withanolide A in the in vitro cultured normal roots as well as native/orphan roots of W. somnifera. Analysis of products by thin layer chromatography revealed that these primary metabolites were incorporated into withanolide A, demonstrating that root-contained withanolide A is de novo synthesized within roots from primary isoprenogenic precursors. Therefore, withanolides are synthesized in different parts of the plant (through operation of the complete metabolic pathway) rather than imported.

Molecular and biochemical characterization of two brassinosteroid sulfotransferases from Arabidopsis, AtST4a (At2g14920) and AtST1 (At2g03760)

Mammalian sulfotransferases (EC 2.8.2) are involved in many important facets of steroid hormone activity and metabolism. In this study, Arabidopsis AtST4a and AtST1 were identified and characterized as brassinosteroid sulfotransferases that appear to be involved in different aspects of hormone regulation. The two proteins share 44% identity in amino acid sequence, and belong to different plant sulfotransferase families. AtST4a was specific for biologically active end products of the brassinosteroid pathway. The enzyme sulfated brassinosteroids with diverse side-chain structures, including 24-epibrassinosteroids and the naturally occurring (22R, 23R)-28-homobrassinosteroids. AtST4a belongs to a small subfamily of sulfotransferases having two other members, AtST4b and -c. Among the three recombinant enzymes, only AtST4a was catalytically active with brassinosteroids. Transcript expression of AtST4 subfamily members was largely specific to the root. AtST4b- and -c transcript levels were induced by treatment with trans-zeatin, while AtST4a was repressed under the same conditions, supporting a divergent function of AtST4a. Co-regulation of AtST4b and -c correlated with their location in tandem on chromosome 1. AtST1 was stereospecific for 24-epibrassinosteroids, with a substrate preference for the metabolic precursor 24-epicathasterone, and exhibited catalytic activity with hydroxysteroids and estrogens. To gain more insight into this dual activity with plant and mammalian steroids, enzymatic activities of human steroid sulfotransferases toward brassinosteroids were characterized. The dehydroepiandrosterone sulfotransferase SULT2A1 displayed catalytic activity with a selected set of 24-epibrassinolide precursors, including 24-epicathasterone, with specific activities comparable to that measured for the endogenous substrate dehydroepiandrosterone. The comparable activity profiles of AtST1 and SULT2A1 suggest a similar architecture of the acceptor-binding site between the two enzymes, and may potentially reflect a common ability to conjugate certain xenobiotics.