Brassinosteroids : A sixth class of Plant hormone

Brassinosteroids: Introduction, Discovery, Biosynthesis, and Metabolism
Introduction: Brassinosteroids (BRs) are a class of polyhydroxysteroids recognized as the sixth class of plant hormones, with potential use as anticancer drugs to treat endocrine-responsive cancers by inducing apoptosis in cancer cells and inhibiting tumor growth. These brassinosteroids were first explored in the 1970s when Mitchell et al. reported that organic extracts of rapeseed (Brassica napus) pollen promoted stem elongation and cell division. Brassinolide, the first brassinosteroid, was isolated in 1979, when it was demonstrated that pollen from Brassica napus promoted stem elongation and cell division, leading to the isolation of the biologically active molecule. Plants are exposed to various biotic and abiotic stresses throughout their life cycle, necessitating the regulation of physiological and developmental processes in response to both internal and external stimuli. These stresses significantly contribute to global crop production losses by influencing the stress tolerance and adaptive ability of plants. Plants rely on signaling molecules, including hormones, to mediate responses to these stresses. Phytohormones play a crucial role in plant growth, development, and defense mechanisms, helping plants adapt to environmental challenges. Brassinosteroids (BRs) are an essential group of plant hormones involved in regulating these processes, aiding in plant adaptation to stress.

Discovery:
Mitchell et al. first reported the promotion of stem elongation and cell division following treatment with organic extracts from rapeseed pollen (Brassica napus). The first BR, brassinolide, was extracted from the pollen of Brassica napus in 1979. Since then, many chemically related compounds have been identified and classified as part of the brassinosteroid family. Discovery of BRs in Different Plant Species: BRs were initially discovered in Brassica napus pollen for their ability to promote growth and were named "brassins." Brassinolide (BL), the most active BR, was isolated in 1979. A significant discovery was the identification of Brassinosteroid Insensitive 1 (BRI1), a receptor kinase that triggers intracellular signaling in response to BR perception. Since the discovery of BL, numerous chemically distinct BRs have been found across the plant kingdom, including in green algae and land plants, indicating that BRs evolved early in plant evolution. BRs were identified as plant hormones following the discovery of BR-deficient mutants in Arabidopsis thaliana. Among the studied BRs, Castasterone (CS), Typhasterol (TY), Brassinolide (BL), 6-deoxocastasterone (6-deoxoCS), 28-norcastasterone (28-norCS), and Teasterone (TE) are commonly found in various plant species across different environments.

Biosynthetic Pathway of Brassinosteroids:
Brassinosteroids are biosynthesized from campesterol. Japanese researchers elucidated the biosynthetic pathway.

1. Brassinoste roids Name of student : Akarshit Dhiman (MSc Fruit Science) Submitted to : Dr . Sajad Ahmed Bhat Sher-e-Kashmir University of Agricultural Sciences and Technology of Kashmir B rassino - -Steroids DIVISION OF FRUIT SCIENCE Sher-e-Kashmir University of Agricultural Science and Technology of Kashmir Discovery B iosynthetic pathways M etabolism

2. Discovery Introduction Biosynthetic Pathways Metabolism Of Brassinosteroids Introduction Discovery Biosynthetic Pathways Metabolism Brassinosteroids : Brassinosteroids  (BRs) are a class of polyhydroxysteroids that have been recognized as a sixth class of  plant hormones   and may have utility as anticancer drugs for treating endocrine-responsive cancers by inducing apoptosis of cancer cells and inhibiting cancerous growth. These brassinosteroids were first explored during the 1970s when Mitchell et al. reported promotion in stem elongation and cell division by the treatment of organic extracts of rapeseed ( Brassica napus ) pollen.  Brassinolide  was the first brassinosteroid to be isolated in 1979, when pollen from  Brassica napus  was shown to promote stem elongation and cell divisions, and the biologically active molecule was isolated.

3. Brassinosteroids : Introduction Plants are exposed to a wide range of biotic and abiotic stresses throughout their life cycle and need to constantly regulate their physiological and developmental processes for responding to numerous internal and external stimuli. Various biotic and abiotic stresses significantly contribute to major global crop production losses by primarily influencing the stress tolerance/adaptive ability of plants. Plants utilize various signaling molecules, including hormones for mediating the plant response to the number of stresses. Phytohormones have been widely considered as the natural activators for plant growth and development. They maintain healthy life in plants, and play an essential role in defense mechanisms against various stresses . Phytohormones initiate a signaling cascade that involves a number of molecular players, which lead to an ideal generic pathway . Brassinosteroids (BRs) are an important group of plant hormones involved in regulating plant growth and development, and they help plants to adapt to the environment  Discovery Biosynthetic Pathways Metabolism Brassinolide , the first brassinosteroid isolated and shown to have biological Activit y :

4. Introduction Commercial Potential Application of brassinosteroids to rice, grapes, tomatoes, wheat, corn, potatoes and melon has shown an increase in their yield. It has the capability to improve the quality and quantity of various horticultural crops. It works better when the plant is stressed rather than in optimal conditions. One of the biggest advantages of using BRs on plants is that it is a natural compound that does not harm the environment. It is referred to as a natural plant-strengthening substance; hence, its use is preferred over pesticides. Discovery Biosynthetic Pathways Metabolism

5. Introduction Discovery Biosynthetic Pathways Metabolism

6. Introduction Discovery Biosynthetic Pathways Metabolism

7. Discovery : Mitchell et al. first reported elongation in stems and promotion in cell division by treatment with organic extracts of rapeseed pollen ( Brassica napus ). The first BR, brassinolide , a biologically active molecule, was extracted from the pollen of  Brassica napus  in 1979. Since then, many chemically related compounds have been discovered and included in the brassinosteroid family. Discovery Introduction Biosynthetic Pathways

8. Discovery of BRs in Different Plant Species : BRs were initially discovered in  Brassica napus  pollen on the basis of their ability to promote growth . BRs have been discovered as stimulants for plant cell elongation and division. BRs were subsequently named ‘ brassins ’. Brassinolide (BL), the most active BR, was isolated in 1979 . The most significant finding was the isolation of Brassinosteroid insensitive 1 (BRI1)—a receptor kinase that triggers an intracellular signaling cascade in response to extracellular BR perception . Since the discovery of BL, a huge number of chemically different BRs have been discovered throughout the plant kingdom, including green algae and land plants, suggesting that BRs evolved early during plant evolution. BRs were identified as plant hormones after discovering BR-deficient mutants in  A. thaliana  . Among all BRs studied to date, Castasterone (CS), Typhasterol (TY), Brassinolide (BL), 6-deoxocastasterone (6-deoxoCS), 28-norcastasterone (28-norCS), and Teasterone (TE) are commonly present in various plant species throughout different environments . Introduction Discovery Biosynthetic Pathways Metabolism

9. Biosynthetic Pathways : The BR is biosynthesised from  campesterol . The biosynthetic pathway was elucidated by Japanese researchers and later shown to be correct through the analysis of BR biosynthesis mutants in  Arabidopsis thaliana , tomatoes, and peas. The sites for BR synthesis in plants have not been experimentally demonstrated. One well-supported hypothesis is that all tissues produce BRs, since BR biosynthetic and signal transduction genes are expressed in a wide range of plant organs, and short distance activity of the hormones also supports this. Experiments have shown that long distance transport is possible and that the flow is from the base to the tips (acropetal), but it is not known if this movement is biologically relevant. A clear understanding of how endogenous BR levels are regulated via synthesis and metabolism is a required component of any molecular model of BR action. The basic features of BL biosynthesis were uncovered utilizing cell suspension cultures of Catharanthus roseus, which were fed deuterated and tritiated putative intermediates in BL biosynthesis followed by analysis with sensitive techniques of GC-MS to monitor conversion of the labeled compounds (Fujioka and Yokota, 2003). Based on side chain structure and stereochemistry, wide distribution in the plant kingdom, and the relative biological activities in bioassays, it was predicted that the plant sterol campesterol would be converted to BL via teasterone , typhasterol and castasterone (Yokota et al., 1991). This skeletal pathway, in addition to several intermediate steps, was confirmed in C. roseus cells and seedlings. Moreover, it was found that C-6 oxidation could occur before (Early C-6 oxidation pathway) or after (Late C-6 oxidation pathway) hydroxylation of the side chain (Fujioka and Yokota, 2003). While campesterol has been shown to be the BL progenitor, other BRs are likely to also be derived from common plant sterols with appropriate side chain structure such as sitosterol, isofucosterol , 24-methylenecholesterol, and 24-epicampesterol (Yokota, 1997). Discovery Introduction Biosynthetic Pathways Metabolism

10. Biosynthetic pathway : The biosynthetic pathway to BL can be divided into general sterol synthesis (cycloartenol to campesterol ), and the BR-specific pathway from campesterol to BL. Besides their role as BR precursors, plant sterols such as campesterol are integral membrane components which serve to regulate the fluidity and permeability of membranes and directly affect the activity of membrane associated proteins, including enzymes and signal transduction components (Clouse, 2002a; Lindsey et al., 2003; Schaller, 2003).   Introduction Discovery Biosynthetic Pathways Metabolism

11. Introduction Discovery Biosynthetic Pathways Metabolism Biosynthetic pathways of sterols from cycloartenol :

12. Introduction Discovery Biosynthetic Pathways Metabolism Biosynthetic pathway from campesterol to brassinolide :

13. METABOLISM Most of the early studies on BR metabolism have been in species other than Arabidopsis (Adam et al., 1999; Bajguz , 2007). In cell suspension cultures of Solanum lycopersicum , 25-hydroxy24-epibrassinolide and 25-b-D-glucosyloxy-24-epibrassinolide constituted the major radioactive BRs seven days after feeding with [3 H]-24-epibrassinolide (Schneider et al., 1994). Further studies with [3 H]-24-epicastasterone showed that tomato cells metabolized this BR not only to the 25-hydroxy form but also to 26-hydroxy-24-epicastasterone and 26-b-D-glucopyranosyloxy24-epicastasterone, and the same system was used to show that direct glucosylation could occur at either of the hydroxyls in the A ring after epimerization (Hai et al., 1996). Introduction The use of specific inhibitors suggested that one cytochrome P-450 and another hydroxylase were involved in C25 and C26 hydroxylation. An Arabidopsis UDP glycosyltransferase termed UGT73C5 was found to catalyze the 23-O-glucosylation of BL and castasterone and overexpression of UGT73C5 resulted in BR-deficient phenotypes, suggesting that glucosylation also regulates BR activity in Arabidopsis Discovery Biosynthetic Pathways Metabolism

14. Another important role for BR metabolism in regulating Arabidopsis growth and development has been suggested by the work of (Neff et al., 1999). The bas1 mutant was identified in an activation-tagging screen for the ability to suppress the long hypocotyl phenotype caused by mutations in the phytochrome B photoreceptor. The bas1 mutant is BR-deficient and accumulates 26-OH-BL, a proposed inactive metabolite of BL. The molecular basis of this event is the overexpression of the BAS1 gene, which encodes a cytochrome P450 enzyme capable of hydroxylating BR to 26-OH-BL. Thus, BAS1 and related P450s may be important regulatory proteins in controlling endogenous levels of active BL in Arabidopsis Besides glucosylated forms, BR conjugation with a variety of fatty acid esters has been reported. Teasterone 3-laurate and teasterone 3-myristate were identified in lily pollen and studies of endogenous BR levels during pollen development suggested that conjugated teasterone may be a storage form which releases teasterone during pollen maturation to allow the biosynthesis of BL. Sulfotransferases from B. napus and Arabidopsis catalyze the O-sulfonation of BRs, leading to a loss of their biological activity in the bean second internode assay (Rouleau et al., 1999; Marsolais et al., 2007). This suggests sulfonation is an additional mode of BR inactivation but the implications of this have not yet been examined in planta. METABOLISM : Introduction Discovery Biosynthetic Pathways Metabolism

15. Thank You For Watching Akarshit (MSc Fruit Science) Division of Fruit Science SKUAST-Kashmir REFERENCE : Brassinosteroids Steven D. Clouse Department of Horticultural Science, North Carolina State University, Raleigh.