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Brassinosteroids (BR) are hormones displaying high activity in stimulation of plant growth and development. They are present at low concentrations in pollen grains, anthers, seeds, leaves, stems, roots and vegetative tissues undergoing early developmental stages in a broad range of species representing various evolutionary groups. The richest sources of brassinosteroids are pollen grains and immature seeds, whereas leaves and stems contain much lower concentration of these hormones. The first identified representative of this class of hormones was a compound characterized as a steroidal lactone brassinolide, extracted in 1979 from pollen grains of Brassica napus. Glucosylation, sulphonization and acetylation of brassinosteroids was noted and these chemical modifications enable their transportation, storing and inactivation. Brassinosteroids are polyhydroxylated derivatives of sterols. Intact metabolism of sterols is crucial for proper embryogenesis, xylogenesis and development of shoot and root apical meristems. Biosynthesis of sterols, which precursor is mevalonian, was elucidated to the greatest extent in Arabidopsis thaliana and divided into three stages: as a result of a series of reactions constituting phase A of this pathway an intermediate is produced, which may serve as a substrate of two further alternative synthesis routes: the first one, known as phase B, leads to production of sitosterol and stigmasterol being an important components of phospholipid membranes, influencing their properties. The last stage of sterol biosynthesis pathway, defined as phase C, comprises series of reactions leading to production of brassinosteroids. As far as genetic regulation of sterol biosynthesis is concerned, late stages of this process leading to brassinosteroid production are much better elucidated by identification of genes encoding enzymes catalyzing reactions constituting phase C, together with characterization of mutant phenotypes, displaying defects in this process. Early stages of sterol biosynthesis pathway are relatively much less understood only some of the genes responsible for these reactions were identified and phenotypes of mutants with abnormalities in this process include impaired embryogenesis. Biochemical and genetic analysis clarified the brassinosteroid signal transduction pathway in A. thaliana, which commences with perception of this hormone on the surface of plasma membrane leading to alteration in genes' expression. Brassinosteroids are perceived by transmembrane polypeptide BRI1 (Brassinosteroid-Insensitive 1) functioning as a serine-threonine kinase, belonging to a vast family of proteins, containing LRR (Leucine-Rich Repeat) domains. As a result of research, several genes were identified, encoding enzymes catalyzing different types of chemical reactions, some of them function as negative regulators of BR signal transduction. Perception of brassinosteroids triggers deactivation of signaling inhibitors and, on the other hand, accumulation and induction of transcription factors, which lead to expression of genes constituting molecular response to perception of this hormone. Brassinosteroid metabolism is maintained in the state of dynamic homeostasis on the basis of feedback mechanism between their synthesis and signaling. Brassinosteroid regulate broad range of physiological processes, including: seed development and germination, cell division and elongation, anther development, microspore germination and pollen-tube growth, differentiation of tracheary elements, proton pumping and membranes polarization, leaf senescence, induction of photosynthesis caused by increase in carbon dioxide assimilation and Rubisco activity. Brassinosteroids modulate plant metabolic response to a broad range of environmental stresses. Positive effect of brassinosteroids on aquaporin activity, responsible for transmembrane water transportation was also reported, as well as stimulation of somatic and microspore embryogenesis described in species from genus Brassica. Brassinosteroids stimulate expression of genes encoding alpha- and beta-tubulin proteins and re-orientation of cortical microtubules, which is essential for proper deposition of cellulose microfibrils, influencing structural properties of a cell wall. Brassinosteroids induce nodulation in Pisum sativum. These hormones when present at high concentration inhibit root growth and lateral root formation. Some of these physiological processes are regulated on the basis of synergistic interactions with auxins. BR regulate photo- and skotomorphogenesis through molecular interactions of brassinosteroid synthesis and signal transduction with photoreceptor-initiated transduction pathway. Alterations in metabolism of brassinosteroids cause abnormalities in morphology and architecture of plants. One of these traits is dwarfism of cereals, which gained an economical importance taking into account that it enables an increase in fertilizers dosage and as a consequence greater yield because of elevated lodging resistance of short stature plants. This paper presents a comprehensive review on the genetic and molecular basis of brassinosteroid metabolism and physiological effects of these hormones on a broad range of morphogenetic processes.
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Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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