Brassinosteroids

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Structural formula of brassinolide

Brassinosteroids ( BR ) are ubiquitous phytohormones . The steroid compounds were first isolated from rapeseed ( Brassica napus ) in 1970 ( 10 mg brassinolide were found in 230 kg rapeseed pollen). About 70 connections are known to date. Brassinosteroids, together with other phytohormones, regulate the development and growth of plants.

structure

The structure and synthesis of brassinosteroids are triterpenes . They are sterane derivatives with a 7-fold ring B containing an oxygen heteroatom. Their structure is very similar to that of cholesterol , the precursor to animal steroid hormones . The most effective brassinosteroid is brassinolide (BL), which was first isolated from oilseed rape ( Brassica napus ) and leads to increased length.

biosynthesis

The biosynthesis probably takes place in the plastids of all plant tissues. It is based on the plastid terpenoid synthesis via the methylerithritol phosphate route (MEP route). Isopentenyl pyrophosphate (IPP) is produced via the MEP route . Condensation turns three IPPs into farnesyl pyrophosphate (FPP), and two FPPs become squalene . Campesterol , a common sterol in plant membranes, is formed from squalene through cyclization and several intermediate steps . This pathway is very similar to the steroid synthesis pathway in animals. However, this is based on IPP from the animal Mevalonatweg (MVA-Weg) in the cytoplasm . Brassinolide is formed from campesterol via a reaction chain in which cytochrome P450 -dependent hydroxylases and a reductase are involved. Inactivation can take place enzymatically by oxidases. The transport of brassinosteroids probably takes place in the form of various intermediate stages in the phloem and xylem of the plant tissue , but they often act near their place of synthesis.

Physiological effects

Brassinosteroids promote the development and growth of plants independently of other phytohormones. An extract from common pitchers contains a relatively large amount of brassinosteroids, which have been shown to have a positive effect on the growth of other plants. They work (also in comparison to other phytohormones) in very low concentrations. Deficiency mutants show dwarfism, reduced apical dominance , increased anthocyanin synthesis and low fertility.

Cell division and elongation

Brassinosteroids promote shoot growth similar to, but additively to auxins , by inducing cell division and elongation growth ( acid growth by activating proton pumps). They also control the formation of the pollen tube for fertilization . However, the root growth is inhibited by brassinosteroids. In addition to auxins, BRs also strengthen apical dominance (which inhibits the sprouting of side buds) and gravitropism .

Cell differentiation

Brassinosteroids lead to the formation of blood vessels, especially to the differentiation of the xylem . Furthermore, they are essential control factors for the Etiolement z. B. in scotomorphogenesis (development in the dark) and inhibit the formation of anthocyanins , which the plant protects against UV light .

Senescence

The senescence of the leaves is increased by brassinosteroids.

Ethylene synthesis

Brassinosteroids can also promote ethylene biosynthesis in the leaf . This leads to slower growth of the underside of the petiole and thus to epinasty (curving of the leaves downwards).

Stress / pathogen resistance

In addition, brassinosteroids seem to be associated with resistance to stress and disease and, for example, increase resistance to cold.

Molecular Effects

The perception of brassinosteroids occurs in the cell membrane (although they can diffuse as lipophilic molecules through the membrane). The membrane receptor responsible for this is named BRI1 ( BRassinosteroid Insensitive 1 ) after the corresponding mutant . It is a homodimer that acts as a serine / threonine kinase . Normally (without brassinosteroids) BRI1 is inactive, while a second kinase (BIN2: Brassinosteroid INsensitive 2) is active in the cytoplasm . BIN2 phosphorylates the two transcription factors BZR1 and BES1, which are then broken down in the proteasome . The action of brassinosteroids activates the BRI1 receptor, which then inactivates BIN2. The transcription factors BZR1 and BES1 are no longer degraded and induce the gene expression of brassinosteroid-regulated genes. Brassinosteroid-regulated genes are used, for example, for the synthesis of cell wall components and the etiolement .

literature

  • JW Mitchell, N. Mandava, JF Worley, JR Plimmer, MV Smith: Brassins: A new family of plant hormones from rape pollen. In: Nature . 225, pp. 1065-1066 (1970).
  • MD Grove, GF Spencer, WK Rohwedder, N. Mandava, JF Worley, JD Warthen, GL Steffens, JL Flippen-Anderson, JC Cook: Brassinolide, a plant growth-promoting steroid isolated from Brassica napus pollen. In: Nature. 281 (1979), pp. 216-217.
  • A. Bajguz: Metabolism of brassinosteroids in plants. In: Plant Physiology and Biochemistry. 45 (2007), pp. 95-107.
  • S. Fujioka, A. Sakurai: Biosynthesis and metabolism of brassinosteroids. In: Physiologia Plantarum. 100 (1997), pp. 710-715.
  • SD Clouse, JM Sasse: Brassinosteroids: Essential regulators of plant growth and development. In: Annu. Rev. Plant Physiol. Plant Mol. Biol. 49 (1998), pp. 427-451.
  • D. Hess: Plant Physiology. 11th edition. Uni-Taschenbücher-Verlag, 2008.
  • A. Bresinski, C. Körner, JW Kadereit, G. Neuhaus, U. Sonnewald: Strasburger - textbook of botany. 36th edition. Spectrum, 2008.

Web links

Individual evidence

  1. ^ Peter Nuhn: Naturstoffchemie , S. Hirzel Wissenschaftliche Verlagsgesellschaft, Stuttgart, 2nd edition, 1990, ISBN 3-7776-0473-9 , p. 496.