Brasinosteroids . They are a family of compounds that act as regulators of plant growth. They are the only plant hormones with a steroidal-type chemical structure in plants and are considered the sixth class of plant hormones. They play an essential role in growth and development, participating in processes of cell expansion, division and differentiation in the young tissues of growing plants.
Summary
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- 1 Story
- 2 Identification and characterization
- 3 synthetic analogues. Structure and biological activity
- 4 Physiological responses
- 5 External links
- 6 Sources
History
Brasinosteroids
In 1968 the isolation reported from methanolic plant extracts distylium racemosum three fractions had an unusual promoter activity of plant growth and were named factors A1, A2 and B. In 1970 , it was reported that a lipoidal extract obtained from the pollen of the turnip ( Brassica napus ) showed a stimulatory activity surprising plant growth. Spectroscopic and X- ray diffraction studies revealed the structure of the compound responsible for this effect such as (22R, 23R, 24S) -2a, 3a, 22,23-tetrahydroxy-24-methyl-B-homo-7-oxa- 5a-cholestan-6-one3. This substance, which was calledBrasinolide is distinguished from other steroidal compounds by having a structure characterized by the presence of a diol group at positions two and three of ring A, a B-ring of type 7-oxa-lactone and a side chain bearing four contiguous chiral centers ( 20S, 22R, 23R, 24S) with methyl groups at positions C-20 and C-24 and hydroxyl groups at positions C-22 and C-23.
After the identification of brasinolide, dozens of compounds with similar structural characteristics have been isolated and characterized, which have shown different degrees of activity to stimulate cell division and elongation. These compounds, in addition to brasinolide, which is the most active representative, make up the family of brassinosteroids, which are generally considered by many specialists as the sixth class of plant hormone.
Identification and characterization
A number of bioassays that have been applied in evaluating the biological activity of natural brassinosteroid, such as “Rooting the tomato “, “First internode from the Bean “, “Weight of the cotyledons of the Radish ” and “length of the hypocotyl of the Radish ». Due to the extremely small amount in which these compounds are present in plants, microanalytical procedures have been developed based on the combination of separation techniques such as gas chromatography and high performance liquid chromatography (HPLC), structural determination techniques Spectrometryof Masses with selective ion monitoring and highly sensitive bioassays, such as the “Inclination of the Rice Leaf”, “Second Internode of the Bean” and the “Unrollment of the Wheat Leaf “.
In general, when a high purity fraction is obtained that responds positively to any of the aforementioned bioassays, it is subjected to analysis using gas chromatography in combination with HPLC, which leads to the identification of the present compound if it is a substance. previously known. Identification of unknown compounds generally requires the combination of several structural determination techniques.
Synthetic analogues. Structure and biological activity
Physiological responses
Of particular interest are the results obtained by a working group based in the Department of Agrochemistry at the University of Tokío that reports obtaining a series of analogues bearing modifications in the B52 ring. The introduction of the functional groupings lactam (24, 25, 26, 30), thioester (27, 28), ether (29), thiolactam (31) and 6-oxa-7-oxo (32) in ring B causes a marked decrease in biological activity. The results obtained corroborate the existing criteria regarding the need for the presence of oxygenated functions of a certain nature in the B ring of the steroidal nucleus.
Physiological responses
The application of brassinosteroids at levels of nanomoles to micromoles causes a pronounced elongation of hypocotyls, epicotils and peduncles in dicotyledons , as well as coleoptiles and mesocotyls in monocots (21, 66, 94). Young tissues are particularly sensitive to BRs, and, in fact, endogenous RBs are directly involved in the control of cell expansion since they are persistent in these tissues and, in addition, accumulate in areas of active elongation