The biological role of boron

In 2013, a group of scientists proposed the hypothesis that boron and molybdenum may have catalyzed RNA formation on Mars , from where life was transferred to Earth by a large meteorite about 3 billion years ago ^[1] .

Some unicellular organisms produce or are capable of producing boron-containing natural antibiotics . The first of the boron-containing antibiotics found in nature was boromycin , synthesized by some soil fungi of the genus streptomycetes ^{[2] [3]} .

For plants, boron is an important trace element necessary primarily to maintain the integrity of cell walls. However, high concentrations of boron in the soil (above 1.0 ppm) lead to necrosis, first of the marginal (marginal) zone of the leaves, and then to necrosis of their calyxes, and to a general slowdown in plant growth and development. Some plants, which are especially sensitive to phytotoxicity of boron, may exhibit the same symptoms with a lower concentration of boron in the soil, but exceeding 0.8 ppm. Almost all plants, even the most resistant to the phytotoxic effect of elevated concentrations of boron in the soil, exhibit at least some signs of boron intoxication with concentrations of boron in the soil exceeding 1.8 ppm. With soil boron concentrations exceeding 2.0 ppm, very few plant species are able to maintain satisfactory productivity and growth rates. Some plants with such a concentration of boron in the soil die ^{[4] [5] [6]} .

Boron deficiency is one of the most common micronutrient deficiencies in plants throughout the world, and causes significant losses in both gross crop yields and crop quality ^[7] . Boron deficiency violates both development and vegetation, and the processes of sexual reproduction (flowering and fruiting) in higher plants. Boron deficiency leads to inhibition of reproduction of plant cells, partial or complete death of shoots (stems), leaves, buds and ovaries, a decrease in the formation of seeds and fruits, and a decrease in the content of sugars and starch in fruits ^[8] .

Plants contain boron both in water-soluble form and in insoluble, immobilized in covalent bonds with large polysaccharide molecules of the outer membrane of cells and membranes of cell organelles. In healthy plants that do not suffer from boron deficiency, the percentage of insoluble boron compounds immobilized in the cell membrane and organelle membranes in tissues remains relatively constant, while the percentage of water-soluble boron compounds in plant tissues may vary in proportion to its current supply from soil and water. The appearance of signs of boron deficiency in a plant is accompanied by a decrease in the content of insoluble cell membranes and cell organelles immobilized in bonds with polysaccharides in the plant tissues. Apparently, it is the insoluble boron compounds with polysaccharides that strengthen the cell membrane and organelle membranes that play a functional role, and it is for their biosynthesis that the plant absorbs and stores water-soluble boron compounds. At the same time, water-soluble boron compounds are an exchange pool of boron, its readily bioavailable supply in plant tissues. If necessary, insoluble functionally useful boron compounds are synthesized from this stock. Thus, water-soluble boron compounds serve to maintain boron homeostasis in plant tissues, but, apparently, they themselves do not play a functional role ^[9] .

Boron is critical for the growth and development of all higher plants. Its main function is to strengthen and maintain the integrity of cell membranes and cell organelles. However, the participation of boron in some metabolic processes in plants is not excluded ^[10] .

There are suggestions that boron plays or may play several important physiological roles in animals and humans. However, the exact physiological significance of boron for humans and animals has not yet been conclusively determined ^{[11] [12] [13]}

In a small study published in 1987, it was reported that postmenopausal women who had previously artificially created a deficiency of boron, the subsequent introduction of 3 mg / day of boron into the body led to a significant decrease in daily urinary calcium excretion and increased blood concentrations of 17-beta-estradiol and testosterone ^{[14] [13]} .

Boron deficiency in pigs , according to Wayne Johnson in 2005 at the Allan J. Lehman Veterinary Conference ^[15], may manifest as osteochondrosis . The development of these symptoms in pigs is prevented by the addition of 50 ppm boron to their normal diet ^[15] . The amount of boron necessary for the normal functioning of another animal or person is not yet fully established.

Despite all these facts, the American National Institute of Medicine and Health (NIH) has not yet confirmed the status of boron as a necessary trace element for humans, and in this regard, has not yet established the norms of recommended intake with food (Recommended Daily Allowance - RDA) or norms of adequate intake (Adequate Intake - AI). The average adult consumption of boron is estimated to be approximately 0.9 to 1.4 mg / day (900 to 1400 μg / day). Boron compounds supplied with food and water are well absorbed (absorption about 90%). Absorbed boron is excreted mainly through the kidneys with urine. The NIH established upper limit of daily intake of boron (Tolerable Upper Limit - TUL), at which obviously no harmful consequences for the body occurs, is 20 mg / day ^{[16] [13]} .

One of the rare types of corneal dystrophy is associated with the gene encoding SLC4A11 (a transporter protein that presumably regulates the intracellular concentration of boron) ^{[17] [13]} .