Effects on growth

Promoting effects of brassinosteroids on the elongation of vegetative tissue have been observed in many species, but only a few have been studied in detail. Overall, brassinosteroids have been tested to evaluate their plant growth promoting activity in more than 20 typical bioassay for the activity of auxin, gibberellins and cytokinins. They have been shown to induce both elongation and cell division, resulting in growth, thickening and curvature oats coleoptiles. In several systems, brassinosteroids interact synergistically with auxin and reported that brassinosteroids can function as auxin at a time and as gibberellins or cytokinins in another (^{Mandava, 1988}). Cell elongation, stimulated by the application of brassinosteroids has been determined to be due to a synergistic or additive effect to that caused by auxins and gibberellins (^{Salgado et al., 2008}; ^{Gudesblat and Russinova, 2011}).

Treatment with plant hormones affect recognized induced elongation brasinolina; gibberellins have an additive effect and an inhibitory effect zeatin. With auxin is a synergism, where they induce brasinolina allows elongation when they are alone ineffective. Exogenous auxin affects the kinetics of the response to brasinolina; however, the synergism found in cucumber can be attributed to an increase in the amplitude of the response to auxin. Interestingly, although both auxin and brassinosteroids promote elongation, its kinetics are very different, as they generally show auxin very short period of time (10-15 min) between the application and the beginning of elongation; however, brassinosteroids have a span of at least 45 min elongation speeds continue for several hours (^{Clouse, 1996}).

Comparing the effects of brassinosteroids with other plant growth regulating substances such as stand characteristics: an activity at extremely low concentrations (0.1-0.001 mg/L), a range 100 times less than that of the other regulators plant growth, stimulate rooting, they do not cause deformations, primarily exert their effect when plants are under adverse growing conditions. Toxicological studies show that these compounds are not genotoxic, nor eco-toxic and neither antigenotoxic (^{Díaz and Fonseca, 1999}).

In studies of morphogenesis, the application of them brassinosteroids has also evidenced positive results; He has been observed than the addition of brassinosteroids to the means of cultivation, reactions of elongation and cell division, cell desdiferenciation with the formation of corn induce, or they stimulate the development of sprouts, bulbs and roots. The brassinosteroids they possess a great potential to increase the development and floral growth; For example, the bulbs of gladioluses and tulips soaked up in epibrasinólida's solution, originate a premature emergency of floral yolks and an increment of the number of flowers and a lofty increase of the number (85%) and mass (68%) of bulbs and yolks bulbs (^{Salgado et al., 2008}).

Level molecular, them the expression of genes and the metabolism of nucleic acids and proteins modify brassinosteroids. In works of morphogenesis in vitro, in addition to the regulators of growth traditionally used (auxins, cytokinins and gibberellins) they include to some brassinosteroids, with positive results fundamentally in the phase of adaptation of the plantelets, in the percentage of germination and vigor of the plantelets (^{Salgado et al., 2008}).

Effects on the biotic and abiotic stress

In addition of his impact in the growth and development of the plants, them brassinosteroids they play an important role in the tolerance to the biotic and abiotic stress, such like the resistance to diseases, tolerance to drought, salinity, heat, cold, pesticides and metals weighed between other (^{Bajguz and Hayat, 2009}; ^{Gomes, 2011}; ^{Vriet et al., 2012}). The plants also reply and they adapt several environmental stress-producing factors between the ones that include the drought, the cold and the salinity themselves, stops to survive even bass severe conditions. As a consequence they induce several physiological answers and biochemistries in the plants; In addition, they right now have described a variety of genes that they answer to the referred stress builders at the same level as the transcription (^{Gonzalez et al., 2005}; ^{Sirhindi, 2013}).

Some results obtained in previous investigations demonstrate than them brassinosteroids they do not have a lot of effect when the conditions of growth are optimal, but yes when plants grow on conditions of stress. The reactions of the plants to different kinds of stress are correlated with the generation of reactive sorts of oxygen, what suggests that these can work like a common sign at the roads of signposting of the answers to the stress of the plant (^{Bajguz and Hayat, 2009}; ^{Kutschera and Zhi-Yong, 2012}; ^{Bajguz and Piotrowska-Niczyporuk, 2014}).

Some studies accomplished in citruses have utilized them brassinosteroids like mechanism for the propagation for embriogénesis somatic, since it has been considered that these biorreguladores are effective in the morphogenetic processes, utilized generally to low concentrations, like substitutes or complements of the auxinas and citoquininas (^{Hernandez et al., 2010}).

The application of abstracts of Brassinosteroids and his analogous they have proven that they help adaptation, example thereby is the presence of a bigger resistance of tobacco, cucumber and tomato to virus and pathogenic mushrooms (Sphaerotheca fuligenia, Botrytis, etc.). Another example is the application of 24 epibrasinoesteroide (24 EBL) that is utilized to preserve cultivations from potato against Phytophthora. To cultivations of barley once 24 EBL were sprinkled with, reduce the incidence of the disease considerably of to the sheet, caused for Helminthosporium teres Sacc. The application of brassinosteroids in plants of tobacco, syringae generates a resistance to Pseudomonas syringae pv. In the case of the cucumbers, posterior to the treatment with brassinosteroids, the resistance to the peronosporosis increased (^{Hayat et al., 2011}; ^{Kutschera and Zhi-Yong, 2012}).

In a study for the improvement of the quality of plantelets in vitro of potato, it was managed to obtain a better acclimatization when they transferred greenhouses and bigger performance of the mini-tubers. Also, it was discovered than application in vitro to low concentrations of the Brasinoesteroide manages to upgrade the resistance to the stress of acclimatization in situ (^{Kowalski et al., 2003}).

In several studies reported that several authors mention the use of brassinosteroids in vitro cultures has proven to be an effective addition of 2.4 D (2 mg / L), as for callus induction rice varieties Pokkali and J-104. They also found that these brassinosteroids in low concentrations favored callogenesis potato (Solanum tuberosum), providing a further breakdown in cell suspensions and promoting the development of meristematic cells and embryonic structures. It was also possible to observe effects induced callus coffee concentrations of 0.01, 0.05 and 0.001 of brassinosteroids and transferred to an embryogenic culture medium where a marked stimulation of the development thereof was observed (^{Rodriguez et al., 2003}).

Brassinosteroids capacity to counteract the inhibitory effects of salinity on the growth of plants has been reported in various investigations. For example, seeds Eucalyptus camaldulensis have been treated with 24-epibrasinolido, which resulted in increased seed germination in saline. Similarly, compounds 24-and 28-homobrasinolido epibrasinolido been used to counteract the inhibitory effect of germination and growth of rice plants generated Salinity. The seed treatment with brassinosteroids very dilute solutions significantly improves the growth of the rice plants in saline (^{Seeta et al., 2002}). With this capability is demonstrated brassinosteroids conferring plant resistance against a wide variety of environmental stresses, which can be established that the role of brassinosteroids in protecting plants against stress or environmental change is an issue of significant research and can contribute greatly to the use of brassinosteroids in agricultural production.

The presented experimental approaches can give an idea of how far the physiological and biochemical changes associated with tolerance are a direct consequence of the effect of brassinosteroid, so it is necessary to expand the range of defined conditions to explore the stress responses of plants as a result brassinosteroids (^{Nuñez and Mazorra, 2001}).

Other effects (germination and aging)

Besides the growth, development and plant resistance, it has also been established that brassinosteroids can promote seed germination. This has been corroborated by several investigations, where the treatment of seeds Eucalyptus camaldulensis with 24 epibrassinolide resulted in a substantial improvement in the percentage of seed germination. Similarly, brassinolide, 24-and 28-homobrasinolida epibrassinolide promoted the germination of peanut. The ability of brassinosteroids to promote germination of seeds has also been observed in rice, wheat, tomato and snuff. Furthermore, the aging phenomenon in plants can also be regulated by plant hormones, and therefore, brassinosteroids also play a crucial role in the regulation of aging. In this regard, it has been reported that compounds brasinólidos accelerated aging Xanthium and Rumex. Brassinosteroids capacity to promote independent senescence in cotyledons of cucumber and bean leaves has also been reported (^{Seeta et al., 2002}).

Mode of action

Different hypotheses have been advanced to explain the effect of brassinosteroids on cell expansion. It was found that the effect of brassinosteroids is genetically determined and are probably involved in all steps in the regulation of cell growth. Furthermore, the mode of action is mainly to an effect on the biosynthesis of enzymes as a result of genome expression and an effect on the wall and cell membrane. It has been shown that these compounds are able to influence the electrical properties of the membrane permeability, the structure, stability and activity of enzymes of the membrane (^{Salgado et al., 2008}). Although numerous studies have been conducted on the effect of brassinosteroids in various plant species there is still no exact basis on its mechanism of action. However, it is understood that, as in all joints ligand-receptor, weak non-covalent interactions play a decisive role in the formation of the hormone-receptor complex.

Studies have shown that a possible receptor protein called BRI1, presents certain rich domains in the amino acid leucine (LRR), which can function in the recognition of extracellular signals that occur on the cell surface. Other studies have shown that the protein is indeed BRI1 receiver brassinosteroids in plants, and at the same time the importance of the extracellular domain of BRI1 regarding recognition of brassinolide demonstrated. In this same study is detailed later found that brassinosteroids can be attached directly or indirectly to the BRI1 protein, to achieve signal transduction so that the binding occurs through a specific region within the extracellular domain of the receptor BRI1. Certain relationship between the extracellular domain and BRI1 LRR receptor, which confers some resistance to disease was also found, which helped to clarify the existence of certain interactions between signaling pathways steroids confer this resistance diseases, so it is said that brassinosteroids increase resistance to attack by pathogens. The presence of a cytoplasmic domain that has kinase activity and is indispensable for the transmission of the signal into the cell was also found, this domain is the one with the potential sites of phosphorylation by serine and threonine, which suggests that these sites may work for the activation of the kinase through homodimerization of the receptor BRI1 (^{Mazorra and Nuñez, 2008}).

Generally the activation mechanism BRI1 receptor complex is unknown, but there are two hypothesis which first suggested the BRI1-induced heterodimerization BAK1 brassinosteroids, and other states that BRI1 monomers interact to form homodimers and not it is initially requires BAK1 for the receiver to recognize ligand.

The study of mutant plants has only it identified a gene signaling brassinosteroids, assumed encoding similar to the steroid receptors in animal protein, cloning of the gene coding for the BRI1 protein and further elucidation of the structure showed that has similarity to known as receptor molecules, but not those that were previously known to participate in steroid signaling events. The BRI1 is a member of a family of transmembrane receptor kinase type having structural similarity to a variety of proteins and are involved in protein-protein interaction. The BRI1 seems to be the main recipient of brassinosteroids in Arabidopsis (^{Salgado et al., 2008}).

Importance of brassinosteroids in agriculture

Currently, the agricultural sciences have alternatives that make less essential chemicals; likewise, the use of bioproducts for plant nutrition has been increasing to the extent that they demonstrate that they are able to minimize the use of chemicals, all of which are of great value today, and to be drawing guidelines to modify the so-called modern agriculture that advocated sustainable agricultural systems, from the point of production, ecological, economic and social terms. The use of brassinosteroids promotes plant growth, germination, nitrogen fixation, senescence, abscission and stress tolerance and others.

At present more than 40 kinds of compounds, structurally and functionally related to brassinosteroids which have applications in agriculture are known; the most important features are its ability to increase yield, crop quality, increase resistance to pesticides and reduce the water, salt, heat and nourishing to stimulate the synthesis of polypeptides stress. For example, the salt stress is one of the environmental factors that limit the productivity of crops. Salinity, besides inducing degradation of agricultural land, reduces the ability of plants to absorb water, causing a rapid reduction in the growth rate and decreased yield and quality of crops (^{Terry et al., 2001}; ^{Pérez et al., 2002}; ^{Zullo and Adam, 2002}; ^{Nuñez et al., 2006}; ^{Serna et al., 2012}; ^{Nuñez et al., 2014}).

Experiments to investigate the potential of brassinosteroids use in agriculture began in the 70's in the United States and the beginning of the 80s of last century, along with other countries like Japan and the countries of the former Soviet Union (USSR), confirming its usefulness as agricultural chemicals. Since then, numerous reports have appeared worldwide and many discoveries have been patented for use of potential practices. In Belarus and Russia epibrassinolide production it has officially registered since 1992 and recommended for the treatment of various important agricultural crops such as tomatoes, potatoes, cucumbers, peppers and barley (^{Khripach et al., 2000}). Even the potential of these compounds on the fields economic effects are immense. Because of its natural and applied to low concentrations they have great ecological importance. It is known that in the United States, Russia, China and Japan were applied to crops of cereals, legumes, potatoes, vegetables and significant yields were obtained (^{Perez et al., 2002}).

Rice cultivation has benefited from the use of these compounds, especially in Japan and China, since the application of brassinosteroids by foliar spray or irrigation, has obtained an increase in crop production 10-15%. Currently, an acceleration achieved in ripening rice plants treated with brassinosteroids, a pronounced for cultivated plants in cold and salt conditions effect was observed. Treatment of rice seedlings with a solution of 5 ppm brassinolide, caused a 22% increase in fresh weight and 31.5% by dry weight of seeds per plant, also increasing the growth rate, the size of the root and stem (^{Khripach et al., 2000}; ^{Zullo and Adam, 2002}). In Mexico has also been carried out to study the effect of brassinosteroids on some types of crops, which have been developed multiple research for use in agriculture, being an alternative to increase the potential of these crops, and that represents a profit for the producer (^{Vargas and Irizar, 2005}).

For example, an investigation whose objective was to determine crossbreeds' answer took effect androestériles and fertile of corn in front of brasinoesteroide's application (CIDEF 4). They evaluated at the Experimental Field Valley of Mexico, in the cycle spring - summer 2004, two fitohormona's dose (30 and 60 g ha^-1) with his respective witness in 10 genotypes For performance variable was no significant applying CIDEF-4 genotype in the overall analysis, regardless of the type of cross difference. While the effect a separating types of crosses, analysis showed that applying CIDEF-4 trilinear hybrids fertile highly significant differences, where the dose of 30 g ha^-1 had yielded 8 083 t ha^-1 compared with the control, the value was 3 858 t ha^-1. Whereas in the male-sterile hybrid trilinear no significant differences for the variables evaluated were presented. However, in the single cross hybrids fertile and male sterile version brassinosteroid forward male and female flowering (^{Torres et al., 2007}).

In another study the effect of brassinosteroid growth regulator was studied in dry matter accumulation and grain yield of acoyote (Phaseolus coccineus L.) variety white Tlaxcala (^{Vargas and Irizar, 2005}). The brassinosteroid increased total biomass and organs in most studied densities compared to treatments without regulator, at densities of 105 and 120 thousand plants per hectare, this increase was greater in roots, stems and leaves. Under the experimental conditions higher performance with population density of 90 000 plants per hectare applying brassinosteroid at a dose of 40 mg was obtained ha-1, with an increase to 68% from production in the same density without brassinosteroid.

As to the ripening of fruits, brassinosteroids stimulate fruit ripening. In cultivation of grape (Vitis vinifera), the maturation period was associated with increased levels catasterone. In addition, the exogenous application of brassinosteroids, improved the rate of maturation of the berry; brassinosteroids are also involved in fruit ripening tomato. Applying tomato pericarp discs, increased lycopene content and carbohydrates and decreasing the levels of chlorophyll and ascorbic acid (^{Vriet et al., 2012}).

Brassinosteroids analogs employed in agriculture

After the discovery of natural brassinosteroids have been synthesized brassinosteroid analogues from various substrates, compounds possessing similar to natural brassinosteroids biological effect, but with advantages in the application, as an effect longer field (^{Kowalski et al., 2003}; ^{Salgado et al., 2008}).

Due to the low concentration of brassinosteroids in plants have been conducted studies aimed at the synthesis of analogs brassinosteroids not present in nature, they have produced qualitatively similar to the effects of natural compounds. These analogues are generally characterized by reproducing only certain structural groups present in natural brassinosteroids, and is known to have a marked effect on the biological activity (^{Yokota, 1997}; ^{Nuñez et al., 2014}). Brassinolide, because of its extremely high biological activity, it is the most important member among brassinosteroids. Its synthesis has been achieved by starting the reaction sequence with pregnenolone, hyodeoxycholic acid and a sterol mixture containing crinoesterol especially stigmasterol.

The homobrasinólida another brasinoesteroide has been synthesized from stigmasterol, is derived from soybean oil, which is relatively inexpensive and available in sufficient quantities (^{Khripach et al., 1999}) commercial product. The epibrassinolide, which is synthesized from ergosterol, is considered the brasinosteroide with the greatest potential in practical applications because it has a combination of major, such as a biologically potent activity characteristics, as its activity is comparable to brassinolide (^{Salgado et al., 2008}). The biobras-16 (BB-16) and biobras 6 (BB-6) have been very positive effects on yield and quality of many tropical crops of great economic importance. Moreover, the MH5 is a new product that is used in the process of in vitro crop (^{Kowalski et al., 2003}; ^{Salgado et al., 2008}; ^{Núñez et al., 2014}).

All these effects are of great interest for agriculture; for that reason, more than a decade has been developing various analogs, including analogs of brassinosteroids espirostánicos, in order to reduce production costs and make products more stable field-level biological activity. One of these analogs espirostánicos is called MH5, which has the characteristic of having three OH groups on the A and B rings of the steroidal nucleus and demonstrated that their addition to the culture media is effective in various biotechnological processes and on increasing the activities of some antioxidant enzymes in tomato seedlings under conditions of thermal stress. Moreover, treatment of rice seedlings at low concentrations of BB-16, another espirostánico brassinosteroid analogue is able to increase the activity of some antioxidant enzymes when grown in medium supplemented with NaCl 75 mM (^{Nuñez et al., 2006}).

Several of these compounds have been applied to plants grown in soil contaminated with metals, in saline soils and high temperature conditions, finding that they give resistance, reducing the effects of stress (^{Salgado et al., 2008}). In some legumes such as soybeans, application of analogues by foliar spraying on the stalks or pouring, it has resulted in increased biomass of leaves, pods and total dry weight at 18, 40 and 10% respectively.

Against this background it is established that the most important feature of brassinosteroids is its ability to increase not only production, but also the quality of the crops (^{Salgado et al., 2008}). Given the current importance of counting on environmentally sound, able to protect the plants to certain types of stress products, it is necessary to continue to deepen the potential of brassinosteroids and their analogues (^{Nuñez et al., 2014}).