Historic review on plant hormones

The growth of the plants is a complex process, but well organized and coordinated. In 1880, Julius Sachs was suspicious of of existence chemical messengers that coordinate the growth between the different parts of the plants. However, the true impulse to this theory came from the book's publication The Power of Movements in Plants for Charles Darwin, that you incorporated some of the observations done by him along with his son Francis Darwin, on the systematic behavior relating to the bending toward the light (phototropism) of the coleoptiles of canary grass. The book suited someone's purposes like diving board to clear the way for a line of investigation, that led to the identification of the hormones in plants. While metabolism provides the energy and the formation of structures for the life of the plants, the ones that regulate the growth rate of the individual parts are hormones, and they integrate these parts to produce the way that we recognize like a plant. Because of this, you became established than the growth and development of the plants is regulated only by five groups of hormones: auxins, gibberellins, cytokines, abscisic acid and ethylene (^{Seeta et al., 2002}).

The plants possess biosintetizar's capability, join a great variety of steroids, they studied whose show like hormones frequently for the following years. However, you did not get over to 1979 than confirmed him the presence of the hormones steroids in the plants. In that year, the North American scientists published the data of a new so-called steroid brassinolide, that the isolated of Brassica napus L. pollen (^{Khripach et al., 2000}).

In reality, you went from at the beginning of the decade the sixty when some investigators already had the hypothesis that the accelerated germination and the growth of the pollen grains would be able to be correlated with promoters presence of the growth. You reported yourself in 1970 than some abstracts of Brassica napus L´s pollen they produced a powerful effect of elongation (lengthening) in the bean stem. This answer was different to the one that another named hormones produce giberelinas to. The substances that promoted the growth of more active way were isolated of Brassica napus L., and brassinos were called for it. Consequently, you assumed the status of plant hormone to the brassinos because they were organic specific compounds, isolated of plants and that they had induced growth when they were diligent in minute quantities to another plants (^{Mitchell et al., 1970}). As from isolation and this brasinolido's identification, the investigations intensified led to go into the effects that this new compound and related others generate in the plants (^{Nuñez y Mazorra, 2001}), in order to consider to the brassinosteroids like fitohormonas's sixth group.

The brassinosteroids's chemical structure

The present hormones so much various structures they meet between possess the peptides of low molecular weight and the steroids in animals and plants. Although hormones similitudes in his biosynthesis and general shows, his sign's molecular mechanisms of recognition and transduction to the cell's nucleus split steroids of both organisms' groups they are different. The phytohormones steroids are the only plant hormones with a chemical structure of this type in the plants, and it groups until now 59 members (^{Coll, 2006}).

The brassinosteroid´s molecules have four rings and a lateral chain (Figure 1), and they form themselves as from the block compaction of five atoms of carbon, named isoprene. The brassinosteroids with principal the ones that possess 28 carbon atoms with different substituents in two rings are presence in plants, that way as in the lateral chain. They have identified 50 brassinosteroids of vegetable sources chemically over, and the brassinolide is until now the one that produces the bigger biological activity of all, since it can be synthesized directly of the campesterol or through the general synthesis of sterols. The vegetable sterols, in addition to his paper like predecessors of the brassinosteroids, music integrating components of the cell membranes, where they regulate his fluidity and permeability (^{Bishop y Yokota, 2001}).

Figure 1 The natural brassinosteroid´s chemical structure (modified of ^{Bishop y Yokota, 2001}). 

The differences in the structure of natural brassinosteroids is due to the presence of an oxygen atom and three additional carbon into two and six of the carbon rings A and B and in the carbon positions 22 and 23 of the side chain (according to the numerical order of the carbons of the steroid analogs brassinosteroids are compounds having a similar structure to natural brassinosteroids and have an activity very similar to brassinolide (^{Bishop y Yokota, 2001}; ^{Zullo y Adam, 2002}).

In general, it can be seen that the chemical structure of steroid plant hormones is similar to that of other hormones, such as mammals or insects. Figure 2 is the chemical structure of brassinolide and castasterona, which are steroid plant hormones compared to testosterone and estradiol, which are sex steroid hormones in mammals, and steroid hormone ecdysone, found in various insect classes (^{Bishop y Koncz, 2002}). Unlike other structures, one can observe a certain number of carbons in the brassinolide that has traces of oxygen, which are important to carry out an appropriate activity of this compound as a steroid hormone.

Figure 2 Structures of steroid hormones (modified of ^{Coll, 2006}). 

After identification of brassinolide, tens of compounds structurally similar characteristics have been isolated and characterized, as dolicólida and epibrassinolide (Figure 3), which have shown different degrees of elongation stimulating activity and cell division. These compounds, in addition to brassinolide, which is the most active representative, form the family of brassinosteroids which generally are considered by many experts as the sixth class of plant hormones (^{Salgado et al., 2008}).

Figure 3 First natural brassinosteroids characterized. A) brassinolide; B) dolicólida; and C) epibrassinolide (modified of ^{Salgado et al., 2008}). 

All natural brassinosteroids even now known are byproducts the 5 colestano's polihidroxilados, and they can present from twenty seven to twenty nine carbon atoms. Functional groups that are supposed to be responsible for the biological activity of these compounds, are concentrated in three main fragments colestánico skeleton; these are the A and B rings (cyclic part) and the side chain (Figure 1). With the maturing conjugates brassinosteroids can hydrolyzed to release free brassinosteroids.

Biosynthesis of brassinosteroids

The biosynthetic pathway can be divided into two major sections: early oxidation and oxidation route later (Figure 4). The first section, covering the formation of sterols, wherein the squalene becomes campesterol, includes a series of 13 biochemical reactions; and in the second section it becomes campesterol brassinolide in 11 additional reactions. In the biosynthetic pathway certain important changes must in order to obtain bioactive molecules, among these changes include the formation of oxo group at position C-6, addition of hydroxyl groups at positions C-22 and C- 23, system diol formation at carbons 2 and 3 of the a ring and the BaeyerVillager oxidation at ring B.

Figure 4 Map of the biosynthesis of brassinosteroids (modified of ^{Bishop y Koncz, 2002}). 

The biosynthetic pathway comprising transforming campesterol brassinolide has two branch points leading to four branches within the biochemical pathway of formation, the first branch point is campesterol, which can follow the route early oxidation or late on route oxidation of carbon 22. the second branch point is the campestanol; it can follow the route of early oxidation or route oxidation of carbon late June, whatever the route to be followed, these are reunited with the formation of the castasterona, it requires more of a reaction to result brassinolide. Concerning the intracellular localization of brassinosteroids, it indicated that plastids are organelles important for these compounds. The stroma can be the site of synthesis while the starch granules are assumed to storage sites these potent growth regulators.

The regulation of stomatal opening is a complex process that depends on many factors, including light, ambient concentrations of CO₂, temperature, relative humidity, cytosolic calcium concentration, hormones and enzymes marker of metabolic pathways involved, which also exert very important influences and performances (^{González et al., 2005}; ^{Salgado et al., 2008}). Some proposals for the biosynthesis of brassinosteroids have already been reported, and possible paths have been established in 2α, 3α-diol. Campesterol is the initial precursor for biosynthesis of brassinolide, due to the similarity of their carbon backbone along with other sterols such as α-sitosterol, and 22-dehydrocolesterol brasicastrol (^{Wada y Marumo, 1981}; ^{Salgado et al., 2008}).

Distribution in plants

Many plant steroids have been identified, but only brassinosteroids are widely distributed throughout the plant kingdom, and possess a unique growth-promoting activity (^{Li y Chori, 1999}). Brassinosteroids mainly found in pollen, leaves, buds, flowers and seeds, in proportions and different ways, characterizing polihidroxiesterpoides compounds. Some of the plants and their parts that brassinosteroids are presented are presented in Table 1 (^{Seeta et al., 2002}).

Brassinosteroids are present in plants at very low concentrations (nanogram levels). Endogenous levels of brassinosteroids vary between plant tissues. Young or growing tissues contain levels higher than mature tissues brassinosteroids. Pollen and seeds are the richest sources with a range of 1- 100 ng/g dry weight, while the shoots and leaves have lower quantities, ie 0.01-0.1 ng/g dry weight (^{Seeta et al., 2002}).

Physiological effects of brassinosteroids

Plants, like other multicellular organisms have developed along its evolution, different mechanisms to recognize and respond to external signals from the environment, which include abiotic and biotic factors and signs of internal environment body, such as the specific case of hormones. Applying brassinosteroids induce a wide range of responses, including an increase in cell leaf expansion, increased stem elongation, pollen tube growth, of leafroll pasture, reorientation of cellulose microfibrils, induces conducting tissue, influencing photomorphogenesis, cell division, either they can stimulate or inhibit rooting, involved in the induction of ethylene biosynthesis in membrane polarization and are substances which positively influence against stress biotic and abiotic (^{Coll, 2006}).

From the isolation and identification of pollen brassinolide Brassica napus research to study the effects that this new compound and related plants generate intensified. As early as 1983, it was reported that treatment of lettuce plants with 22, 23, 24-triepibrasinólida increased crop yield when it was grown in soil fertilized not optimally. Later, in assessing the practical uses of brassinolide is considered an important action of this compound was accelerating resistance to stress, such as low temperatures, fungal infection, herbicide injury and soil salinity (^{Nuñez and Mazorra, 2001}).

The plant response to brassinosteroids include effects on signaling systems for defense against insects and fungi in cell elongation and stem cell division, vascular and reproductive development, the polarization of the membranes and pumping protons, the source/consumption site and modulating stress relationships. Also it reported their influence on gravitropism and delaying abscission of leaves and fruits (^{Clouse, 1996}; ^{Izquierdo, 2011}).

Because of the substantial effects of brassinosteroids on growth and development of plants, the economic potential of brassinosteroids in agriculture was recognized in the early eighties. The synthesis of brassinosteroid analogues confirming the structure-activity relationship provides a method for preparing large amounts of active brassinosteroids for use and evaluation greenhouse and field level. Extensive testing of a synthetic brassinosteroid, 24 epibrassinolide in China, Japan and Russia showed that despite being an exogenous compound has the ability to increase performance in a variety of plant species, the results are variable depending on the mode application, growth stage and environmental conditions (^{Divi and Krishna, 2009}; ^{Izquierdo, 2011}).