Brassinosteroids: Practical Applications in Agriculture and Human Health

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Brassinosteroids (BRs) regulate plant growth and development and show structural similarities to animal steroidal lactones. They are ubiquitously distributed throughout plant kingdom and regulate a broad spectrum of plant developmental and physiological processes, including gene expression, cell division and expansion and differentiation at nanomolar to micromolar concentrations. Exogenous applications of BRs revealed their enhancing effects on yield and quality of crops, vegetables and fruits. Besides, BRs have also been reported to play a significant role in stress-protection in both biotic and abiotic stress in plants. Recently, BRs have attained worldwide attention for their bioactivities in diverse test systems as well as in agricultural applications. Burgeoning studies have divulged antiviral, antifungal, antiproliferative, antibacterial, neuroprotective and immunomodulatory properties of BRs in animal systems. In human cells, BRs and their analogues are reported to inhibit cell growth in cancer cell lines. Keeping in view the agricultural, stress protective and medicinal properties of BRs, they are emerging as potential candidates for use in human welfare.

Brassinosteroids is a new group of plant growth regulators with significant growth promoting influence which was first isolated and characterized from the pollen of Brassica napus L. Subsequently, they were reported from monocots, dicots, gymnosperms, pteridophyte and alga. Brassinosteroids are now considered as plant hormones with pleiotropic effects as they influence varied developmental processes like growth, germination of seeds, rhizogenesis, flowering, senescence and all kinds of stresses. This chapter reviews the effect of brassinosteroids in mitigating water and saline stresses.

Brassinosteroids (BRs) comprise a specific class of low-abundance, natural polyhydroxy steroidal lactones and ketones now recognized as a new class of phytohormones. These steroids of ubiquitous occurrence in plants are known to stimulate stem elongation and to control apical dominance as well. In this chapter, we describe the use of BRs to significantly improve protocols for micropropagation of woody species, more specifically, the marubakaido apple rootstock and a clone of Eucalyptus, through the stimulation of shoot elongation and formation of new shoots. It is also shown in this chapter that these BR-induced changes in shoot architecture do not result from changes in the endogenous levels of any single metabolite and do not rely on broad changes in the metabolite profile, as well.

From passive life forms to living organisms that can sense a myriad of external signals and alter their development, our perception of how plants interact with the environment has changed profoundly. We now know that plants can promptly respond to biotic stressors by quickly reallocating resources from growth to defensive traits. Which signals do plants use to finely tune their “growth versus defense balance”? Hormones seem to fulfill this role, since they are associated with almost every process in plant development, and also with stress responses. For this reason, plants are constantly modulating hormonal pathways in order to better allocate internal resources. Here, we discuss how brassinosteroids (BRs), steroidal plant hormones known to be potent growth regulators, work as strong mediators of plant biotic stress responses. Interaction between BRs and other stress hormones, like jasmonates and salicylic acid, important to build-up defensive barriers necessary to cope with insects and microbes are also discussed. Finally, we present evidence that these plant steroids are not only directly involved in defense responses against pest and pathogens, but they are also key regulators in the resource allocation decision.

The studies discussed here include the effect of brassinosteroids (BRs) on seed germination, growth, and development, yield characteristics, nitrogen metabolism, including nitrate reductase activity, chlorophyll content, photosynthesis and antioxidant system of plants grown under different levels of heavy metals. This chapter is expected to provide an easy access to the pivotal role of BRs on heavy metal detoxification.

In this chapter, we reviewed the antiviral activity of natural and synthetic brassinosteroids (BRs). Brassinolide and other natural BRs such as 28-homocastasterone present a broad antiviral spectrum against RNA and DNA viruses. Since the concentration of BRs in the plant tissue is very low, isolation of these compounds from plant sources is impractical, and they had to be obtained by chemical synthesis. The antiviral activity of a group of synthetic analogues against Herpes Simplex Type 1 and 2 (HSV-1 and HSV-2), Measles (MV), Vesicular Stomatitis Virus (VSV), Polio Virus (PV) and Arena Viruses was also determined. Several of the tested compounds showed selectivity indexes 10- to 18- fold higher than ribavirin, a broad spectrum antiviral compound, for Junín Virus (JUNV), a moderate activity against HSV-1 / HSV-2 and good anti-MV, anti-PV and anti-VSV activity, with antiviral selectivity Index (SI) values also higher than ribavirin, a reference drug. Structure activity relationship Studies were performed in order to determine which stereochemistry, type and position of functional groups are needed to develop a potent and selective class of viral inhibitors. The antiviral mode of action of the BRs against HSV, JUNV and VSV was also investigated. For all assayed viruses, the antiviral compounds adversely affect virus protein synthesis and mature viral particle formation.

Many viral infections are associated with the development of immunopathologies and autoimmune diseases, of difficult treatment, for which no vaccines are available yet. Obtaining compounds that conjugate both antiviral and immunomodulating activities in the same molecule would be very useful for the prevention and/or treatment of these immunopathologies of viral origin. Within this chapter, we present the evaluation of biological properties of synthetic brassinosteroids (BRs) analogs, and stigmastane and androstane derivatives, as potential antiviral and immunomodulating agents. We chose three synthetic BRs which had exhibited high antiviral activity against different human pathogenic viruses. The new steroidal molecules were designed with chemical modifications from the three BRs, in order to improve their antiviral activity. We also included some structural moieties responsible for the immunommodulating activity of well-known anti-inflammatory steroids.

Molecular and cellular effects of two groups of antiproliferative agents, natural brassinosteroids (BRs) and their synthetic derivatives, were examined in different human cancer cell lines and in primary endothelial cells in vitro. Natural and synthetic BRs caused growth inhibition, cell cycle arrest and initiation of apoptosis in many different cancer cell lines. The inhibition of proliferation and migration of human endothelial cells by BRs was demonstrated and evidences were obtained that BRs initiate cell death by apoptosis. And, analogues of BRs were found to be more effective than natural BRs. Observed inhibition of migration and tube formation demonstrated the antiangiogenic activity of BRs. These findings indicate a potential use of BRs in the prevention of metastasis development. Investigation of the mechanisms of action of BRs in human cancer and endothelial cells using cellular and molecular techniques indicated the possible involvement of steroid receptors in BR action. However, BRs were shown not to bind directly to steroid receptors which demonstrate that BRs act via steroid receptor-independent pathway(s). Concluding, BRs and their derivatives are capable to inhibit growth of several human cancer cell lines and to inhibit angiogenesis-like behaviour of primary endothelial cells in vitro, as well.

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