Science in Society Archive

Plant Steroids in Transgenic Plants Safe for Humans?

There are too many similarities between plant and animal steroids for them to be presumed safe. Prof. Joe Cummins

For many years, it was presumed that the regulators of growth and development in plants are distinct from the hormones that regulate growth and development in animals. Yet, many of the pesticides used to control insects and fungi were found to disrupt the endocrine systems of animals.

In 1979, steroid compounds called brassinosteroids (BRs) were discovered in the pollen of Brassica napus (rapeseed). Later studies showed that BRs are ubiquitous among plant species [1]. BRs are similar in structure to animal steroid hormones, including ecdysone, progesterone and testosterone. Steroid hormones regulate transcription through nuclear receptors and by membrane bound cell surface receptors. Trans-membrane BRs receptors have been recognized in plants [1]. Recently, nuclear transcription factors regulated by BRS were recognized [2]. Like animal steroids, BRs act on a number of aspects of growth and development, and function in a broad range of disease resistance as part of an innate immunity system [3].

A number of BR inhibitors and analogues have been produced [4], and BR analogs with high growth promoting activity have been patented [5].

BR analogues proved active in the control of mosquitoes through their resemblance to the insect molting hormone [6]. Synthetic BR analogues selectively inhibited multiplication of herpes simplex virus by interfering with late viral protein synthesis [7, 8]. BR derivatives were also effective in inhibiting replication of the measles virus [9]. These finding suggest that BRs are active in both invertebrate and vertebrate animal viruses including those infecting humans. Might they not interfere with the synthesis or action of human steroid hormones?

As in the synthesis of animal steroids, synthesis of BR is achieved using certain cytochrome p450 molecules. Researchers from the Salk Institute patented transgenic plants altered to either impair or over-express a gene for cytochrome p450 activity. The resulting transgenic plants could be manipulated to produce insect resistance, dwarfism and darker green foliage compared to wild type plants [10,11]. Another cytochrome p450 gene was isolated from dwarf plants and that gene and protein patented  [12,13]. Cotton plants with improved fibre characteristics resulted from treatment of cottonseeds with BRs prior to planting, or by producing cotton fibre in liquid cell culture. Transgenic cotton plants also produced fibres with improved characteristics when sprayed with BR analogues [14,15].

BRs are versatile and allow the production of greater cell mass if desired, as for example in forage. BRs may also provide dwarfism for grain crops to avoid lodging of the crops. BRs appear to be associated with multiple disease resistance. However, manipulation of BRs may create unexpected toxins as byproducts of the modified p450 genes.

BR analogues may provide the happy prospect of making crops grow larger, while warding off insect pests and curing genital herpes. But it is imperative that BRs and transgenic crops producing them should be tested thoroughly for untoward toxicity for human beings and other non-target species.

Article first published 30/03/06


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