The response I have had to my last column (Sept 10, 2009) on the plant Sanjeevani has been gratifyingly large. While some named a few other locales where plants with the same name are found, others named competitors to Selaginella for this distinction.
But the most scientifically rigorous and illuminating response has come from the famous plant biologist Dr. Ramesh Maheshwari of Bangalore. He pitched Selaginella as Sanjeevani from a biochemical angle. His reason focused on this plant’s ability to play dead for months and “resurrect” itself into life in full bloom when water is sprinkled on it.
This remarkable ability is shared by a few other plants such as Rudanti or D.fimbriatum found in the Western Ghats, and Myrothamnus of Zimbabwe in Africa. The logic used by Drs. Ganeshaiah and colleagues in suggesting that Selaginella or D. fimbriatum could be Sanjeevani was based on the traditional medicinal belief “Like cures Like”.
Since Sanjeevani is a plant that resurrects itself from inertness to bustling life, it is believed to resurrect near-dead people into active life. But what is the mechanism by which it is able to “go dead” and revive itself when conditions are favourable?
Professor Maheshwari points out that such “resurrection plants” are the only plants that contain the sugar called trehalose, instead of the usual sugar sucrose. And trehalose has some remarkably unique properties that no other sugar has. And the secret of resurrection plants lies in their ability to synthesize trehalose and store it as a preservative agent.
What is it about the sugar trehalose that is special? Trehalose is a molecule made up of two glucose molecules stitched together chemically; it is thus a dimer of glucose or a diglucoside. Now, a glucose molecule has five hydroxyl groups, using any one of which it can stitch itself (or chemically bond with) another glucose.
Given ten such hydroxyls, and each of them spatially disposed in one of two ways, the number of possible glucose dimers is large indeed. The resultant shape of each diglucoside, with its hydroxyl groups flung out in space, is thus a mélange or medley of molecular shapes that could teach gymnasts like Nadia Comaneci some lessons.
If maltose adopts a characteristic shape because of the way its atoms are put together, cellobiose has a different shape of its own and trehalose its own. And it turns out that the way that some of the relevant hydroxyl groups of trehalose are disposed in space exactly mimics the way water molecule is shaped — as a tetrahedral prism.
In a soup or cellular cytoplasm containing trehalose and water, when the water is evaporated off by dehydration, trehalose appears to take over, attaching itself to the cellular molecules, membranes and other units in exactly the same geometrical way that water does; and the system does not seem to realize it has lost water.
Professor James Clegg of the University of California at Davis is the pioneer in studying this phenomenon of how trehalose takes up the role of water in maintaining the integrity of cells, tissues and even organisms as they are desiccated.
He showed over fifty years ago that a particular type of shrimp called Artemia Salina produces embryos in the form of capsules or cysts that can survive complete dehydration, at a state where metabolism, as we understand it, ceases. He called this phenomenon “anhydrobiosis” or life in the dry state.
When the cysts are rehydrated, they rapidly imbibe water, resume active metabolism and develop into full grown shrimps. Since this remarkable discovery of Clegg and the role that trehalose plays in “playing water”, keeping the cellular components in “deepfreeze” as it were, others have tried using trehalose as a preservative for cells, blood, vaccines.
The idea here is to dispense with the “cold chain” or refrigerated storage and transport. Trehalose stabilized free-dried products are currently in various stages of development and clinical trials for human use.
Turning now to plants, not all plants synthesize trehalose and store them within themselves. It appears that the class that the plant Selaginella belongs to is the one that does so with ease. And for good reason — since they are among the oldest plants known to us (Dr. Jo Ann Banks of Purdue University traces them to at least 400 million years ago), and have had to survive climate catastrophes of various kinds.
And apart from making trehalose to let them keep alive, they also have been able to gain genes that produce molecules to fight infection win over natural and invader–induced stress, to repel predator plants and animals, and produce plant hormones that help them grow well.
And some of these molecules are now seen to be of use in medicine, as Lord Lakshmana found out. Incidentally, I cannot resist pointing out at the same time that Sanjeevani, with its ability to go to sleep for long, beats Kumbhakarna hollow!