How Plants Wax Their Leaves
Plants have a waxy coating on their leaves, some more and some less, a fact many gardeners may notice without much thought. A recent paper by two plant biologists in Science1 reveals that even this seemingly ordinary feature comes about only through a complex process in plant cells. The waxy coating, called the cuticle, is composed of three distinct layers including water-resistant wax crystals that are synthesized by epidermal cells. Burkhard Schulz and Wolf B. Frommer, commenting on research on this subject, note that over 100 transporter genes of a class named ABC have been discovered in plants, some of which carefully move the insoluble wax molecules to the surface. They describe the process as major effort in transporting a multitude of large, complex molecules. Their diagram shows a multitude of molecular machines that take part in the construction of the “elaborate structure” of the cuticle. Yet they assume this cuticle, with its varied and essential functions, and all the machinery required to product it, arose through a “sloppy” evolutionary history:
When plants moved from water to land 450 million years ago, they needed to develop a sealed surface to protect themselves against water loss in the “dry” air environment. To solve this problem, plants invented an epicuticular wax layer that covers the entire surface of the plant that is exposed to air. This protective wax cuticle also serves a multitude of other functions. Its elaborate micro- and nanostructure prevents water and other particles from sticking to the surface of leaves, keeping them clean and so enhancing their ability to trap light for photosynthesis. Adhering water droplets and other particles are washed away in a self-cleaning process called the lotus effect. The wax layer also filters out damaging ultraviolet rays, prevents volatile chemicals and pollutants from sticking to leaves and stems, and protects plants against attack by microbes and herbivores
Schulz and Frommer want to know “what were the evolutionary steps that led to this innovation?” They figure that early plants somehow co-opted existing transporter machinery for this new function, because the plants needed it:
How did land plants invent wax secretion? The genomes of living land plants contain more than 100 ABC transporter genes. Because transporters seem to be sloppy with respect to their substrate specificity, it is feasible that when plants crept out of the water, they turned a member of the ABC transporter family into a lipid exporter by ensuring that it became localized to a different cellular compartment. Perhaps this is an example of an evolutionary principle in which sloppiness is transformed into flexibility.
It’s only a suggestion, they end; “Obviously, there is more work to be done….”
1Burkhard Schulz and Wolf B. Frommer, “A Plant ABC Transporter Takes the Lotus Seat,” Science, Science, Vol 306, Issue 5696, 622-625, 22 October 2004, [DOI: 10.1126/science.1105227].
Stupid Evolution Quote of the Week, easily. At the rate the Darwin Party is turning up the propaganda, we’re going to have to make this a daily award. So plants invented something because they needed it when they crawled out of the water onto the land, and used existing machinery that just happened to be in their toolbox. This is going to sound so stupid to everybody some day, just like it already does to anybody that cleans his ears of Charlie Ear Wax.