Many aspects of plant biology, especially anatomical topics, are oft-neglected or simply overlooked. This item is an opportunity to give a bit of publicity to the outermost layer of the aerial parts of plants, the cuticle – a thin continuous membrane consisting of a polymer matrix (cutin), polysaccharides and associated solvent-soluble lipids (cuticular waxes).
Although stomata are pre-eminent when it comes to controllable loss of water – transpiration – from plants, it falls to the cuticle to reduce uncontrolled water loss from plant parts when the stomata are shut, or on above-ground plant surfaces devoid of stomata. The cuticle is arguably the plant’s ultimate defence against dehydration.* Given the importance of this layer’s role, it may seem surprising that it is only a few micrometres thick. Which makes it all the more essential that the cuticle is fit for purpose.
One of the habitats where an efficient cuticle is of paramount importance is in hot deserts, which are not only hot, but also low in water, with the result that both getting and retaining water is a major issue for plants. Plants in such places often have many modifications that enable them to survive in such a water-challenging environment and are often classified as xerophytes. One such adaptation is often a thicker-than-normal cuticle. But is thicker necessarily the same as effective?
Bearing in mind that cuticles are basically coverings of fats, and that fats become more mobile when warmed up (think of how much easier it is to spread butter when removed from the refrigerator, after it’s been allowed to warm up to room temperature), might cuticles become more fluid and less effective as barriers for water loss with increased temperature? Or, and more scientifically put, what is the relationship between how well cuticles can resist movement of water under the high temperatures in a desert environment and cuticle chemistry? Do such cuticles differ from those of plants from more mesic environments where water is more readily available?
Surprising as it may seem there appears to be little information on these important points, which dearth of desert data Ann-Christin Schuster et al. aimed to put right. Using Rhazya stricta (an evergreen, dwarf shrub of the Apocyanaceae that lives in arid areas from the Arabian Peninsula to southern Iran to northwest India, they investigated the effect of increased temperature on the permeability of its cuticle to water. Although this property increased – i.e. at higher temperatures (up to 50 °C) more water was lost through the cuticle than at lower temperatures (from 15 °C) – this increase was much less than for plants living in non-Arabian desert conditions.
The team propose that this reduced ‘leakiness’ is due to the large amounts of triterpenoids they found in R. stricta’s cuticle. Specifically, they suggest that those organic molecules restrict the thermal expansion of the polymer (i.e. they stabilize the cuticle’s structure), thereby reducing the disruption that accompanies elevated temperatures in cuticles of other plants. In this way, the integrity of the plant’s barrier to uncontrolled water loss is kept intact.**
The above finding represents an evolutionary dimension – adaptation of plants to their environment – in a flowering plant, one of the crowning glories of plant evolution. An even longer-term evolutionary angle is present in the next paragraph…
Just as retaining water in hot desert environments is important to the survival of highly-developed plants, development of a cuticle and the ability to survive on dry land is one of the pivotal ‘moments’ along the evolutionary path that allowed ‘plants’ to colonise the terrestrial habitat and ultimately give rise to the land flora (which we know better as the Plant Kingdom). A major conclusion arising from the examination of cuticles of mosses (plants that represent some of the earliest examples of terrestrial vegetation as part of the great evolutionary journey from aquatic green algal ancestors to angiosperms) by Lucas Busta et al. was that ‘overall, wax composition and coverage on Funaria hygrometrica were similar to those reported for some vascular plant species, suggesting that the underlying biosynthetic processes in plants of both lineages were inherited from a common ancestor.’ Another piece of evidence supporting the evolutionary view of the origins of the Plant Kingdom.[Ed. – this item reminds me of a question to ask your undergraduates: Where on a plant would you find the cuticle? One would certainly expect the answer ‘on the aerial parts’. However, the more informed/better read student(s) might volunteer the information that a cuticle can also be found in the seeds of some angiosperms (!). For more on this phenomenon, the open-access article by Julien De Giorgi et al. might be a good starting point].
* Amongst other roles. The cuticle also protects the plant in other ways (for more information, see pages like this).
** Since cuticles are famously not just barriers to liquid water, but to water vapour, they are also gas-proof barriers to a great extent. So, they don’t just decrease water vapour loss from a plant but also reduce gas exchange between the plant and the external environment. If ‘water’ permeability increases at high temperatures, mightn’t permeability to other gases also increase? Could this be a route by which plants in such hot desert habitats gain extra CO2 from the atmosphere, which could be used for photosynthesis? Could this be another ingenious way that plants get around limitations to photosynthesis when stomata are closed, but when light is abundant…?