The Drought Alert System of Terrestrial Plants has an Underwater Origin

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For plants, the transition from water to land was a monumental achievement that changed our world forever. Such a transition was fraught with unique challenges, not the least of which being the ever present threat of desiccation. A new study now suggests that those early land plants already had the the tools to deal with drought and they have their aquatic algal ancestors to thank.

One of the keys to being able to survive drought is being able to detect it in the first place. Without some sort of signalling pathway, plants would not be able to close up stomata and channel vital water and nutrients to more important tissues and organs. As such, elucidating the origins and function of drought signalling pathways in plants has been of great interest to science.

One key set of pathways involved in plant drought response is collectively referred to as the “chloroplast retrograde signaling network.” I’m not even going to pretend that I understand how these pathways operate in any detail but there is one aspect of this network that is the key to this recent discovery. It involves the means by which drought and high-light conditions are sensed in one part of the plant and how that information is then communicated to the rest of the plant. When this signalling pathway is activated, the plant can then begin to produce enzymes that go on to activate defense strategies such as stomatal closure.

Chara braunii  - a modern day example of a streptophyte alga

Chara braunii - a modern day example of a streptophyte alga

The surprise came when researchers at the Australian National University, in collaboration with researchers at the University of Florida, decided to study the chloroplast retrograde signaling network in more detail. They were interested in the inner workings of this process in relation to stomata. Stomata are tiny pores on the leaves and stems of terrestrial plants that regulate the exchange of gases like CO2 and oxygen as well as water vapor. To add some controls to their experiment, the team added a few species of aquatic algae into the mix. Algae do not produce stomata and therefore they reasoned that no traces of chloroplast retrograde signaling network enzymes should be present.

This is not what happened. Instead, the team discovered that the enzymes in question also showed up in a group of algae known as the streptophytes. This was exciting because streptophyte algae hail from the lineage thought to be ancestral to all land plants. It appears that the tools necessary for terrestrial plants to survive drought were already in place before their ancestors ever left the water.

Why this is the case could have something to do with the streptophyte lifestyle. Today, these algae are known to tolerate very tough conditions. Though outright drought is rarely a threat for these aquatic algae, they nonetheless have to deal with scenarios that resemble drought such as high salinity. Streptophyte algae found growing in ephemeral pools must cope with ever increasing concentrations of salinity as the water around them evaporates. It is possible that this drought signalling pathway may have evolved as a response to hyper-saline conditions such as these. Regardless of what was going on during those early days of plant evolution, this research indicates that the ability for terrestrial plants to deal with drought evolved before their ancestors ever left the water.

The closer we look, the more we can appreciate that evolution of important traits isn’t always de novo. More often it appears that new innovations result from a retooling of of older genetic equipment. In the case of land plants, a signalling pathway that allowed their aquatic ancestors to deal with water loss was coopted later on by organs such as leaves and stems to deal with the stresses of life on land. As the old saying goes, “life uhhh… finds a way.”

Photo Credits: [1] [2]

Further Reading: [1] [2]

Big Things Come In Small Packages

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Meet Blossfeldia liliputana, the smallest species of cactus in the world. With a maximum diameter of only 12 mm, this wonderful succulent would be hard to spot tucked in among the nooks and crannies of rock outcrops. Its species name "liliputana" is a reference to the fictional island of Liliput (Gulliver's Travels) whose inhabitants were said to be rather small. If its size alone wasn't interesting in and of itself, the biology of B. liliputana is also downright bizarre.

B. liliputana is native to arid regions between southern Bolivia and northern Argentina. It appears to prefer growing wedged between cracks in rock as these are usually the spots where just enough soil builds up to put down its roots. Root formation, however, does not happen for quite some time. Most often new individuals bud off from the parent plant. They emerge not from the base, but rather from apical tissues, yet another unique feature of this cactus. What's more, this cactus produces no spines. Instead, its numerous areoles are covered in a dense layer of trichomes that are rather felt-like to the touch.

As you can clearly see, this species is small. It only ever becomes conspicuous when it comes time to flower. Imagine a bunch of tiny white to pink cactus flowers poking out of a crevice. It must be a remarkable sight to see in person. Despite their showy appearance, its is believed that most are self-fertilized.

As mentioned, the size of this cactus isn't the only interesting thing about its biology. B. liliputana is categorized as a poikilohydric organism, meaning it doesn't have the ability to regulate its internal water content. Researchers have found that individual plants can lose up to 80% of their weight in water and can maintain that state for as long as two years without any negative effects. As such, colonies of these tiny cacti often appear shrunken or squished. Once the rains arrive, however, it springs back to its original rounded shape with seemingly no issues. Amazingly, a significant amount of water uptake happens via the fuzzy areoles that cover its surface, hence it does not harm the plant to hold off growing roots for quite some time. 

Speaking of water regulation, B. liliputana holds another record for having the lowest density of stomata of any terrestrial autotrophic vascular plant. Stomata are the pores in which plants regulate water and gas exchange so having so few may have something to do with why this species loses and gains water to such a degree that would kill most other vascular plant species.

Another peculiar quality of this cactus are its seeds. Unlike all other cacti whose seeds are hard and relatively smooth, the seeds of B. liliputana are hairy. Attached to each seed is a small fleshy structure called an aril, which aids in seed dispersal. As it turns out, B. liliputana relies on ants as its main seed dispersers. Ants attracted to the fleshy aril drag the seeds back to their nests, remove and eat the aril, and then discard the seed. This is often good news for the cactus because its seeds end up in nutrient-rich ant middens protected from both the elements and seed predators, often in much more suitable conditions for germination.

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Needless to say, B. liliputana is a bit of an oddball as far as cacti are concerned. Its highly derived features coupled with its bizarre biology has made it difficult for taxonomists to elucidate its relationship to the rest of the cactus family. It certainly deserves its own genus, to which it is the only member, however, it has been added to and removed form a handful of cactus subfamilies over the years. The most recent genetic analyses suggests that it is unique enough to warrant its own tribe within Cactaceae - Blossfeldieae.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3]