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]

Are Algae Plants?

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I was nibbling on some nori the other day when a thought suddenly hit me. I don't know squat about algae. I know it comes in many shapes, sizes, and colors. I know it is that stuff that we used to throw at each other on the beach. I know that it photosynthesizes. That's about it. What are algae? Are they even plants?

The shortest answer I can give you is "it depends." The term algae is a bit nebulous in and of itself. In Latin, the word "alga" simply means "seaweed." Algae are paraphyletic, meaning they do not share a recent common ancestor with one another. In fact, without specification, algae may refer to entirely different kingdoms of life including Plantae (which is often divided in the broad sense, Archaeplastida and the narrow sense, Viridiplantae), Chromista, Protista, or Bacteria.

Caulerpa racemosa , a beautiful green algae.

Caulerpa racemosa, a beautiful green algae.

Taxonomy being what it is, these groupings may differ depending on who you ask. The point I am trying to make here is that algae are quite diverse from an evolutionary standpoint. Even calling them seaweed is a bit misleading as many different species of algae can be found in fresh water as well as growing on land.

Cyanobacteria are photosynthetic bacteria, not plants.

Cyanobacteria are photosynthetic bacteria, not plants.

Take for instance what is referred to as cyanobacteria. Known commonly as blue-green algae, colonies of these photosynthetic bacteria represent some of the earliest evidence of life in the fossil record. Remains of colonial blue-green algae have been found in rocks dating back more than 4 billion years. As a whole, these types of fossils represent nearly 7/8th of the history of life on this planet! However, they are considered bacteria, not plants.

Diatoms (Chromista)

Diatoms (Chromista)

Diatoms (Chromista) are another enormously important group. The single celled, photosynthetic organisms are encased in beautiful glass shells that make up entire layers of geologic strata. They comprise a majority of the phytoplankton in the world's oceans and are important indicators of climate. However, they belong to their own kingdom of life - Chromista or the brown algae.

To bring it back to what constitutes true plants, there is one group of algae that really started it all. It is widely believed that land plants share a close evolutionary history with a branch of green algae known as the stoneworts (order Charales). These aquatic, multicellular algae superficially resemble plants with their stalked appearance and radial leaflets.

A nice example of a stonewort ( Chara braunii ).

A nice example of a stonewort (Chara braunii).

It is likely that land plants evolved from a Chara-like ancestor that may have resembling modern day hornworts that lived in shallow freshwater inlets. Estimates of when this happen go back as far as 500 million years before present. Unfortunately, fossil evidence is sparse for this sort of thing and mostly comes in the form of fossilized spores and molecular clock calculations.

Porphyra umbilicalis   - One of the many species of red algae frequently referred to as nori.

Porphyra umbilicalis  - One of the many species of red algae frequently referred to as nori.

Now, to bring it back to what started me down this road in the first place. Nori is made from algae in the genus Porphyra, which is a type of Rhodophyta or red algae. Together with Chlorophyta (the green algae), they make up some of the most familiar groups of algae. They have also been the source of a lot of taxonomic debate. Recent phylogenetic analyses place the red algae as a sister group to all other plants starting with green algae. However, some authors prefer to take a broader look at the tree and thus lump red algae in a member of the plant kingdom. So, depending on the particular paper I am reading, the nori I am currently digesting may or may not be considered a plant in the strictest sense of the word. That being said, the lines are a bit blurry and frankly I don't really care as long as it tastes good.

Photo Credits: [1] [2] [3] [4] [5] [6]

Further Reading: [1] [2] [3] [4]

 

The World's Only (Known) Photosynthetic Vertebrate

You may be asking yourself right now why I have posted a picture of a salamander this morning. This is a plant blog after all! Well, what I am about to tell you may seem a bit crazy, but I assure you this discovery has opened up some doors that science never really considered a possibility before. The yellow spotted salamander (Ambystoma maculatum) is the first and only (known) photosynthetic vertebrate ever discovered!

That's right. You heard me. A photosynthetic animal. More accurately speaking, it is the embryos of this species that undergo photosynthesis. To understand why this happens we must back up a little bit. Yellow spotted salamanders are a species of mole salamander that can be found in wet areas of eastern North America. They spend most of their adult lives underground, hiding beneath logs and rocks in the forest, feeding on any manner of invertebrates. Once a year (around this time) adult yellow spotted salamanders undertake a massive migration down to the pools where they mate. On the first few warm, rainy nights, thousands of salamanders can be seen trucking their way to vernal pools and ponds to breed. It is an amazing sight to behold.

The thing about yellow spotted salamanders is they will only breed in fishless ponds. Their larvae would be an easy meal for many predatory fish species. The problem that arises out of this breeding strategy is that fishless ponds tend to be very low in oxygen. It has long been known that the eggs of this species form a symbiotic relationship with an algae. The algae produce oxygen for the developing embryo and the embryo feeds the algae via its nitrogen rich waste and CO2. This relationship was always thought to be external, that is until Ryan Kerney of Dalhousie University in Halifax, Nova Scotia discovered that embryos of a certain age actually had algae living within their cells.

They algae don't seem to start off inside the cells though. This may be why this relationship wasn't discovered earlier. Roger Hangarter at Indiana University found that it isn't until parts of the salamander's nervous system begin to develop that the algae move into the embryo and set up shop. The algae then reside near the salamander's mitochondria, which are the powerhouses of the cell. So where are the algae coming from? While more research needs to be done, Karney also discovered the presence of algae in the oviducts of adult female spotted salamanders. It is looking like mother salamanders are actually passing the algae on to their offspring. 

Though this is the first and only instance we know of this sort of photosynthetic relationship in vertebrate animals, this discovery has opened the door for exploring the possibility of other photosynthetic symbionts. It has also allowed scientists a different avenue to explore just how cells recognize and deal with foreign bodies. We live in such an amazing world!

Further Reading: [1] [2] [3]