How plants conquered the land

It’s difficult to imagine a land without plants, but at one-time plant life was only found in water.

New research has uncovered how aquatic plants evolved into the land plants we know today, through a hormone that prevents dehydration.

The University of Leeds is part of an international team led by Hebrew University of Jerusalem, which has unlocked one of the secrets that enabled plants to colonise land approximately half-a-billion years ago.

About 500 million years ago, aquatic green algae - pond slime - started to establish on land. Aquatic algae are typically single-celled or consist of simple filaments. Left without water, they dry out quickly and usually don’t survive, but all today’s land plants are descended from what was probably an ancestral accident - the alga being washed up on the shores of an ancient freshwater lake and being able to withstand dehydration.

Dr Andrew Cuming, from the University of Leeds’ School of Biology, said: “We have identified a key part in a complex jigsaw that tells us how aquatic plants moved onto the land approximately 500 million years ago. This was an event that changed the face of the planet.”

The work is published today in the journal Proceedings of the National Academy of Sciences (PNAS). It describes the evolutionary origins of a hormonal mechanism used by plants to survive dehydration – a necessary prerequisite for growth on land.

Key stress hormone

The most ancient land plants are anatomically simple - moss tissues are typically one-cell thick, but many species are incredibly hardy and can colonise bare surfaces (rocks, walls, roofs) exposed to continual cycles of dehydration and rehydration. This is because they use a stress hormone to trigger a genetic programme that results in the cells accumulating protective proteins and sugars that enable the cells to survive. 

The hormone is called abscisic acid (ABA) and it’s found in all plants. In the flowering plants it can still activate a set of genes that enable dehydration tolerance - but usually only in seeds – and it regulates a host of intermediate responses to water stress that enable today’s plants to inhabit a wide range of habitats.

The genes activated by abscisic acid are present in all plants, from algae to artichokes, but their activation by abscisic acid only occurs in the land plants. This is because only land plants contain a receptor protein that recognises abscisic acid, raising the question of just how the land plants suddenly gained a receptor gene.  

The answer was provided by collaborators from the University of Goettingen, who discovered a gene encoding an ancestral receptor-like protein in the alga Zygnema – one of the algae most closely related to land plants. 

This protein doesn’t interact with abscisic acid, explaining why algae can’t respond to the hormone, but they found that it interacts with and activates the other components of the dehydration response in the absence of the hormone. This is a low-level activation, but it’s enough to explain how that ancestral alga may have survived that initial exposure on land. 

During the course of evolution the receptor acquired changes by mutation that caused it to bind abscisic acid and activate the dehydration response more powerfully. 

This hormonal trigger – coupled with an increase in the numbers of genes encoding the receptor - enabled plants to diversify and conquer the terrestrial environment.