Researchers Identify Protein That Evolved To Enable Photosynthesis In Land Plants

Evolutionary plant biologists at the University of Toronto (U of T) have identified a protein that evolved approximately 500 million years ago, enabling plants to convert light into energy through photosynthesis as they moved from aquatic environments to land.

The discovery provides a target for sustainable herbicides against parasitic plants and other weeds and may help boost food security by increasing the efficiency of photosynthesis in crops.

Using genome analysis and CRISPR gene editing, the researchers pinpointed Shikimate kinase-like 1 (SKL1) as a protein present in all land plants— but no other organisms — and showed the protein evolved from the Shikimate kinase (SK) enzyme to play an essential role in forming the chloroplasts needed for photosynthesis.

“One of the fundamental questions we investigate in this study is: what were the initial events that contributed to simple aquatic organisms moving onto land” says Michael Kanaris, lead author of the paper published recently in Molecular Biology and Evolution.

“A role for SKL1 in chloroplast biogenesis has previously been determined in Arabidopsis, a flowering plant studied extensively in modern laboratories. However, the biological function for SKL1 has not been established in early land plants.”

Kanaris conducted the research with Professor Dinesh Christendat in the Department of Cell & Systems Biology in the Faculty of Arts & Science at U of T, whose work focuses on the evolution of new protein functions. When DNA replication errors result in two identical copies of a protein, one copy may take on new functions as organisms adapt to changing environments over millions of years of evolution.

One example is the SKL1 protein in flowering plants, which originated as a copy of the SK protein, but gained a new function. Christendat’s prior research determined that flowering plants — evolving approximately 130 million years ago — became stunted and albino without SKL1 due to defective chloroplast development that impairs photosynthesis.

To look even further back into the evolution of land plants, the researchers used CRISPR genome editing to disrupt SKL1 function in common liverworts, which were among the first plants to colonize land about 500 million years ago. The team then put liverwort SKL1 into an albino flowering plant lacking SKL1, which resulted in seedlings that grew a green set of leaves with rescued chloroplasts.

The result was so unexpected that the researchers repeated the experiment several times. They confirmed that liverworts with disrupted SKL1 are pale and have stunted growth, just like flowering plants lacking SKL1, suggesting SKL1 might have the same function in chloroplast development in a plant significantly older than more modern flowers.

“My colleagues and I were astonished because liverworts are a very ancient plant species,” says Christendat. “We assumed that the way SKL1 functions in liverwort would be very different to a more recently evolved plant. Not only is SKL1 function conserved over 500 million years of plant evolution, it is also essential for their existence on land.”

The researchers note that while all land plants have SKL1 — as revealed by an analysis of gene sequences from diverse liverworts, ferns, mosses and flowering plants — ancestors to modern-day plants including water-living algae have only the original SK protein.

“The inability to identify SKL1 in organisms predating land plants suggests an important role for this gene coinciding with the emergence of terrestrial plants,” says Kanaris.

Christendat says knowing the role SKL1 plays in photosynthesis could both improve the ability to grow crops and make it a more effective target for new generations of herbicides, as the metabolic pathway that involves the SK protein is the current target of most herbicides. “Certain domains of the SKL1 protein vary across plants, so it may be possible to target SKL1 from specific plants to ensure safety and agricultural sustainability.”

Shikimate Kinase-Like 1 Participates in an Ancient and Conserved Role Contributing to Chloroplast Biogenesis in Land Plants, Molecular Biology and Evolution (open access)

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