Researchers in Dinesh Christendat’s lab have pinpointed a protein that all land plants need to harvest sunlight and grow on land. Using sequence analysis and CRISPR/Cas9 gene editing, they identified a protein that is present in land plants but not other organisms and showed that this protein evolved to sustain photosynthesis when plants first moved onto land approximately 500 million years ago.
Given plants’ fundamental need for photosynthesis, this research provides a target for sustainable herbicides against parasitic plants and other weeds. This protein can also enhance the structures that gather light for photosynthesis to allow more productive crops.
The results are published in Molecular Biology and Evolution as “Shikimate Kinase-Like 1 Participates in an Ancient and Conserved Role Contributing to Chloroplast Biogenesis in Land Plants“.
SKL1 gains novel protein function
“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’” asserts Dr. Michael Kanaris, a former PhD student in the Christendat lab.
The evolution of new protein function is a particular fascination of the Christendat lab. When genes duplicate leading to two identical copies of a protein due to errors in DNA replication, one copy may take on new functions as organisms adapt to environments over the course of millions of years evolution.
As one example, the SKL1 protein in flowering plants is a copy of the SK protein that has gained a new function. Whereas SK is involved in making specialized compounds, Dr. Christendat’s prior research determined that flowering plants are stunted and albino without SKL1 due to defective chloroplast development that impairs photosynthesis.
Dr. Christendat’s new research probes the function of SKL1 in earlier plants. Flowering plants evolved about 130 million years ago, so Dr. Christendat decided to look further back within liverworts, which were among the first plants to colonize land about 500 million years ago.
Conserved role of SKL1 in liverworts surprises
Dr. Christendat’s team used CRISPR/Cas9 genome editing in liverworts to disrupt SKL1. The result was so unexpected that Dr. Christendat asked his team to repeat the experiment several times. They confirmed that liverworts with disrupted SKL1 are pale and have stunted growth, just like flowering plants lacking SKL1. They realized SKL1 might have the same function in chloroplast development in a plant even older than flowers!
To confirm that liverwort SKL1 truly had the same function, the team put liverwort SKL1 into a flowering plant lacking SKL1 that is albino. Remarkably, the resulting seedlings grew with a green set of first leaves with rescued chloroplasts!
“My colleagues were astonished when I showed them, saying ‘Wow, that’s really cool!’” Dr. Christendat asserts, “because liverworts are a very ancient plant species. And we were assuming that the way SKL1 functions in liverwort would be very different to a more recently evolved plant.”
All plants have SKL1, as revealed by an analysis of gene sequences from diverse liverworts, ferns, mosses and flowering plants, whereas ancestors to modern-day plants including water-living algae have only the original SK protein. Dr. Christendat’s team was excited to realize that not only is SKL1 function conserved over 500 million years of plant evolution, but it is also essential for their existence on land!
SKL1 structure suggests future applications
The team turned to protein structure analysis to address what provides this novel function to SKL1. They determined that structural reorganization of the shikimate binding site of SK resulted in the evolution of a new ligand binding site in SKL1.
Future work on SKL1 and its potential ligands could improve our ability to grow crops. The metabolic pathway involving the SK protein is the target of herbicides including Roundup, so the SKL1 protein may be a more effective target for new generations of herbicides given its fundamental function. Certain domains of the SKL1 protein vary across plants, so it may be possible to target SKL1 from specific plants to ensure safety and sustainability.
Microscopically, flowering plant chloroplasts containing liverwort SKL1 are enriched in the structures that capture light for photosynthesis in specific light conditions. SKL1 could be targeted to improve the ability of crops to grow in light conditions that are better suited for the environment, which are topics under investigation within the Christendat lab.