a day ago
How does a plant's first shoot rise safely through soil, into daylight?
Researchers from the Indian Institute of Science Education and Research (IISER), Bhopal, have found that a single protein helps plants time their first step from darkness into light.
When a seed sprouts in darkness under the soil, its stem curves into a small hook shape that protects the delicate shoot tip as it pushes upward. The hook needs to stay 'closed' until the seedling breaks through the soil and meets light. In the study, the team wanted to know how two common signals — ethylene, a plant hormone that builds up underground, and light — work together to decide exactly when the hook opens.
The team focused on what a gene called BBX32 really does in the model plant Arabidopsis thaliana. By comparing seedlings modified to lack BBX32, to churn out extra copies, to carry extra mutations, or to glow blue or green when the gene was activated or its protein moved around, the scientists could pinpoint how the protein made by the gene helps keep the hook closed.
The team also grew seedlings indarkness, red, blue, far-red light, and normal light, in plates with or without a compound that raises ethylene levels, and in thin layers of sand to imitate soil pressure.
They photographed three-day-old seedlings and used software to measure the hook angle as it opened over time. They also used genetic tools to track the performance of the BBX32 gene and counted how many seedlings breached a sand layer and turned green.
The findings were published in New Phytologiston May 28. The team comprised Nevedha Ravindran, Kavuri Venkateswara Rao, and Sourav Datta of the Department of Biological Sciences at IISER Bhopal.
They found that ethylene turns BBX32 on and that light protects BBX32 from being destroyed. The role of BBX32 itself is to keep the hook closed for longer. Without extra ethylene, BBX32 mutants behave like normal plants whereas with high ethylene or a sand cover, the hook opens too soon.
BBX32 was found to work by raising the activity of the PIF3 protein, which switched on HLS1, which kept the hook closed. If PIF3 was missing, BBX32 couldn't prevent the hook from opening.
In the sand test, only about a quarter of seedlings ever reached the surface compared to 40% of normal seedlings and 80% of over-expressors. Keeping the hook closed just a bit longer clearly helped a sprout survive its climb.
The researchers also worked out why BBX32 accumulates only when it's most useful. In total darkness, an enzyme called COP1 latches onto BBX32 and sends it to be degraded, keeping the hook flexible. Ethylene partially shields BBX32, but once the emerging seedling first senses daylight, COP1 activity drops, allowing the protein to build up on the concave side of the hook and hold it shut a little longer.
This finely tuned handshake offers a way to breed crops whose seedlings can breach denser soils — a trait that may be valuable as climate change brings heavier rains.
