Plants cannot move to chase comfort, yet they still need to choose the right season to reproduce. A new study explains how a small plant uses blue light and cool air together to decide when to flower.

A research team led by Adam Seluzicki from the Salk Institute for Biological Studies mapped a genetic switch that listens for both signals at once.

The study shows how one pathway senses blue light and another senses low temperature, and how they meet to control the transition to flowering.

When do plants start flowering?

Arabidopsis thaliana uses a blue light sensor called PHOT2 and a helper protein named NPH3 to read light signals. When the air is cool, a temperature responsive gene switch called CAMTA2 turns on another gene called EHB1.

The EHB1 gene then connects with NPH3, which brings the light and temperature signals together. The plant starts flowering only when both conditions are met – blue light is present and the air is cool.

“Here we show that signals from the PHOTOTROPIN2 (PHOT2) blue photoreceptor combine with low temperature information to control flowering,” wrote Seluzicki.

The timing of plant flowering

Seasonal timing depends on multiple cues, not a single trigger. A recent review outlines how plants sense temperature through changes in cell membranes, protein activity, and chromatin.

Day length, light color, and light intensity also shape flowering. These signals run through several networks at once, which is why combining them gives a more reliable readout than using either one alone.

Blue light pathway

Plant photoreceptors are proteins that detect wavelengths and pass along messages. The blue light sensor PHOT2, located at the cell membrane, helps direct growth and movement.

NPH3 works after the PHOT proteins and helps control how their signals are used. In the system described in the new research, NPH3 acts like the main hub. It changes its response depending on what the temperature pathway is doing.

Temperature pathway

When it is cool, the gene switch CAMTA2 becomes active. It turns on the gene EHB1, which then slows down NPH3 at the right moment.

Other studies on NPH3 show that its behavior depends on where it is in the cell and how it is modified. These details explain why EHB1’s link with NPH3 can change the balance of signals that decide when the plant flowers.

Revelations about plant flowering

When the days were long and the air was cool, plants without PHOT2 grew more leaves before making flowers compared to normal plants. At warmer temperatures, this delay disappeared, showing that PHOT2 matters most in cooler conditions.

Previous studies found that PHOT2 and other phototropins help plants adjust to daily cold at sunrise. This supports the idea that blue light sensors play an important role in how plants respond to low temperatures.

These results together make the connection between PHOT2, CAMTA2, NPH3, and EHB1 more convincing.

How the plant system functions

PHOT2 picks up blue light and passes the signal to NPH3. When it is cold, CAMTA2 turns on EHB1, and EHB1 connects with NPH3 to help control flowering.

In warm conditions, CAMTA2 does not activate EHB1 the same way, so the system works differently. In that state, flowering is not delayed, which explains why the effect only shows up in cooler weather.

Arabidopsis uses a signal called FT, also known as florigen, to send the message from the leaves to the shoot, where flowers develop. FT levels go up when day length and light quality favor flowering, and this signal also takes temperature into account.

Other blue light sensors besides PHOT proteins also affect flowering time. One called FKF1 works with the plant’s internal clock and a protein named CONSTANS to boost FT under the right day length. This shows that several blue light pathways feed into the same flowering signal.

What sets this research apart

Most past studies linked flowering to sensors called cryptochromes, phytochromes, or to the FT pathway.

This new research shows that PHOT2, a sensor usually known for guiding growth, moving chloroplasts, and opening stomata, also plays a role in flowering when it is cool.

In addition, the study shows how temperature signals through CAMTA2 and EHB1 connect directly to NPH3, the hub of the light pathway. This direct link explains why flowering is affected only when both light and temperature signals are present.

Implications for crops

Climate change is altering seasonal light patterns and nighttime temperatures in many places. A system that only triggers when both signals line up can help prevent plants from flowering too early in unpredictable springs.

The Salk team’s results highlight genes that plant breeders could adjust to control flowering in cooler conditions.

Small changes in PHOT2, CAMTA2, NPH3, or EHB1 could help time flowering to match better conditions for pollination and seed production.

Future research directions

Future work can map where, within a leaf, this module is most active. Tissue specific data would show how the signal flows into FT production and transport.

It will also be important to test crop species. If a similar module exists in brassicas or cereals, targeted edits could help stabilize yields when cold snaps interrupt the start of the growing season.

The study is published in the journal Nature Communications.

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