Seeds germinate only when they find the right environmental conditions. Once germinated, the seedling starts developing in preparation for emerging above the ground and commencing life as an independent entity. But what happens when the conditions are not optimum after the seed has already germinated? One way that this can happen is if the seedling gets buried under the soil, resulting in prolonged dark conditions.

In a recent study, Sourav Datta, Associate Professor, Indian Institute of Science Education and Research (IISER), Bhopal and his group show that just as a seed does not germinate if the conditions are not right, seedlings also do not grow unless certain conditions are met. In particular, the authors studied the importance of the presence of light for the growth and survival of seedlings.

The researchers uncovered a mechanism by which seedlings can perceive when it is dark, resulting in a pause or arrested growth in the absence of sufficient light. This involves a plant growth hormone called abscisic acid (ABA).

Plant cells receive signals from the environment (light, temperature, pH, humidity), through sensors present on their surface. They also secrete plant hormones like ABA, auxins, gibberellins, etc. that serve a variety of functions. Signals from the sensors as well as the plant hormones instruct the cells to carry out the appropriate metabolic activities required for a given stage of plant growth. 

This is facilitated through a series of signals passed on from one molecule to another, like a baton in a relay race. Every environmental cue results in activation of a specific set of signalling pathways, helping the plant make complex decisions. Identifying the molecular players in this relay race is vital to studying plant functions and growth. 

When light is optimum for seedling growth, a certain signalling pathway is activated through light-sensing photoreceptors on the cell surface, providing a ​“go” signal for growth and development. Datta’s group found that in darkness (caused by the seedling getting buried under soil layers), a molecule called COP1 communicates with the plant hormone ABA, arresting the growth of developing seedlings. 

Datta explains that while seedlings normally slow down their growth in dark conditions, the presence of excess ABA makes seedlings more sensitive to growth arrest. This observation prompted them to hypothesize that there might be a signalling molecule activated only in the dark, working hand-in-hand with ABA to promote growth-arrest. 

Their search pointed towards COP1, a protein found to be active in dark conditions, while less active in the presence of light. The researchers found that when COP1 levels are high in seedlings, they are more responsive to ABA-mediated arrest in seedling development, compared to when it is absent. Using mutant plant lines, they found that seedlings lacking COP1 did not lower their growth rate even in the presence of ABA. These studies suggested that COP1 was the primary player responsible for making seedlings sensitive to ABA-mediated growth arrest.

Early stages of plant growth are highly regulated and crucial for the survival of seedlings. Newly germinated seedlings face stresses like low light, water scarcity, abnormal pH or salinity of soil, etc, all of which compromise their ability to make food. With limited energy stored in their tiny leaves, they have to conserve energy to adapt and survive in stressful conditions. Thus, prioritizing survival over growth, the seedlings choose to remain in an arrested state under unfavourable conditions, while keeping defence mechanisms active. 

The researchers used the plant model Arabidopsis thaliana for their experiments, but this molecular interplay could be investigated for similar roles in other plants, as COP1 and many components of ABA-associated signalling are conserved across plant species.

Jitendra K. Thakur, Scientist at National Institute of Plant Genome Research (NIPGR), New Delhi, who was not associated with this study, adds ​“This study may have implications in agriculture as the optimum timing of seed germination and seedling establishment is critical to ensure high plant yields, which can be utilized to enhance crop productivity especially under changing climatic conditions.”