From Soil to Sunlight

Mango season is at its peak. Ethylene, which acts as a gaseous hormone in plants, plays an important role in ripening of mangoes both naturally and artificially. However, this phytohormone does much more in plants. A team of scientists at the Indian Institute of Science Education and Research (IISER) in Bhopal (which included Nevedha Ravindran, Kavuri Venkateswara Rao and myself) have uncovered an interesting role of ethylene in detecting soil depth and successful emergence of seedlings from underneath the soil during germination. Ethylene production is increased in seeds when they perceive mechanical pressure of soil over them. In natural conditions, when a seed germinates and tries to emerge out of the soil, pressure of soil induces ethylene production and the seedling slowly gets exposed to light.

In a scientific study that we published in the journal New Phytologist, we detail how scientists identified that a protein BBX32 is promoted by ethylene and light, to optimize the timing of successful emergence out of the soil. Soil pressure leads to the production of more ethylene that increases the stability of the BBX32 protein. Light also promotes the accumulation of BBX32 which helps to maintain a hook-like structure at the tip of the growing seedling. The hook allows the seedling to pierce through the abrasive soil particles without damaging the plant tip which gives rise to stems and other above ground parts of the plant. The protein BBX32 helps the plant maintain the hook until conditions are favourable for emergence.

The study was conducted using Arabidopsis thaliana, a widely used model plant. The team compared seedlings with normal levels of BBX32 against those that either lacked the protein or overproduced it. The plants were grown under controlled conditions: on plates with and without ethylene exposure, under layers of sand to mimic soil pressure, and under various wavelengths of light including white, red, blue and far-red. The goal was to determine how ethylene and light interact to influence the behaviour of BBX32 and the timing of hook opening.

The researchers monitored hook angles at different stages of seedling development as the plants transitioned from darkness to light. They also measured the percentage of seedlings able to emerge from beneath the sand. The findings were clear: the more BBX32 present, the higher the success rate of emergence. BBX32, in concert with another protein, PIF3, induces HLS1, which keeps the hook closed until the time is right.

The study offers new insight into how seeds navigate their journey from the subterranean dark to the light above. For farmers, this is more than just academic, given that seedling establishment failures account for a significant share of crop loss. Understanding and manipulating this process could have far-reaching implications for agricultural productivity.

In future, then, ethylene may be valued for more than just ripening fruit. Its role in helping plants emerge into the light could prove just as vital.

(The writer is a Professor at the Department of Biological Sciences, IISER Bhopal.)