Understanding the mechanisms that let plants precisely sense which way is up
Without any eyes, ears or other familiar sensory organs, plants keep extremely close tabs on which way is up. Or, more accurately, they can sense exactly which way is down, generally so that they can grow in the opposite direction, as leaves and stalks aimed underground would otherwise be a waste of resources. Aside from the occasionally unreliable information we get from our eyes, humans sense up and down with tiny grains called otoliths in our inner ears. As it turns out, plants rely on a similar concept, just without needing an ear or brain to make it work.
Specialized cells called statocytes grow close to a plant’s vascular systems, both near stems and near roots. Inside those cells, organelles called statoliths behave a bit like analogs to our own otoliths. They can move freely within the cell, accumulating in the direction of the Earth’s gravitational pull. This then triggers changes in the plant’s allocation of growth hormones, shaping the direction of the plant’s overall growth.
Moving like a liquid
As similar as this solution is to what’s in our inner ears, plants may have an edge on the gravitropism, or gravity-sensing, abilities found in humans. The number of statoliths found in a single statocyst cell didn’t seem like they could provide the amazing sensitivity to “down” observed in plant growth. Most small particles tend to stick, clump and tumble in a group, which in this case would mean that plants would only get an accurate reading on gravity after the statoliths tumbled over, leaving the plant open to problems like vertigo. Strong wind that shook and regrouped the statoliths would leave plants in danger of constant disruption as well.
The answer seems to be that the statoliths aren’t as passive as we expected. They possess a mechanical ability that lets them avoid clumping altogether, constantly separating and resettling themselves into an optimal position. The result is that while a single statolith can be compared to a grain, a group of them in a statocyst operate more like a fluid, always filling the available space in the bottom of their container cell. This gives the plant an extremely precise and sensitive way to know the exact angle it’s growing at.
The best option isn’t always up
Even with this amazingly precise mechanism in their cells, plants sometime skip worrying about what’s up and what’s down. Light-sensing photochromes help a plant orient itself towards a light source, even when that means growing at an angle that “disagrees” with feedback from statoliths. As different photochromes picked up red or blue spectrum light, they can further inform a plant which direction to grow in, especially when that direction is more likely to help the plant photosynthesize its food.
Source: Why plants are so sensitive to gravity: The lowdown, Phys.org