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Plant Biology

What keeps trees grounded?

While exploring the root cause of trees' gravity sensing, the Morita lab stumbled upon proteins called ‘LAZY’ and dissected their association with the starch-filled packets in roots to dissolve a long-standing dilemma – is gravity sensed in terms of the force that the heavy starch-filled packets exert on the cell content or is it their position that send signals dictating where the Earth is?

Credits: Unsplash
by Rucha Kulkarni | PhD student

Rucha Kulkarni is PhD student at University of Geneva .

Rucha Kulkarni is also an author of the original article

Edited by

Margaux Héritier

Senior Scientific Editor

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Views 931
Reading time 3 min
published on Nov 24, 2023
When asked to sketch a tree, you'll likely draw branches reaching towards the sky and roots delving into the earth. Sounds alright, but have you ever wondered how trees know where the earth is? 

If you were to tip over a potted plant, its roots wouldn't grow along the surface but instead curve downwards, following the pull of gravity. Now, imagine if some plants had genetic modifications that disrupted their gravity-sensing ability, causing their roots to grow erratically. This intriguing possibility led scientists to explore which genes help plants detect gravity. 

Wait, what are genes? - They're like the instruction manuals for the tiny factories inside all living organisms called cells. These factories keep everything in our bodies running smoothly. Genes usually contain instructions for building proteins, the engines of these factories. The shape and place of proteins in cells are super important for how they work. Scientists often remove a gene to understand how it affects the protein it makes and its function in the cell. It's a bit like taking a piece out of a machine to see what goes wrong. 

By studying different genetic changes that made plants unresponsive to gravity, scientists previously discovered a few layers of column-like cells at the root tip that detect gravity. These unique cells are called 'columella' cells because of their shape. Inside these columella cells, there are starch deposits in small packets. These packets always sink to the cell's bottom due to their weight. However, if it is their weight that the cell senses or their position that defines the 'bottom' of these cells remained a question. 

While exploring this question, a team of scientists from the National Institute for Basic Biology in Japan found a group of genes known as 'LAZY’ genes, that produce the ‘LAZY’ proteins. When plants lacked these genes, they grew in random directions, earning them the name 'LAZY'. We find these LAZY proteins inside starch-filled packets and also in the cell's outer layer, called the plasma membrane. Interestingly, these LAZY proteins are always near the 'bottom' of the columella cells. When plants changed their orientation, so their roots didn't align with gravity, these proteins moved to the 'new lower' side of the cells' plasma membrane. This showed they played a role in sensing gravity. But what exactly was their role? 

To answer this question, another one needed an answer first: why do these proteins stick to the plasma membrane?. Often, proteins have built-in signals that guide them to specific spots. In the case of LAZY proteins, they had two positively charged patches on their surface that helped them stick to the negatively charged plasma membrane. When Nishimura and colleagues replaced these patches with uncharged versions through genetic changes, the LAZY proteins no longer stuck to the plasma membrane. This confirmed that these positively charged patches were crucial for the proteins to attach to the membrane and determine their direction. It also confirmed that LAZY proteins could naturally stick to the membrane due to their structure. 

Now, the question was how these proteins identified the 'lower' side of columella cells. Remember, LAZY proteins are also in starch-filled packets. To explore the connection between starch-filled packets and LAZY proteins, the Japanese scientists studied LAZY protein localization in plants without starch. These starchless plants lacked genes for starch production, making the supposed starch-filled packets lighter and preventing them from settling at the bottom of the columella cells. In these starchless plants, LAZY proteins localized to the supposed starch-carrying packets and the plasma membrane, but they couldn't find the 'lower' side of the cell and instead were found all over the cell. So, the presence of starch-filled packets was crucial for LAZY proteins to locate the bottom of columella cells. 

Additionally, to understand the timing of events in gravity sensing, the scientists closely observed the movements of LAZY proteins, starch-filled packets, and the influx of auxin - a growth hormone - before and after reorienting the root under a microscope. It took 3, 15, and 30 minutes for the starch packets, LAZY proteins, and auxin influx, respectively, to adjust to the 'new lower' side after root reorientation. 

From the knowledge gained so far, it seemed logical to think that LAZY proteins followed the starch-filled packets in columella cells. But how could we confirm this idea? Imagine if one could move the starch-filled packets without changing the root's direction! Well, Nishimura and colleagues achieved this using a strong laser beam to push the starch-filled packets inside plant cells. With these 'optical tweezers', they moved the starch-filled packets against gravity and watched how LAZY proteins reacted. Voila! LAZY proteins followed the starch-filled packets and gathered near them on the plasma membrane, in the opposite direction of gravity. 

In other words, the LAZY proteins set up polarity in the columella cells by sensing where the starch-filled packets are (and, not the force they exert on the cells). This further triggers signals that draw growth hormones toward the roots, helping them grow towards gravity. And, this is how trees stay grounded! 
Original Article:
Nishimura T, Mori S, Shikata H, Nakamura M, Hashiguchi Y, Abe Y, et al. Cell polarity linked to gravity sensing is generated by LZY translocation from statoliths to the plasma membrane. Science. 2023;381(6661):1006-10.

Edited by:

Margaux Héritier , Senior Scientific Editor

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