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I read the article on the New York Times about the work of Simon Gilroy. I love plants and I would like to know when he mentions that plants deliver Calcium to their leaves in case of danger, where do they take this Calcium, it is stored there or does it come from the soil right at that moment?
Question Date: 2018-09-21
Answer 1:

Your question sent me off to the internet to read the article. It’s really interesting because we don’t usually think of plants as having a fast response. I am not a botanist and didn’t know the answer to your question, so I did what I usually do to figure something out in biology. First, I took a quick look at papers in science journals by this author. He is really interested in how the plant cells communicate with each other, so the papers I saw did not mention the source of the calcium.

Then I went back to thinking about what I know about plants. Plants pull calcium out of the soil. They use energy to pump calcium from the soil into root cells. Then the plant needs a way to get calcium up to the rest of the cells. There are tiny tubes that run up the plant from the roots. Plants don’t have hearts, so how do they move things up against gravity?

They use the power of the sun to pull things up from the roots. Here’s how it works:

First, the sun provides heat that causes water to evaporate out of tiny holes in the leaves (these are called stomata). Water has a lot of amazing properties. One is that water molecules stick to each other. Another is that water molecules stick to other things, like the sides of those tiny tubes. So when one water molecule evaporates, it drags another one up behind it. That one drags another one up, and it’s sort of like a chain of water molecules, all dragging up the molecules below. Calcium and other nutrients get dragged along too.

So how does that help answer your question? This whole evaporation thing (it’s called evapotranspiration) takes time. To see how long, try the old demonstration of cutting celery off at the bottom and putting it in a cup of colored water. Gilroy’s plants were moving calcium very quickly because of an emergency. So this is what I think happens, the calcium is being delivered to most cells slowly through evapotranspiration, so every cell has some. The damage could happen anywhere. Once a group of cells is damaged, a signal is sent to all the nearby cells to send their calcium over in a hurry.

Imagine that your cafeteria kept all the napkins in one place and everyone would grab one and walk to a table that may be far away. Suddenly, someone spills their milk and everyone at the table quickly hands over their napkin. After the spill, everyone might get up and walk all the way to where the napkins are. (Okay, maybe only one person would go get the napkins, but you can see what I mean.)

So why do you think the plant doesn’t just load up EVERY cell with calcium?

Thanks for asking,

Answer 2:

I couldn't find the answer to your question, so I emailed Simon Gilroy. Here's what he said:

"Hi Helen,
We think the calcium is coming into the cells from the cell wall. It is most likely being taken up from the soil well before the wound signal is generated and then that store is being released into the cell from the cell wall as the wound signal needs to be generated.
Cheers,
Simon"

Here's what I wrote you before emailing Simon:
I read about that research in Science magazine. Here's a link to news about it:
plants communicate distress

Calcium is stored in the apoplast. That's the space inside the cell wall. Calcium gets into the plant through the xylem, 'from root to shoot'. Xylem is in all higher plants [vascular plants] but they didn't get it from their common ancestor. Different groups of plants developed xylem independently. Simon Gilroy's article is about Arabidopsis, a tiny 'weed' from the mustard family that's really useful for plant research. It doesn't have nearly as much xylem as woody trees.

References:
1. woody trees - Arabidopsis thaliana as a model species for xylem hydraulics: does size matter?
2. 'root to shoot' and apoplast - Cell-Specific Vacuolar Calcium Storage Mediated by CAX1 Regulates Apoplastic Calcium Concentration, Gas Exchange, and Plant Productivity in Arabidopsis



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