植物の栄養供給の重要性
Hello everyone, welcome to SCIEN-SPOT. SCIEN-SPOT is a podcast that shines a spotlight on the
latest scientific technology from Japan. Your host is REN from SCIEN-TALK.
Today we are diving into a truly groundbreaking discovery in the world of plants. Something
that hasn't happened in astonishing 160 years. Before we dive into today's episode, let
me tell you about a special collaboration called SCIENCE PODCAST DAY. It's a monthly
project where science podcasters, mostly those who talk about science in Japan, come together
to create episodes under a shared theme. This month's host is a podcast called American
Night Gold, and the theme is simply rice. There's even a curated Spotify playlist featuring
all participating shows. Check the link in the episode's description if you're curious.
Most of the episodes will probably be in Japanese, but I think it's a great way to discover new
and fascinating science podcasts. And speaking of rice, today's topic is closely related,
and I'll take a look at new research that may change how we understand rice cultivation
itself. Okay, so let's dive into today's episode. For many living creatures, including us humans,
for a baby to grow big and strong, it absolutely needs nutrients from its mother. In our case,
this vital supply comes through the umbilical cord connecting to the mother's placenta.
But what about plants? Inside a flower, there's a part called the ovule. When this ovule is fertilized,
it becomes the future baby, the seed. For this seed to grow large and healthy, it too needs to
receive nutrients from the mother plant, and this transfer of nutrients is an incredibly crucial
process in plant reproduction. If this nutrient supply fails, the seed won't develop properly,
which could lead to reduced crop yields. Since around 2005, scientists have known that for a
plant seed to grow large, fertilization is necessary. However, the exact mechanism behind
植物の受精過程の謎
this was like a black box. Fundamental questions about plant reproduction, such as why is fertilization
necessary, and how does the plant sense a successful fertilization and efficiently
begin to supply nutrients, had remained a long-standing mystery.
And just imagine if our mother plant kept sending precious nutrients to an ovule that had failed to
be fertilized. Wouldn't that be a massive waste of energy for the plant? This is precisely why
researchers believe that plants must possess a clever mechanism to intelligently determine if
fertilization was successful, and to use their limited resources in the most efficient way
possible. And now, an astonishing new organ has been discovered, unveiling this very mystery.
A research group led by a specially appointed associate professor,
Ryushiro Kasahara at Nagoya University, achieved this monumental discovery.
They named it the Kasahara Gateway, a new umbilical cord for plants, absolutely indispensable for seed
formation. The discovery of a new plant's organ is a truly historic event, something that hasn't
happened in the remarkable 160 years. It's no exaggeration to say this has the same impact as
discovering a completely new organ in an animal. The research group started by meticulously
observing ovules that had been successfully fertilized, and those that unfortunately had
failed. They were looking for any cellular differences. Initially, besides the fertilized
ovules simply growing larger, they struggled to find concrete distinctions, and their research
remained in a state of confusion. It was a frustrating period, searching for something
that seemed to elude them. Then, one day, through an unexpected turn, they noticed something highly
intriguing. This happened while they were examining pollen tubes,
the tubes that carry sperm cells in plants, using a reagent called aniline blue,
カサハラゲートの発見
which specially stains a substance called callose. Typically, research is focused on the
micropillar side of the ovule, where the pollen tube enters, but what they coincidentally
discovered was a strange, strong signal appearing on the opposite side of the ovule,
called the chalazal side. And surprisingly, this signal became particularly strong when
fertilization failed, and the substance indicated by this signal was none other than callose.
Callose is a glue-like substance that temporarily accumulates between the cells
and the cell wall and the cell membrane during various plant developmental processes.
This accidental discovery led to a major breakthrough.
The researchers uncovered a new principle. When fertilization fails, a large amount of
callose accumulates on this chalazal side, acting like a dam to physically block the inflow of
nutrients from the mother plants. Indeed, when they used a special substance called
nutrient markers to visually observe the nutrient flow, they clearly saw that nutrients
flowed smoothly into successfully fertilized ovules. In contrast, for failure ovules,
due to this callose, nutrient inflow was completely blocked. It's as if there's a gate
that opens and closes only when necessary. And this very gate was later named the Kasahara
gateway, and the plant's new umbilical cord organ. Plants cleverly use this gate to avoid
wasteful investment. So how does this gate control its opening and closing?
To unveil this mechanism, the research team focused on a specific gene whose expression
level increased particularly in fertilized ovules. This gene is called ATPG-PPAP.
But it was found that this gene encodes the enzymes that break down callose,
which we just discussed. So when fertilization is successful, the ovule senses this
fertilization and activates the ATPG-PPAP gene. This leads to the production of the
植物の栄養供給メカニズム
callose-degrading enzyme, which dissolves the callose acting like a dam, opening the gate,
and allowing nutrients from the mother plant to be taken in. A highly rational mechanism.
To scientifically prove the correspondence of this hypothesis, the researchers used a
cutting-edge genome-editing technology called CRISPR to intentionally inactivate this gene,
creating a mutant. As a result, in this mutant, callose was not sufficiently broken down and the
nutrient inflow was severely restricted. And the seeds that eventually formed were
an astonishingly 8% smaller compared to the wild type or normal plant.
This clearly demonstrates how closely the ATPG-PPAP gene, callose, and the nutrient supply
interconnected. Conversely, what happened when they created plants that overexpressed this gene
beyond normal levels? They found that callose was continuously more easily broken down and the
nutrient saturation became very smooth. As a result, these overexpressed plants
formed seeds that were impressive 16% larger than the wild type.
Even more remarkably, this method was effective not only in Arabiodopsis saliana,
a commonly used research plant, but also in rice, a staple food for much of the world,
successfully making rice seeds 9% larger. This is a truly significant achievement,
indicating its potential for application in food production.
And detailed microscopic observation further revealed that the Kasahara Gateway is composed
of a unique group of multiple cells and is closely associated with phloem.
This is the sieve tubes and the plant's nutrient transport pathways.
This organization was confirmed to have two distinct and crucial functions, removing callose
after fertilization to open the gate and delivering nutrients directly to the main
body of the ovule. It is truly a clever plant mechanism, much like an animal's umbilical cord,
and they are responsible for nutrient supply and efficiency.
植物の臍帯の発見
In summary, the newly discovered Kasahara Gateway acts like a plant's umbilical cord,
precisely sensing successful fertilization and efficiently supplying nutrients only to
the necessary ovules. By withholding nutrients from unfertilized ovules and thus completely
eliminating waste, this discovery reveals a highly intelligent survival strategy that
plants have acquired through countless generations of fierce competition.
This finding not only opens new avenues for fundamental research in plant seed farming,
but is also expected to significantly contribute to the development of new breeding methods for
enlarging the seeds of food and industrial crops like rice by applying the newly identified
ATBGPP gene. With global food security being the growing concern, I think it's truly exciting to
consider how this technology might lead to a stable food supply and more abundant agriculture in the future.
So that's all for today's science report, and it's very remarkable and fascinating research,
and this podcast is broadcast daily on weekday morning in both Japanese and English.
I hope today's discovery has changed how you think of rice or some seeds, and I'd love for
you to listen to the podcast and post your notes and thoughts with the hashtag sciencepot. See you next time.
