Feeling nervous? Not if you're a plant

May 3, 2017

Plants are an incredibly versatile group of organisms that can survive in some very inhospitable locations, from acidic and nutrient-poor bogs to deserts and tundra.  Despite having to face some extreme conditions, plants have developed many ingenious methods of tolerating, modifying, responding to and thriving in their diverse habitats.

 

Unlike animals, plants don’t tend to move very far other than leaves changing their position relative to the sun in order to increase photosynthesis.  There are some, such as tumbleweed, that move around after they’ve set root, but these are a rarity. 

 

When animals get too warm, they can seek shade.  When under attack, animals can run away or at least immediately fight back.  Plants don’t have this luxury, yet they are able to stay in the same place, sometimes for up to thousands of years, actively responding to the suite of stresses that go along with surviving while standing still.

 

Even more impressively, plants don’t have nerves.  However, much like animals and all other life forms, plants can sense their environment.  Plants can feel touch, heat, cold, salt, osmotic stress, and light, as well as being able to measure the amount of nutrients available in the soil. 

 

Plants can detect when they are under attack from pests or pathogens, or even when a favourable fungus or bacterium comes along in order to work together to collect nutrients such as nitrogen, sulphur and phosphorus. 

 

How do they do this?

 

Calcium:  The universal cell signal

 

One way, universal to all eukaryotes (algae, protozoa, plants, fungi and animals), is calcium signalling.

 

Calcium is the fifth most abundant element in the earth’s crust and is vital to the survival of all cellular life.  Not only is calcium an important constituent of cell walls and cell membranes in plant cells but it is an incredibly versatile signalling molecule.

 

The reason that calcium is so useful is that it is very toxic to cells at high concentrations.  Since the dawn of life cells have therefore had to strictly regulate the amount of calcium that can enter through the membrane.  This regulation is so tight that the resting concentration of calcium in the cytoplasm of plant cells is a tiny figure of about 100nM.

 

Calcium homeostasis involves protein channels and transporters that reside either at the plasma membrane or in the membranes of organelles, including the vacuole, endoplasmic reticulum, chloroplast and the nucleus.

 

The calcium channels allow calcium to enter a cell along a concentration gradient, while transporters actively remove calcium from the cell using energy (ATP) and protons (H+) – either pumping calcium out entirely or storing calcium in organelles.

 

 

Nature’s Morse code

 

The calcium channels in the plasma membrane take on a range of forms, some being activated by changes in voltage and others activated by such things as stretching – from wind or from touch. Once a calcium channel has been activated, calcium rushes into the cytoplasm.

 

A combination of calcium channels, calcium transporters and calcium binding proteins then alter the amount of calcium rushing into the cell depending on the stimulus, so that each stress causes a specific calcium response.

 

Calcium homeostasis in the cell:  in and out.  Calcium (Ca2+) enters the cytoplasm (light green) along a concentration gradient via calcium channels (CC).  Ca2+ is actively pumped out of cells using energy (ATP) via calcium pumps (CP), also known as Ca2+-ATPases.  Ca2+ can also exit the cytoplasm in exchange for other ions, sodium (Na+) via Na+/ Ca2+ exchangers (NCX) and protons (H+) via Ca2+ / H+ exchangers (CAX).  Ca2+ can enter the cytoplasm from the external milieu or from internal stores in organelles.  Chl – chloroplast, ER – endoplasmic reticulum, Mt – mitochondria.

 

You can think of this to be somewhat like Morse code.  For each different stress – cold, heat, light, touch, wounding, wind, saltiness, heavy metals or disease – plants produce calcium increases in specific patterns.  Sometimes the calcium oscillates continuously in response to a stimulus, other times there is a swift, sharp “spike” and other times calcium can be continuously elevated.  To draw a mental picture, on a graph the signals tend to resemble a trace on a heart monitor.

 

Each pattern, with its unique shape (frequency and amplitude), encodes the information that plants can use to produce the correct response.  This is known as calcium signalling.

 

Decoding the signal

 

In order to read all of these different signals, plants are equipped with an impressive range of molecules that can bind to calcium, as well as to DNA and other proteins, which help cells respond to stress.

 

Some of the calcium binding proteins are attached to a phosphate group, which can be used to activate or deactivate other proteins such as enzymes.  Some can also cause genes to be turned on or off in the nucleus (or chloroplast and mitochondrion) either by binding directly to DNA or by activating other transcription factors (proteins that bind to DNA and allow gene expression).

 

Many of these calcium binding proteins, much like the signal, are specific for different stresses, and cause activation of specific genes involved in the response to a specific stress.  We can even artificially replicate a plant calcium signal in response to cold or salt and trick plants into producing the correct response and activating cold and salt-responsive genes.

 

 

A typical Ca2+ signalling pathway.  A “stress,” e.g. cold, pathogen, abscisic acid (ABA) causes specific increases in Ca2+ known as a Ca2+ signal.  This signal can be detected by sensors, e.g. calcineurin B-like protein (CBL), calmodulin (CaM), calcium-dependent protein kinases (CDPK), calmodulin like proteins (CML) and calcium-independent protein kinases (CIPK).  These proteins relay the signal via activating other proteins, e.g. transporters at the cell membrane, or by increasing or decreasing gene expression through either directly binding to DNA or via transcription factors (TF).  Protein kinases (CDPK, CIPK) target downstream proteins by supplying a phosphate group (P) which can cause enzymes to change their structure and become active (i.e. reveal an active site).

 

Rapid responses

 

Some plant responses are particularly rapid and rival our own reaction times.  Two great examples can be found among the carnivorous plants, Venus flytraps and sundews, and their reflexes render them some the fastest plants on earth. 

 

The Venus flytrap possesses modified leaves which snap shut in response to touch.  How they do this is still down to some debate, but we do know that on the inside of each leaf are tiny hairs.  When two of these hairs are triggered within about 20 seconds of each other the leaf will snap shut.  This is an impressive mechanism that prevents leaves from closing too regularly due to something like an errant rain drop.  Instead, the plant is almost certain to detect a wriggling insect, spider, or even (rarely) a tiny frog.

 

The snapping shut is based on a number of factors.  The first and second touches of the hairs on the inside of the leaf trigger an action potential, dependent on calcium, which then causes a change in turgor pressure inside the leaf cells, causing the leaf to snap shut.

 

There is also a large degree of elasticity involved.  The leaf is primed, a little bit like a jumping popper toy, in a convex resting state.  Once the cells expand a certain amount, the leaf quickly snaps into a concave form, trapping its prey from escaping with its prison cell-like lattice of hairs.  The whole process occurs in under one second.

 

The catapult plant

 

Quicker still, however, is a type of sundew.

 

Unlike most species of sundew, Drosera glanduligera does not rely on insects simply flying into its sticky death-fingers.  Instead, this speedy sundew has two types of finger-traps.  The first type is like any other sundew – tentacle-like fronds with sticky glue, ready to entangle and digest any prey that might be unfortunate enough to land within its grip.

 

The second type is ingenious.  These fingers are much longer than the sticky ones and protrude out from the plant into the path of any oncoming beasts.  When stepped on, these fingers act as a quasi-catapult and hurl the passing insect into its sticky web of digestive glue.  The trap is incredibly fast, snapping shut in fractions of a second.

 

The even more impressive aspect of these traps is that they don’t rely on inherent elasticity, as with the Venus flytrap.  A flytrap snaps shut in 100ms because the snapping action is somewhat similar to a strained bow releasing an arrow, from being bent out of shape the leaf snaps back into form.  The D. glanduligera catapult relies only on hydraulics, a bit like the “sensitive plant” Mimosa pudica, yet it is super-speedy nonetheless.

 

This might be explained by its incredible morphology.  The tentacles of D. glanduligera are a perfect catapult.  At the tip of the tentacle is a head, raised above the main tentacle and separated by connecting tissue – the most likely trigger for the snap trap.  Leading from the head is a “conducting strand” which leads to a hinge point - the pivot for the tentacle’s catapult action. 

 

 The death catapult.  Hinged tentacles extrude from the sundew plant, ready for an insect to trigger the trap via the head.  Once triggered, a change in water pressure causes the trap to snap via a rupture point at the hinge region, enabled by a disconnected “conducting strand” which provides mechanical support.  This catapults prey toward the sticky, glue-tipped fingers of the sundew, which manoeuvre the prey into the middle of the leaf for digestion.

 

Although it looks like a nerve, the conducting strand most likely provides mechanical support to the trap tentacle so that it remains flat.  In normal glue-fingers, the conducting strand is complete, therefore the fact that it ends at the hinge zone in catapult tentacles probably aids the trap to rupture at the hinge zone and act as a catapult. 

 

It takes just 75ms for the catapult trap to snap to action, hurling its prey into the waiting glue-fingers, which then suck insects into the centre of the sundew in less than 2 minutes for digestion.  Each catapult trap lasts only once, but long enough to provide a nutritious, nitrogen-rich meal for the plant.

 

Rapid response plants.  A)  The modified leaf of a Venus flytrap with its trigger hairs visible in the centre of the leaf.  B)  The sticky tentacles, catapults and flowers of a Drosera glanduligera sundew plant in Australia.  C)  The touch-sensitive leaves of the “sensitive plant” Mimosa pudica.

 

The art of attraction

 

Not limited to movement, however, the reactions of plants to their environment are varied and incredible.  One particular plant response - that of the detection and counter-attack against predatory aphids - is among the most impressive defence mechanisms on earth.

 

Aphids are a pest, ravaging plants by sapping them of nutritious phloem.  Plants can, however, detect that they are being eaten by aphids – and some plants take matters into their own hands.

 

All plants produce an impressive range of chemical compounds with all plants combined producing over 50 000 secondary metabolites.  Some of these chemicals are scents, which are famously released from flowers to attract pollinators.  As well as attracting pollinators, some of these scents have other handy uses.

 

One such chemical, jasmone, is one of the suite of scents produced by plants.  This chemical is released upon wounding by a pest, such as an aphid.  An obvious reason for this is to put off the aphid, which is indeed what happens.  However, concomitantly, the jasmone is also partly responsible for attracting ladybirds.  If this wasn’t enough, the volatiles produced by plants being grazed can also signal to other plants in the vicinity to guard against pests and in turn that they should alter their own gene expression and scent release.

 

Untapped potential

 

So, all-in-all, plants are able to not only detect and attempt to put off aphid predators but even attract the predators of their pests, while simultaneously warning other plants in the area that there are aphids on the loose.

 

Plants can react to touch in just a fraction of a second, using this sensitivity to protect leaves from herbivory, or even to catapult prey into their carnivorous leaves and digestive juices.

 

The intricate and complex signalling mechanisms which allow plants to respond to these varied stimuli are only just beginning to be unravelled and there are plenty more fascinating discoveries to be made.

 

However, for now let’s appreciate the magnificent feat that plants have achieved.  Survival, while standing still.

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