Quick plants

Plant & zo

The science of plants and more


Quick plants

One, two, gotcha, that one is not going anywhere. Who wants to catch flies needs to be super quick, like Venus flytrap. A real hunter, this plant.

Where most plants are preyed by hungry insects, is this meat-eater eating them. This makes Venus flytrap already an exception within plants. Another difference, most plants are moving super slow, so slow that you can not see it. You can only see it when you film a plant for a day, and play this movie at an increased speed. But not Venus flytrap, she reacts super vast, within 100 milliseconds.

Realtime recording of a Venus flytrap reaction to a fly. This film was recorded by Procko et al., 2021 eLife ;10:e64250

Venus is luring flies to its trap with a for the fly attractive smell. The trap is made off two so called trap-leaves, which are surrounded by thorns, like a mouth with sharp teeth. As soon as a fly lands on these trap-leaves, the trap springs. The thorns reach into each other and the fly can not escape. Venus has the time to digest the fly. How the plant manages to react so quickly is a question that researches try to answer for a long time. Already in 1875 Charles Darwin, they guy of the evolution theory, was already thinking about this.

Each trap-leaf has three or four touch sensitive trigger hairs, like whiskers, to enable Venus to react that quickly. When one of these trigger hairs bends, an electric current, a signal, goes to the point where the trap-leaves connect. A single signal can be caused by a falling twig, nothing to react to. But are two signals arriving quickly in a row, then the trap springs.

But how can bending of a trigger hair result in a signal? Now we have the answer. A trigger hair consists of three parts. A foot, which connects the trigger hair with the trap-leaf. A long unbending lever. And a bending part that connects the lever with the foot. American researchers found in the cells of this bending part a channel, that they called Flycatcher. A channel is like a door between cells. Normally this door is closed. But when a trigger hair bends and the cell stretches, Flycatcher opens and allows a current to pass.

Other plants have channels that look like Flycatcher. We do not know why they do not react to touch that quickly. Maybe something to find out. In the meantime, Venus will catch another fly. One, two, gotcha.

Literature

Carl Procko, Swetha Murthy, William T Keenan, Seyed Ali Reza Mousavi, Tsegaye Dabi, Adam Coombs, Erik Procko, Lisa Baird, Ardem Patapoutian and Joanne Chory (2021) Stretch-activated ion channels identified in the touch-sensitive structures of carnivorous Droseraceae plants. eLife 10:e64250

Snelle planten

Plant & zo

Plantenwetenschap en meer


Snelle planten

Een, twee, hebbes, die vliegt niet meer weg. Wie vliegen wil vangen moet vliegensvlug zijn, en dat is venusvliegenval. Een jagende plant.

Waar de meeste planten ten prooi vallen aan gulzige insecten, eet deze vleeseter ze juist op. Daarmee is venusvliegenval al een uitzondering onder de planten. Een ander verschil, de meeste planten bewegen héél langzaam, zo langzaam dat je het niet kunt zien. Je ziet het pas als je de plant een dag filmt, en die film daarna versnelt afspeelt. Venusvliegenval daarentegen reageert heel snel, binnen 100 milliseconde.

Realtime video opnamen van het reageren venusvliegenval op een vlieg. Deze video is opgenomen door Procko et al., 2021 eLife ;10:e64250

Met een voor de vlieg aantrekkelijk geurtje lokt venus vliegen in de val. De val bestaat uit twee zogenaamde vangbladeren, die omringd zijn met stekels, zoals een mond met scherpe tanden. Zodra de vlieg tussen deze vangbladeren landt, klappen ze dicht. De stekels grijpen in elkaar en de vlieg kan niet meer weg. Venus heeft dan alle tijd om het insect op te peuzelen. Hoe de plant het voor elkaar krijgt om zo snel te reageren is een vraag die liefhebbers van venusvliegenval al lang willen beantwoorden. Charles Darwin, die van de evolutietheorie, dacht er al in 1875 over na.

Om te reageren op een vlieg, zitten aan de binnenkant van elk vangblad drie of vier aanrakingsgevoelige prikkelharen, een soort snorharen. Wanneer een van deze prikkelharen buigt gaat er een elektrisch stroompje, een seintje, van de prikkelhaar naar het punt waar de vangbladeren samen komen. Een seintje kan een vallend takje zijn, daarop reageert venus niet. Maar komen hier twee seintjes, vlak achter elkaar zoals wanneer een vlieg landt, dan klapt de val dicht.

Maar hoe zorgt een buiging voor een seintje? Nu is er een antwoord. Een prikkelhaar bestaat uit drie delen. Een voet, waarmee de prikkelhaar aan het vangblad vastzit. Een lange onbuigzame hendel. En een buigzaam stukje dat de hendel met de voet verbindt. In de cellen, van dat buigzame stukje, vonden Amerikaanse onderzoekers een kanaaltje dat ze Flycatcher, vliegenvanger, noemden. Zo’n een kanaaltje is een soort deur tussen cellen. Normaal is deze deur dicht. Alleen wanneer de prikkelhaar buigt en de cel zich uitstrekt, gaat Flycatcher open en laat een stroompje door.

Andere planten hebben kanaaltjes die op Flycatcher lijken. Maar waarom die niet zo snel op aanraking reageren weten we niet. Misschien iets om uit te zoeken. Venus vangt ondertussen nog een vlieg. Een, twee, hebbes.

Literatuur

Carl Procko, Swetha Murthy, William T Keenan, Seyed Ali Reza Mousavi, Tsegaye Dabi, Adam Coombs, Erik Procko, Lisa Baird, Ardem Patapoutian and Joanne Chory (2021) Stretch-activated ion channels identified in the touch-sensitive structures of carnivorous Droseraceae plants. eLife 10:e64250

Regulating enzymes

Plant & zo

The science of plants and more


Regulating enzymes

Regulation can happen in many ways. For metabolic pathways three main regulatory mechanisms can be distinguished. A) The production of its enzymes, B) the degradation of these enzymes and C) regulating the activity of these enzymes. Where the regulation through the production and degradation of enzymes is crude and imprecise, regulation through regulating enzyme activity allows for precise reactions of developmental and environmental cues. The recent study of Yokoyama and colleagues on the regulation of the shikimate pathway illustrates this nicely.

The shikimate pathway and subsequent biosynthesis of the aromatic amino acids tryptophan, tyrosine and phenylalanine

Yokoyama and colleagues studied the Arabidopsis DHS enzymes. DHS is the enzyme that catalyses the first reaction of the shikimate pathway, whose products are the aromatic amino acids tyrosine, tryptophan, and phenylalanine. These amino acids are not only needed for protein synthesis but are also functioning as precursors for various secondary metabolites, including auxin, lignin, and flavonoids.

Arabidopsis has three DHS enzymes, DHS1, DHS2 and DHS3. Al three have roughly the same activity, with DHS1 doing slightly better that DHS2 and 3. The main difference between these enzymes is when they are expressed. DHS2 is mainly expressed in seedlings, whereas DHS1 and DHS3 did not show any expression in seedlings but are expressed in mature leaves. Moreover, their expression is also induced in response to stress.

Yokoyama and colleagues also looked at how the DHS enzymes are regulated. They did this trough looking which downstream products inhibited their activity. They found that surprisingly the amino acid phenylalanine did not inhibit any of the DHS enzymes and that the amino acids tryptophan and tyrosine only inhibited DHS2. As DHS2 is the only DHS enzyme active in seedlings, DHS2 inhibition by tryptophan and tyrosine but not phenylalanine likely reflects the needs of the growing seedling, needing amino acids for proteins and lignin, a downstream product of phenylalanine, for its cell walls.

That the activity of DHS1 and 3 are not inhibited by any of the aromatic acids might be correlated with them showing increased expression during stress. The downstream products of all three aromatic amino acids are contributing to the plant stress response, as such it would be damaging to the plant if the production of the aromatic amino acids is limited by their own abundance.

Yokoyama and colleagues did find however, that the activity of all three DHS enzymes is inhibited by chorismate, an intermediate of the shikimate pathway. But that this inhibition has a kind of fail safe for DHS1 and 3. For these enzymes, chorismate inhibition is inhibited by arogenate, the direct precursor of tyrosine and phenylalanine. Kind of telling the enzymes, don’t worry, the chorismate build up is not due to a reduction of flow through the shikimate pathway.

For DHS2 however, arogenate also inhibits its activity. This was also found to be the case for caffeate, a downstream product of phenylalanine and a precursor of lignin. Again, this can be seen as reflecting the need of a growing seedling that does not want to waste its carbon on metabolites that are not used.

The limited photosynthetic activity of seedlings requires them to be thrifty. In contrast mature leaves with their more abundant photosynthetic activity and  carbon supply can afford waste carbon on secondary metabolites that come in handy for a range of processes ranging from stress response to pollinator attraction. The DHS enzyme production and inhibition reflects this, fine tuning the shikimate pathway, allowing it to respond to both developmental and environmental cues.

References

Ryo Yokoyama, Marcos V V de Oliveira, Bailey Kleven, Hiroshi A Maeda (2021) The entry reaction of the plant shikimate pathway is subjected to highly complex metabolite-mediated regulation, The Plant Cell, koaa042

The art of bending

Plant & zo

The science of plants and more


The art of bending

How to bend, or more precisely, how do plants bend? This was the question Baral and his colleagues set to answer. The bending of plants occurs as a result of many stimuli, such as wind, the search of nutrients, or obstacles. These can all occur at various stages of plant development. However, there is type of bending that always takes place at the same moment, the formation of the apical hook through bending of the hypocotyl just after seedling germination. The apical hook is formed to protect the shoot meristem during the process of emerging from the soil. This process is so important for the young seedling that it even occurs when there is no soil to emerge from.

It is not the first time that researches looked at apical hook formation. In the past researches have looked at how cell elongation, hormones and gene transcription regulation affect hypocotyl bending. All in a laboratory stetting, growing seedlings on top of agar, thereby ignoring the potential influence that mechanical force might have.

For Baral and his colleagues it became clear that mechanical force was playing a role when they observed that in the absence of the protein katanin, which affects cortical microtuble organization, hypocotyl bending was absent when seedlings were grown on agar plates, but not when grown on soil.

Which aspects of bending need a mechanical cue?

Looking for which aspects need a mechanical cue, Baral and colleagues found that the auxin asymmetry required for bending, needed a mechanical cue. In line with this, PIN transporters (auxin efflux carriers) needed a mechanical cue to change their localization. The change of localization of the PIN transporters results in changes of the auxin flow, leading to asymmetric auxin distribution, what in turn leads to bending of the hypocotyl.

As auxin has a hand in almost every process of plant development, the next thing they looked at was how auxin was regulating the process of hypocotyl bending. There were two options

  1. The classical auxin response pathway via the transcription factors ARF7 and ARF19.
  2. The alternative auxin response pathway whereby the plasma membrane localized receptor TMK1 which upon perception of high auxin levels sent a C-terminal fragment of itself to the nucleus to stabilize specific AUX/IAA transcription factors.

While mechanical cues still caused hypocotyl bending in the arf7 arf19 mutant, no hypocotyl bending was observed for the tmk1 mutant. Indicating that it is the alternative auxin response pathway that is needed for the regulation of the process of hypocotyl bending.

To find out how precisely requires more research. But what the Baral and colleagues also show is that even for such a well studied process such as hypocotyl bending, there are still discoveries to be made that give new insights in how these processes are regulated.

References

Baral, A., Aryal, B., Jonsson, K., Morris, E., Demes, E., Takatani, S., Verger, S., Xu, T., Bennett, M., Hamant, O., & Bhalerao, R. P. (2021). External Mechanical Cues Reveal a Katanin-Independent Mechanism behind Auxin-Mediated Tissue Bending in Plants. Developmental cell, 56(1), 67–80.e3

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