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

Pollinated by roaches

Plant & zo

The science of plants and more

Pollinated by roaches

Thinking about the pollination of flowers most of us generally assume bees do all the hard work. While bees are busy as bees pollinating flowers, they are hardly the only ones recruited for the job. Loads of other insects are helping out, with bats and hummingbirds also part of the pollination workforce. A recent paper by Xiong and colleagues tells us that even cockroaches are also doing their bit. These roaches were shown to be essential for successful pollination of the endangered climbing vine species, Vincetoxicum hainanense.

In their quest to find out which floral visitors pollinate this climbing vine, the first step was to look at the flower itself. The flowers of these climbing vines are small, pale green, and open during the night. The most noticeable though was their scent, the researches describe this as “a heavy, nauseatingly sweet scent, reminiscent of rotting fruit”. Opening of the flowers enhanced this scent, at night, allowing it to attract potential pollinators.

When sitting down for the night to observe which potential pollinators were visiting the flowers, it was noticed that two cockroach species were frequent visitors of the flowers, but not the only ones. With flowers also being visited by flies, ants, beetles and crickets. Although there was a wide variety of species visiting our climbing vine flowers, only cockroaches, ants and beetles were detected to carry any pollen with them after a visit.

But do they actually use the pollen to pollinate the flowers?

To test this the researches looked to the fruit development of open and bagged flowers. Only the open flowers developed any fruit, indicating that the flowers get pollinated with the pollen of another flower. When looking which insects were responsible, the researches found that a single cockroach species was involved in most of the fruit development observed. As both the nymphs and the adults of this cockroach species visited the flowers, they looked at them separately. With the nymphs only visiting flowers of the same plants they were mildly successful in pollinating the flowers, the adults, however, also visited flowers from other plants were the most successful pollinators. Making these roaches the main pollinators of the climbing vine.

While as the authors tell us that the importance of this study lies in the fact that it is the first study in which shows us that cockroaches are recruited by plants to pollinate their flowers. I think it also highlight that plants have strange bedfellows, and that the loss of insect species, even one as detested by us as cockroaches, might unwittingly also contribute to the loss of plant species. 

If you want to know more about pollinators check out Jeff Ollerton’s blog

References

Xiong, W., J. Ollerton, S. Liede-Schumann, W. Zhao, Q. Jiang, H. Sun, W. Liao, and W. You. 2020. Specialized cockroach pollination in the rare and endangered plant Vincetoxicum hainanense in China. American Journal of Botany 107(10): 1–11.doi:10.1002/ajb2.1545

Gene expression ≠ protein accumulation

Plant & zo

The science of plants and more

Gene expression ≠ protein accumulation

I was remined lately that people forget that gene expression does not equal protein accumulation. And also that protein accumulated is not equal to protein that is actual able to do what it is made for. So here an explaination about gene expression, protein accumulation, why are they not the same, and why protein accumulation is not telling anything about functionality of the protein.

Gene expression

As I explained to my mum last night, you can think about it like this: when playing with LEGO, if you are making one type of car over and over again you can measure how fast you assemble that car. This measurement of assembly we call expression, so in this case it is LEGO-car expression we measure, for the expression of genes it is called gene expression.

Protein accumulation

Expression is however is telling you nothing about the number of cars of that type of car are actually there. That is not only dependent on the speed of car assembly (LEGO-car expression), but also on the speed of car disassembly. If assembly and disassembly are in balance it does not matter if the assembly speed (LEGO-car expression) is high or low because the total number available cars does not change. The total number of available we call accumulation, for our LEGO-cars it is LEGO-car accumulation, when we talk about the total number of available proteins it is protein accumulation.

Protein functionality

This explains how protein accumulation is dependent on more than just gene expression. In turn protein functionality is dependent on more than just protein accumulation. It is also dependent on something called posttranscriptional modifications. Back to our cars. If you have 5 vans that you use to deliver goods, but three are stuck at the petrol station waiting for fuel, then effectively you have only 2 vans that are able to go on their delivery rounds. In this example you can see being stuck at a petrol station as a posttranscriptional modification that takes away the functionality of the protein (3 vans are not functional) without changing to overall car accumulation (there are still 5 vans). The simple measure of supplying fuel (posttranscriptional modification) will increase the number of functional proteins without changing the overall number of vans (car accumulation).

Hopefully this will help you remember that gene expression does not equal protein accumulation and that protein functionality is dependent on more that just protein accumulation. No just remember, protein does not equal car.

PLC sandwich

Plant & zo

The science of plants and more

PLC sandwich

Another genius bit of biological design comes from the enzyme PLC. PLC is a phospholipase whose job it is to cleave of the headgroup of PI(4,5)P2, a membrane lipid. Like tethering proteins, PLCs are made up out of a couple of domains. At its core there are three domains. There is the catalytic domain, which is doing all the action. Then there is a lipid binding domain, C2 domain, whose job it is to attach the protein to the membrane, preferably near PI(4,5)P2. Last there is a EF-hand domain, which likes to interact with other proteins. The order of these domains in the proteins is as follow:  EF-hand domain – catalytic domain – C2 domain. Mammalian PLCs may have a couple of other domains, so called regulatory domains, on either end. But plant PLCs don’t so it is unknown how exactly their activity is regulated.

chematics PLC
A scematic overview of the core of PLC proteins.

However, there have been some studies done on which domains are required to be there for which action, and how the protein looks when it is all folded up. First its structure, in contrast to its sequence order, when folded up the C2 domain sits in the middle. With on one side the catalytic domain, to which it is loosely attracted to. On the other side is the EF-hand domain to which it feels quite some attraction so it huddles close to it. By doing so it stretches the linker between the EF-hand domain and the catalytic domain, shoving the catalytic domain close to the C2 domain. In doing so, when bound to the membrane, it places the catalytic domain just right so it can grab hold of a PI(4,5)P2 headgroup to cleave it off.

scematics structure PLC
A schematic overview of how the core of the PLC protein looks when folded up, with the EF-hands binding to the C2 domain, forcing the C2 domain and catalytic domain close

This by itself is just brilliant already. Looking at what is required for what function, expectantly they found that for PLCs membrane localization its C2 domain was just enough. However, looking at what is needed to chop of the headgroup of PI(4,5)P2 things became puzzled. It turned out that with just the catalytic domain, nothing happens, likely because the membrane could not be found. But having the catalytic and the C2 domain is not enough either. The EF-hand domain needs to be there, holding them all close together, so the catalytic domain can find PI(4,5)P2.

This suggests that when the EF-hand domain is not bound to the C2 domain, the catalytic domain is just hanging of the C2 domain, away from the membrane, unable to do its job. Implying that controlling the interaction between the C2-domain and the EF-hand domains is a way of regulating PLC’s activity.

Realizing this was quite an eyeopener for me. However, what surprised me more was that although this was all discovered over 15 years ago. No subsequent research seem to have been done to find out the details of how the interaction between the C2 domain and EF-hand domain is regulated.

Literature

Essen LO, Perisic O, Cheung R, Katan M, Williams RL. Crystal structure of a mammalian phosphoinositide-specific phospholipase C delta. Nature. 1996;380(6575):595-602. doi:10.1038/380595a0

Otterhag L, Sommarin M, Pical C. N-terminal EF-hand-like domain is required for phosphoinositide-specific phospholipase C activity in Arabidopsis thaliana. FEBS Lett. 2001;497(2-3):165-170. doi:10.1016/s0014-5793(01)02453-x

Jiménez JL, Smith GR, Contreras-Moreira B, et al. Functional recycling of C2 domains throughout evolution: a comparative study of synaptotagmin, protein kinase C and phospholipase C by sequence, structural and modelling approaches. J Mol Biol. 2003;333(3):621-639. doi:10.1016/j.jmb.2003.08.052

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