Tag Archives: #pollentube

Overlooked Helpers: Membrane Building Blocks Play a Decisive Role in Controlling Cell Growth (Botany)

Lipids are the building blocks of a cell’s envelope – the cell membrane. In addition to their structural function, some lipids also play a regulatory role and decisively influence cell growth. This has been investigated in a new study by scientists at Martin Luther University Halle-Wittenberg (MLU). The impact of the lipids depends on how they are distributed over the plasma membrane. The study was published in “The Plant Cell”.

If plant cells want to move, they need to grow. One notable example of this is the pollen tube. When pollen lands on a flower, the pollen tube grows directionally into the female reproductive organs. This allows the male gametes to be delivered, so fertilisation can occur. The pollen tube is special in that it is made up of a single cell that continues to extend and, in extreme cases, can become several centimetres long. “This makes pollen tubes an exciting object for research on directional growth processes,” says Professor Ingo Heilmann, head of the Department of Plant Biochemistry at MLU.

For the current study, Heilmann’s team focused on the phospholipids of pollen tubes, which, as the main component of the plasma membrane, are responsible for separating the cell’s interior from its surroundings. “Lipids are generally known to have this structuring function,” says Dr Marta Fratini, first author of the study. It has only recently come to light that some phospholipids can also regulate cellular processes. The scientists from Halle have now been able to show that a specific phospholipid called phosphatidylinositol 4,5-bisphosphate (“PIP2”) can control various aspects of cell growth in pollen tubes – depending on its position at the plasma membrane. They did this by labelling the lipid with a fluorescent marker. “We found it is either distributed diffusely over the entire tip of the pollen tube without a recognisable pattern, or is concentrated in small dynamic nanodomains,” Fratini explains. One can imagine a group of people on a square: either individuals remain 1.5 metres apart as currently prescribed, or they form small groups.

Pollen tube with swollen tip (green: one of the enzymes responsible for lipid production, magenta: the lipid nano domains discovered and described in the study) / Foto: Marta Fratini

It appears that different enzymes are responsible for the varying distribution of PIP2. “Plant cells have several enzymes that can produce this one phospholipid,” explains Heilmann. Like the lipids, some of these enzymes are widely distributed over the membrane and others are concentrated in nanodomains, as shown by the current study. Depending on which of the enzymes the researchers artificially increased, either the cytoskeleton – a structure important for directed growth – stabilised and the pollen tube swelled at the tip, or more pectin – an important building material for plant cell walls – was secreted. This made the cell branch out at the tip. To make sure that the distribution of the lipids was indeed responsible for these growth effects, the biochemists artificially changed the arrangement of the enzymes at the plasma membrane – from clusters to a wide scattering or vice versa. It turns out they were able to control the respective effects on cell growth. 

“As far as I know, our study is the first to trace the regulatory function of a lipid back to its spatial distribution in the membrane,” says Heilmann. Further research is now needed to clarify exactly how the membrane nanodomains assemble and how the distribution of PIP2 at the membrane can have such varying effects. 

The research was funded by the Deutsche Forschungsgemeinschaft (German Research Association, DFG) through various programmes, including the Research Training Group 2498 “Communication and Dynamics of Plant Cell Compartments”, and supported by the Centre for Innovation Competence HALOmem at MLU. 

Featured image: A pollen tube that grows out of a pollen grain (green: one of the enzymes responsible for the production of lipids that control cell growth, magenta: actin cytoskeleton). / Foto: Marta Fratini

Study: Fratini et al. Plasma membrane nano-organization specifies phosphoinositide effects on Rho-GTPases and actin dynamics in tobacco pollen tubes. The Plant Cell (2020). doi: 10.1093/plcell/koaa035

Provided by Martin Luther University Halle-Wittenberg

We Now Know, How An Enzyme Ensures The Correct Growth Of Pollen Tubes In Flowering Plants (Botany)

New insight on how an enzyme ensures the correct growth of pollen tubes in flowering plants has been published today in the open-access journal eLife.

Pollen grains deposited on receptive papilla cells lacking KATANIN. Credit: Lucie Riglet (CC BY 4.0)

The study reveals an unexpected role of KATANIN in moderating the mechanical properties of the papilla cell wall in Arabidopsis thaliana (A. thaliana), thereby preventing disordered pollen tube growth and allowing the tube to find its correct path to the underlying female plant tissues. These findings suggest that KATANIN has likely played a major role in the success of flowering plants on earth more widely.

Seeds are produced in flowering plants when male and female germ cells called gametes fuse together. Male gametes are contained in the pollen grain while female gametes are found in the ovules, which are embedded in a female reproductive organ called the pistil. For successful seed production to happen, pollen grains need to meet with the surface of the pistil, which is composed of a layer of elongated cells called papillae. When a pollen grain lands on a papilla, it rehydrates and then produces a tube that will carry the male gametes toward the ovules.

Pollen tubes grow first within the papilla cell wall, exerting a physical pressure on the cell. After crossing the papilla layer, they then grow in the intercellular space of underlying tissues. The pistil then produces compounds that guide the pollen tube to the ovules where it reaches the female gametes. But how the tube orients itself when it emerges from the pollen at the papilla surface remains unknown.

Mechanical forces are known to play a major role in plant cell shape by controlling the orientation of cortical microtubules, which in turn mediate the deposition of cellulose microfibrils. For their study, Riglet and her team combined imaging, genetic and chemical approaches to show that the enzyme KATANIN, which cuts microtubules, also acts on cellulose microfibril orientation and confers mechanical properties to the papilla cell wall that allow for correct pollen tube orientation.

According to Thierry Gaude, Group Leader at the Laboratory of Plant Reproduction and Development, ENS Lyon, by forcing the pollen tubes to take the right direction from their early places in the papilla, KATANIN has likely played a major role in the success of flowering plants on earth by promoting fertilization. As KATANIN is found in most organisms, including humans, it is possible that the enzyme plays a role in regulating mechanical properties in other processes—but this is a fascinating question that remains to be explored.

References: Lucie Riglet et al, KATANIN-dependent mechanical properties of the stigmatic cell wall mediate the pollen tube path in Arabidopsis, eLife (2020). DOI: 10.7554/eLife.57282 link: https://elifesciences.org/articles/57282