A herringbone look for the growing plant cell

A herringbone pattern in plant cell walls is critical to cell growth, US researchers believe.

It’s formed – as the picture on the left shows – by the protein CSI1 and the alternating angle of the cell wall’s layers. Without the protein, the layers are all oriented in the same direction (right) and growth is restricted, according to a team from Pennsylvania State University, US.

“When plant cells grow, they tend to expand considerably along their length while not increasing much in width,” says Ying Gu, lead author of a paper in the Journal of Experimental Botany. 

“It is generally thought that microtubules – structures that form the skeleton of the cell –wrap around the cell like rings on a barrel, restricting growth in width. 

“We wanted to know what regulates growth in the cell’s length – and found that the story is more complicated than just rings on a barrel.”

Working with a mutant model of thale cress (Arabidopsis thaliana), they first confirmed that CSI1 (cellulose synthase interactive 1), a protein previously identified as important to the creation of cellulose, the main component of cell walls, is also important to cell growth.

“On a whim, one of my graduate students decided to perform a creep test, where plant cells are stretched out under acidic conditions,” Gu says. “Acidic conditions usually instigate cell growth, but the mutant lacking CSI1 didn’t elongate during the test. This suggested that the issue may be with the cell wall or the cell architecture, which was a surprise.”

Plant cell walls have many layers – 10 to 20 in thale cress up to 100 in other species. Each is composed of proteins as well as cellulose microfibres. 

The microfibres in a given layer are deposited at about a 60-degree angle compared to those in the previous layer. The alternating angles in each layer create a herringbone pattern and produce what scientists call crossed-polylamellate walls.

“CSI1 acts as a linker protein, helping the cellulose synthase complex deposit microfibres,” says Gu. “We thought that without CSI1, microfibres in the mutant would be deposited in random orientations. Instead, we found that the microfibres in each layer were all deposited in the same direction, which was a huge surprise.”

The researchers then pharmacologically disrupted the herringbone pattern in a normal plant’s cell walls, which prevented cells from growing normally even in the presence of a growth hormone. 

“It’s possible that CSI1 helps initiate the change in angle between layers,” said Gu. “We plan to investigate this hypothesis in the future.”

The findings already suggest, the researchers say, that existing theories about cell growth are incomplete. 

Improving understanding of how plant cells build cellulose and cell walls, they add, could eventually help scientists more easily break it apart for use in biofuels.

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