Plant stem cells push cytoskeleton away to divide asymmetrically

A recent study published in Science has shed light on how plant cells distribute different materials to their daughter cells during division. While animal stem cells use the cytoskeleton to physically pull materials into each daughter cell, plant cells have a unique approach. Researchers at Stanford University discovered that plant cells actually push the cytoskeleton away instead of pulling on it, effectively signaling the cells not to divide in a certain way. This novel finding has implications for engineering plants that are more adaptable to environmental changes, which is crucial in the face of climate change. Just as understanding animal stem cell division has influenced human disease research and medical advancements, gaining insight into plant stem cell division could have future applications in engineering.

Blocking wall construction by threatening catastrophe

The research team led by Dominique Bergmann, the Shirley R. and Leonard W. Ely, Jr. Professorship in the School of Humanities and Sciences professor of biology, embarked on their study by examining polarity complexes, which are clusters of proteins crucial for leaf development in each cell. These complexes play a vital role in guiding the orientation of dividing leaf stem cells.

“We found that stem cells utilize these polarity proteins to determine the site of division,” explained Muroyama. “While we were aware of their involvement in division, the molecular mechanisms behind this process remained unknown.”

To delve into the functioning of these proteins, the scientists generated plant cell lines that expressed fluorescent versions of both polarity complex and cytoskeletal proteins. They then dedicated countless hours in a dark room, meticulously tracking the movements of these glowing proteins as the cells underwent growth, division, and replication.

The interactions between a polarity protein, BASL (magenta), and microtubules (black). Note how microtubules can grow into the polarized region but rapidly shrink back, creating a microtubule-depleted zone within the plant cell. Credit: Andrew Muroyama

The researchers made a significant observation during their investigation - some plant cells were not following the conventional “shortest wall rule” that typically governs cell division in plants. Normally, plant cells divide by constructing the smallest, most energy-conserving walls possible. However, in certain cases, the polarity complex, a cluster of proteins, was positioned exactly where the wall should have been formed, obstructing its construction. Through a series of meticulous experiments, the scientists determined that the polarity complex was actively pushing away the microtubules that are responsible for wall formation.

Bergmann explained, “The polarity complex essentially said, ‘If any of you microtubules dare to encroach upon my territory, I will forcefully repel you. I will create a catastrophe, a technical term for completely disrupting microtubules, so that you cannot invade this region.'”

Management for a changing climate

Bergmann’s research team has a keen interest in understanding how plants exhibit resilience in the face of changing environments. Plants possess the ability to adapt by modifying their leaves, branch patterns, respiration rates, or sugar storage mechanisms.

Muroyama emphasized the potential applications of their research, stating, “This study could pave the way for manipulating stem cell behavior, such as altering plant architecture or aiding plants in adapting to a shifting climate.”

Stem cells play a crucial role in making decisions based on environmental signals. Within this process, the polarity complex serves as a construction manager, providing instructions that ensure proper division of stem cells.

Bergmann likened the polarity complex to a construction manager who receives environmental signals, determines the appropriate course of action, and instructs the cell, saying, “‘Yes, you should divide.’ However, it also directs the divided cell, saying, ‘Now that you have divided, go forth and fulfill your destiny.'”

With a better understanding of how this construction manager operates, the researchers can explore its involvement in upstream and downstream processes and explore methods to harness its capabilities.

Bergmann expressed the need to further comprehend the precise mechanisms by which the polarity complex functions, stating, “We still have more to uncover about how the polarity complex operates. How do plants generate specialized cells with unique shapes, cells that produce intriguing chemicals, or cells that respond to specific stimuli? Can we engineer these processes?”

Source: Stanford University

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