Team unravels regulatory mechanism that prevents stem cell differentiation and maintains gender balance in vascular plants

Yun Zhou, associate professor of botany and plant pathology, and five Purdue collaborators recently published their results in Current Biology. The team's work focused on Ceratopteris, a fern species often used in botany studies, to reveal fundamental processes that regulate stem cell proliferation at both the cellular and molecular levels. In plants, undifferentiated stem cells are found in actively growing tissues, known as meristems.

The researchers discovered that a gene called CrHAM connects with a critically important process shared by all plants, animals and humans -- stem cell proliferation. All of these multicellular organisms possess undifferentiated stem cells.

"Undifferentiated means they have the ability to divide and supply cells that eventually differentiate into all kinds of organs," Zhou said. "That's why maintaining stem cell population and meristem development is important for plant growth and reproduction."

Although plant scientists already knew that stem cells play a vital role in plant growth and development, "in this work, we find that CrHAM regulates meristem indeterminacy, which means that their main job is to prevent stem cells from differentiation," Zhou explained.

This plant species can develop as either male or hermaphrodite. Hermaphrodites, which have both male and female reproductive organs, develop and maintain a meristem. Males, however, do not develop meristems. The findings reported in Current Biology identify CrHAM as a key mechanism that prevents meristem tissue in hermaphrodites from forming male organs. Without this regulation, meristem tissue would always differentiate into male organs, leading to the loss of the hermaphrodite identity and ending reproduction in the species.

Zhou's group used a combination of rigorous molecular genetic analysis, genome-wide expression profiling across different sex types and genotypes, and quantitative analysis of cell division using live-imaging confocal microscopy to collect data and test their hypothesis. The technique developed in Zhou's lab allowed researchers to observe how individual Ceratopteris cells divide and differentiate into different organs without disturbing the plant's normal growth.

The previous results obtained using this technique were published in a 2022 Communications Biology paper. The lead author of both the Current Biology and Communications Biology papers was Yuan Geng, a Purdue PhD alumna who graduated from Zhou's lab and is now a postdoctoral scholar at the California Institute of Technology.

This Current Biology paper is the most recent in a series of findings that Zhou and his team have published since he joined the Purdue faculty. Zhou's research aims to understand the cellular and molecular mechanisms that plants use to maintain stem cell division and sustain organ formation. How multicellular organisms balance these processes generates many questions.

"In my lab, we have established two model systems to address these questions," Zhou said. One of them is the fern Ceratopteris, a seed-free plant model. The other is the shoot meristem of Arabidopsis, a model for flowering seed plants. In related Arabidopsis research involving stem cells and other HAM family genes published in Science in 2018 and in Nature Communications in 2020, Zhou's group discovered regulatory principles that scientists may eventually exploit to optimize shoot meristem activity and increase crop yields.

While Zhou's research doesn't directly focus on crops, the knowledge gained could aid in enhancing biomass for energy production and improving yield for food production. Additionally, a deeper understanding of stem cell behavior and regulation could eventually benefit human health through stem cell-regenerated organs or stem cell-based therapies.

"They're all directly related to this fundamental mechanism underlying stem cell division and differentiation," he said. "The fundamental knowledge we learn in this model system can definitely be applied to make people live better," Zhou said.

As vascular plants, both ferns and seed plants transport water and nutrients via specialized tissues. However, unlike seed plants, ferns generate spores rather than seeds to reproduce.

The lineages of ferns and seed plants separated several hundred million years ago. After all this time, the HAM gene family still works toward keeping meristem cells undifferentiated.

"These genes in different plant species sometimes cooperate with distinct signaling pathways, but at the cellular and developmental levels, they work to achieve the same goal. It's somewhat surprising and raises new questions for our future research," Zhou said.

This work was funded by the National Science Foundation (NSF). The Zhou group's stem cell research using plant models is currently funded by the NSF and National Institutes of Health.