How parasitic, asexual plants evolve and live

(Press-News.org) There are plants that are neither green nor sexually reproductive, but precisely because of that they teach us a lot about what it means to be a plant. New research with Kobe University participation took a close look at Balanophora to learn how such non-green, asexual plants evolve and live.

“My long-standing aim is to rethink what it truly means to be a plant,” says Kobe University botanist SUETSUGU Kenji. He continues, “For many years I have been fascinated by plants that have abandoned photosynthesis, and I want to uncover the changes that occur in the process.” Balanophora species are an extreme example, being non-green plants that feed off the roots of others. They live underground and emerge above ground only during the flowering season — and some species even reproduce exclusively asexually. “However, while there have been individual studies on changes in the plants’ genomes, their ecology and their reproduction, we don’t know how they relate to each other,” explains Suetsugu.

For Suetsugu, the challenge therefore was to connect three levels of analysis that have never been integrated for Balanophora: He needed to robustly establish how the plants of that group relate to each other, how they modified their plastids (a part of a plant’s cells which in green plants serves as the cell’s “solar panels”), and how their reproduction fits into their ecology. On what made this particularly challenging, he says: “These plants are rare, patchy and often restricted to steep, humid forests. But years of experience with studying Balanophora both in the lab and in field studies, as well as long-standing relationships with local naturalists made this project possible.” And to complement this intimate knowledge of the species with expertise on highly reduced genomes, he partnered with researchers from the Okinawa Institute of Science and Technology.

In the journal New Phytologist, the team now publishes their results. They found that all plants in the group had an extremely reduced plastid genome (DNA not kept in the plant cell’s nucleus but directly in the plastid), and that this must have happened in their common ancestor, before the plants diversified into different species. Suetsugu says: “It is exciting to see how far a plant can reduce its plastid genome, which at first glance looks as though the plastid is on the verge of disappearing. But looking more closely we found that many proteins are still transported to the plastid, showing that even though the plant has abandoned photosynthesis, the plastid is still a vital part of the plant’s metabolism.”

In contrast, asexual reproduction likely evolved repeatedly in the group, as the research team showed. They found that the plants possibly evolved the additional ability to create seeds even without fertilization early on, and that this proved an advantage as they colonized the archipelago spanning from mainland Japan via Okinawa to Taiwan. “Over the past decade I have studied Balanophora pollination and seed dispersal where camel crickets and cockroaches play an unexpected role, but I also noticed that asexual seed production often ensured reproduction when mates or pollinators are scarce,” explains Suetsugu. Eventually, this form of reproduction might have become permanent in some species.

For the Kobe University botanist, the study marks an important step in his effort to understand how non-photosynthetic plants function and persist in ecosystems. Suetsugu says, “For someone who has spent many hours observing these plants in dark, humid forests, seeing their story unfold at the genomic level is deeply satisfying. My next goal is to connect these results with biochemical measurements to find out what Balanophora plastids actually produce and how these products help sustain the parasitic plants’ growth within the roots of their hosts.”

This research was funded by the Japan Society for the Promotion of Science (grant 23K14256), the Human Frontier Science Program (grant RGEC29/2024), the Japan Science and Technology Agency (grants JPMJPR21D6, JPMJFR2339) and the National Science and Technology Council, Taiwan (grant 109-2311-B-845-001). It was conducted in collaboration with researchers from the Okinawa Institute of Science and Technology and the University of Taipei.

Kobe University is a national university with roots dating back to the Kobe Higher Commercial School founded in 1902. It is now one of Japan’s leading comprehensive research universities with over 16,000 students and over 1,700 faculty in 11 faculties and schools and 15 graduate schools. Combining the social and natural sciences to cultivate leaders with an interdisciplinary perspective, Kobe University creates knowledge and fosters innovation to address society’s challenges.

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