LIFE SCIENCE

Demystify the mystery of the evolution of the “Ghost Orchid”


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Some of the fungal heterotrophic purple-tongued labyrinth (a) and the complete fungal heterotrophic Guangdong glossy lip orchid (b) were presented in the image

The orchid gives the impression of being elegant and noble, a pot in the room, full of fragrance. However, there is an orchid that grows all its life under the ground, like a ghost, and only when it blooms will it stick out of the ground, which is called “ghost orchid”.

What’s even more amazing is that this orchid is different from other plants in that it doesn’t need leaves, roots and sunlight to grow. Because of this, they can go deepest into the forest and create their own niche in a dark and dark habitat.

So, how does “Ghost Orchid” live peacefully in such an environment, and what special ability does it have?

On April 21, Nature Plants published online the important research results of the orchid team led by Professor Lan Siren and Professor Liu Zhongjian of Fujian Agriculture and Forestry University in the field of fungal heterotropy, which unveiled the mystery of the evolution of “ghost orchid”. Nature Portfolio published a research brief online at the same time.

A pair of “sister” orchids

The emergence of land plants is an important event in the long history of life evolution, which has promoted the evolution of the entire terrestrial ecosystem. Alliances with soil fungi are considered a key innovation in plants to “conquer” land – plant-fungus symbiosis.

Scientists divide plants into three types according to the intensity of nutrients provided by plants dependent on fungi: first, the initial fungal symbiotic type, the early developmental stage of the plant is completely dependent on fungi to provide carbon sources, such as moss, stone pine and fern gametophyte stage and the germination stage of seeds such as orchids; second, part of the fungal symbiotic type or called mixed nutrient type, at least one stage of plant life history part of the carbon source provided by the fungus, and part comes from its own photosynthetic products; third, the complete fungal symbiosis type or called saprophytic, The carbon source for the plant’s entire life history is provided by fungi.

The origin and evolution of fungal heterotrophicity is a scientific question that has been debated and speculated for more than two centuries. Due to the lack of suitable autotrophic and heterotrophic related contrasting patterns of plants, the understanding of them is still lagging behind.

Orchid seed germination relies on fungi to provide nutrients, belongs to the typical fungal symbiotic plant, is the highest diversity of fungal heterotrophic plant species (about 28,000 species in the whole family), and includes a large number of partial fungal heterotrophic and complete fungal heterotrophic independent evolutionary branches, is an ideal taxon for studying fungal heterotrophicity.

“To figure out the molecular mechanism of orchid fungus heterotrophicity, you need to find two similar species for comparison.” Liu Zhongjian, co-corresponding author of the paper, said in an interview with China Science News. Coincidentally, in 2018, the research team discovered a pair of “sister” orchids with partial fungal heterotrophism (purple tongue orchid) and complete fungal heterotrophic (Cantonese glossy lip orchid) and assembled their genomes.

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Purple-tongued lip orchid Courtesy of the interviewee

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Photo courtesy of the Interviewee

Purple-gold lip orchid and Guangdong glossy lip orchid are the two largest genomes in the current whole genome sequencing orchid, and are also the first two orchids in the orchid family to be sequenced, filling a gap in the evolutionary research of orchids and having important scientific significance.

Evolutionary events triggered by gene loss

“We’ve always wondered why all orchid seeds are dusty and have no endosperm, and the completely fungal heterotrophic rootless and leafless orchids, such as the Cantonese glossy orchid, are built in form and how they get the nutrients they need. The mechanism of action behind this complex trait has been unclear and has been called the mystery of the ‘ghost orchid’. Lan Siren, co-corresponding author of the paper, said.

The team found that when orchids went from initial fungal heterotrophism to partial fungal heterotrophy to complete fungal heterotrophy, genes involved in photosynthesis, leaf and root development, and nutrient absorption in dark environments were lost, and the rate of base replacement increased.

“Specifically, photosynthetic genes, photoreceptors genes, and auxin transporter genes are all missing, resulting in completely fungal heterotrophic orchids not growing leaves and roots, occupying the lightless ecological niche, and the fungi are the most numerous in lightless habitats.” Liu Zhongjian said.

Through the analysis of evolutionary trees, researchers believe that the purple and golden tongue orchid and the Guangdong glossy lip orchid diverged about 12 million years ago, and the above genes were slowly lost with the differentiation of species, and the formation of orchid morphology is closely related to the loss of genes.

Plants need photosynthesis to survive, need to rely on roots to absorb nutrients, and orchids lose these skills, rely on what “eat”?

“In such evolutionary events, orchids add trehalose hydrolase genes, enabling orchids to digest trehalose in the hyphae, so that partially and completely fungal heterotrophic orchids can use fungi as a source of nutrients, which is a very critical innovation for heterotrophic fungi orchids.” Liu Zhongjian introduced.

Autotrophic and fungal heterotrophicity of orchids is associated with the expression of trehalose hydrolase genes, and it has been suggested that fully saprophytic orchids are a continuation of the orchid prococcoidal stage. This also explains why complete heterotrophic fungi in orchids occur more frequently than other plants. The nutrient supply of associated fungi may provide adaptive advantages for facultative saprophytic orchids, allowing atypical photosynthetic plants to explore new niches, such as extending deep into low-light forests. The ancestors of mixed vegetative plants adapted to the dark environment one step ahead, lost genes associated with light response and photosynthesis, and terminated the development of leaves and roots, resulting in the eventual production of complete fungal heterotrophic orchids

“In general, the complete fungal heterotrophic orchid has uncovered the mystery of the evolution of the ‘ghost orchid’ by expressing trehalose hydrolase genes and activating the phosphorus and nitrogen response mechanisms.” Lan Siren said.

You have to survive by “robbing”

In the plant-fungal symbiosis, the vast majority of plants use fungi to help obtain moisture and minerals from the soil and return photosynthesis to fixed carbon-source organic matter to fungi.

However, this symbiotic reciprocity has had multiple subversive reversals in all terrestrial plant lineages, including bryophytes, pinus, ferns, and seed plants. The water, minerals and carbon source organic matter that depend on plant growth and development are mainly obtained by fungi, and even some taxa do not photosynthesis, and the entire life cycle is completely dependent on fungi to provide carbon sources and other nutrients.

“In the traditional view, there are also many people who think that orchids and fungi are a harmonious relationship of mutual benefit and reciprocity.” Liu Zhongjian said, “This is not the case, our research found that in the orchids that are completely fungal heterotrophic, the orchids do not produce energy, there is nothing to return to the fungus, and completely rely on the nutrition of the ‘robbing’ fungi for their own survival.” ”

“Unlike self-reliance, the business of living by plunder is not large, so the form of completely fungal heterotrophic orchids is often relatively small.” Liu Zhongjian said.

According to Dr. Feiwei Li of Cornell University, “This is a fascinating and well-conceived study. The comparison of two closely related species of the genus Aylocarposa, one completely fungal heterotrophic and the other partially heterotrophic, is very ingenious and does provide many important insights into the evolution of fungal heterotrophicity. ”

Nature-Plants was accompanied by news commentary, and journal editors gave it a high evaluation: “Biologists have long been interested in the evolution of orchid species. This study caught my attention because it provided high-quality genomes for two species of orchids in the orchid subfamily partially or completely heterotrophic fungus, and also revealed how the orchid genome changes with the evolution of fungal heterotrophic lifestyles, and which genetic evolutionary events contributed to their symbiotic relationship with fungi. (Source: China Science Daily Zhang Qingdan)

Related paper information:https://doi.org/10.1038/s41477-022-01127-9



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