After 10 years of climbing mountains and crossing rivers, it was finally proved that this fruit originated in China

Yunnan, Hubei, Hunan, Guangxi, Guangdong, Jiangxi... From the tropical rainforest of Xishuangbanna to Mangshan and then to Maoer Mountain, for more than 10 years, Xu Qiang, a professor at the College of Horticulture and Forestry of Huazhong Agricultural University (hereinafter referred to as HAU), has led a team to search for traces of wild citrus species in the mountains of the south. Just because they believe that the oldest citrus plants in the world grow there. Recently, Nature Genetics published the team's latest research paper online. They revealed the origin and evolution of the genus Citrus and the subfamily Citrus, mapped the pan-genome map of the subfamily Citrus, identified the key genes that regulate the accumulation of citric acid in citrus fruits, and confirmed that citrus originated in China. This study has important scientific significance for understanding the molecular regulatory mechanism of citric acid accumulation in plant fruits, and at the same time provides important genetic resources for fruit flavor breeding.

The citrus family. Photo provided by the interviewee

“Tell the story of the origin of citrus clearly”

"The emperor planted the favorite tree, the tangerine. It obeyed the order and did not move away, it was born in the south..." Many scholars have proposed that China is the origin center of the world's citrus fruit. However, previous research was mainly limited to commercial varieties widely grown around the world, such as tangerines, oranges, grapefruits, and lemons, and there was a lack of systematic understanding of the wild and related species of citrus, which restricted research on the origin and evolution of citrus. The claim that citrus originated in China has always lacked conclusive evidence.

Around 2013, Deng Xiuxin, an academician of the Chinese Academy of Engineering, led Xu Qiang and others to visit the southern citrus distribution areas many times, visited ancient trees and searched for resources, and urged them: "Collect and identify them carefully. Such a large number of wild citrus resources can be used as materials for origin research."

At that time, Xu Qiang noticed that there were some very old citrus resources in the northern foothills of Nanling Mountains, Mangshan Mountain in Hunan, and Hubei, such as the "Yichang Orange" named after Yichang, Hubei. "At that time, I thought that this might be an important origin of citrus, and we should go to these places for actual research and in-depth study of the genetic background of these citrus." Xu Qiang said.

Xu Qiang led the team to draw up a detailed research plan right away.

Doctoral students Huang Yue and Xu Yuantao began to collect resources and data. They cooperated with the Institute of Tropical and Subtropical Economic Crops of Yunnan Academy of Agricultural Sciences and the Institute of Horticulture of Hunan Academy of Agricultural Sciences to go deep into Yunnan, Hunan, Guangxi, Sichuan, Jiangxi and other southern citrus distribution areas in my country to find ancient trees, visit old farmers and collect resources.

After a long period of cooperation, they have established partnerships with scientific researchers and even fruit farmers in Yunnan, Hunan and other places. This friendship has significantly improved the efficiency of collecting and observing wild citrus resources.

Xu Yuantao clearly remembers that once, when Xu Qiang heard that there might be precious materials missing for the project research in Ruili, Yunnan, he immediately flew from Wuhan to Baoshan, Yunnan, and then drove for three hours to the wild citrus resource distribution area under the guidance of his partners. He hiked up the mountain and searched for more than two hours before finding the samples he needed. Every summer vacation, they must spend a long time in these ravines, collecting branches, leaves, flowers, etc. of citrus species, and they have to go to the same place to collect their fruits from October to December. Sometimes they are unlucky, the fruits fall off when ripe or are eaten by wild animals, and they cannot observe and obtain the fruits smoothly.

"Because we needed to collect wild plant resources, we climbed wild mountains and encountered all kinds of dangers and met all kinds of animals." Xu Qiang said that once they almost fell into a valley, "If we had really fallen down, we might have been seriously injured."

Xu Qiang said that the wild germplasm resources they collected would be grafted in the greenhouse of Huazhong Agricultural University, and more than 300 living germplasm resources have been preserved so far.

However, around 2018, a US-led team published research results in Nature, clearly proposing the "theory" that citrus originated from the southeastern foothills of the Himalayas.

This caught Xu Qiang's attention. "The claim that it originated from the southeastern foothills of the Himalayas is questionable, or at least inaccurate," "It is worth re-examining using the research paradigm of wild materials and genomics," "We can tell the story of the origin of citrus more objectively."

Huang Yuezheng and Xu Qiang were on a train to Yunnan when they discovered this "loophole". The master and apprentice set up a computer on the small table on the train, opened the ancient continental plate movement model and the distribution map of China's citrus resources, and began to study it carefully.

Correcting the classification of the genus Citrus

Throughout the project, they also unexpectedly discovered two new species.

In 2016, a staff member of the Agriculture and Rural Affairs Bureau of Chenzhou City, Hunan Province asked Deng Xiuxin to identify a fruit.

This fruit is very special. You can smell a unique fragrance from a long distance away. The seeds are also very large and look a bit "strange".

Deng Xiuxin thought it was worth studying. Later, Xu Qiang's team went to Linwu County, Chenzhou for several researches and tested the fruit, finding that it tasted sour and bitter, very "wild". However, the fruit only had fruit, no flowers, leaves, or branches, so it was impossible to tell what species it was.

Coincidentally, Xu Qiang's team later found similar germplasm materials at the Horticulture Research Institute of Hunan Academy of Agricultural Sciences. Its leaves are different from those of other species of the genus Citrus. Citrus leaves are usually hard and leathery on the outside, but the leaves of this "weird" plant are soft like mint leaves and have hair on the surface.

"From the genome point of view, it is a separate genus and a transitional species in the evolution from Yichang orange to tangerine," said Xu Qiang.

Another new species was discovered in Yunnan, where there is a large tree that bears very large and sour fruits. Local people don't know what it is and don't care what it is.

It was not until 2019 that Xu Qiang's team came here and saw it. After research, they believed that this was an ancient species before the differentiation of citron and citron species.

"Over the years, it has hardly changed, only evolved a little bit, but because its growth cycle is particularly long, it has retained its ancient genome," said Xu Qiang.

When the two new species were first discovered, they were not sure they were new species. "The classification of the genus Citrus and the subfamily Citrus is a bit confusing," Xu Qiang said. For the genus Citrus, some American scholars believe there are 12 to 14 species, while some Japanese scholars believe there are 162 species.

It was not until Xu Qiang's team collected the whole genome data of 312 citrus subfamily germplasm resources worldwide and conducted a comprehensive review that they confirmed that these were two new species. From this, they proposed that there are 15 species in the genus Citrus.

After arduous exploration, they speculated that the ancient Indian plate may be the origin center of the Citrus subfamily, southern China may be the primary origin center of the Citrus genus, and the southeastern foothills of the Himalayas and Australia and nearby islands are the two secondary differentiation centers of citrus.

"On the Huaihai coast, Yangzhou is the place where tangerines and grapefruits are offered as tribute." In the Xia Dynasty, tangerines and grapefruits were precious tributes to Emperor Yu. This "rectification of the name" seems to coincide with the history of Chinese citrus cultivation described in the "Book of Documents: Tribute to Yu" more than 2,000 years ago. They have "reunited" across time and space.

Citrus is one of the most important fruit trees in the world and in my country. In 2022, China's citrus planting area exceeded 45 million mu, and the output exceeded 55 million tons. The citrus industry has played an important role in the development of the national economy and rural revitalization. Xu Qiang's team hopes to continue to work hard to transform scientific research results into germplasm resources to empower China's seed industry security and rural revitalization.