Remarkable Symbiotic Plant-Fungus
Gastrodia (tianma) is a very unusual plant. It contains no chlorophyll and has no green color in it. Lacking chlorophyll, it cannot produce its nutrients from sunlight as do most plants. Moreover, it has no roots; in the ground is a rhizome (tuber) that appears sealed shut from the soil environment. How then does it get nourishment and grow?
Gastrodia is entirely dependent on a soil fungus, Armellaria mellea, for its nutrition. This fungus can develop into the fruiting body commonly called "honey mushroom," but exists in far greater amounts in filamentous forms. When yet another fungus (Mycena osmundicola) invades the seed of gastrodia in the soil, it produces an orchid tuber; with enough input from the Armellaria fungus, which invades this rudimentary component, the underground portion grows and then seasonally sprouts into a luxurious flowering spike. So, in a field of gastrodia, there may be numerous rhizomes unseen in the ground, with only a few sprouting to form the orchid, namely those being adequately infiltrated by Armellaria.
Armellaria fruiting body, the honey mushroom, infesting a tree stump.
Armellaria mycelium growing in laboratory Petri dish. This mycelium is grown in huge batches to yield a substitute for gastrodia tuber used in Chinese herbal medicine.
The Armellaria fungus doesn't seem dependent on the orchid, but feeds on other plants, mainly trees. A portion of the mycelium forms dark-brown to black, shoe-string like objects called rhizomorphs. They attach to the surface of tree roots, feeding off the tree's nutrition supplies. These rhizomorphs are capable of growing outward through the soil long distances from infected tree to reach an uninfected tree. They attach themselves onto healthy root systems and begin the process of infection, colonization, and decay, eventually leading to the death of the tree. So, Armellaria is known as a serious pathogen for trees. The rhizomorphs that are seen on infected tree roots are the source of one of the names for the disease affecting the trees: shoe-string root rot. Also, the rhizomorphs can extend far enough from forest land to invade adjacent cultivated fields, damaging crops, especially potatoes, which have tubers similar to those of gastrodia. Armellaria does not, however, require live trees and crops, at least, not all the time, as Armellaria mycelium in the soil can live off decaying plant matter and the remains of trees.
Shoelace-like strands of Armellaria on a tree root.
|Trunk of a Scots pine killed by Armellaria fungus. The cord-like rhizomorphs travel from the soil up the trunk, growing through the living tissues between the bark and the heartwood, eventually destroying them and killing the tree.|
A fascinating aspect of the orchid is that it pacifies the appetite of the fungus for trees, thus preserving the uninfected forest trees from attack where the orchid grows. The process by which it inhibits the fungus is not known, but it is expected that substances acting as fungal inhibitors, phytoallexins, are produced by the infected gastrodia rhizomes in quantities sufficient to slow the fungal spread but not damage it. And, while Armellaria destroys trees and potato crops, it doesn't appear to cause any harm to gastrodia; to the contrary, it feeds it. While it is evident that gastrodia benefits from being parasitized by Armellaria, it is unclear what benefit Armellaria gets from gastrodia.
While Gastrodia elata, the species used in Chinese medicine, produces only tiny flowers, another variety of gastrodia (Gastrodia cunninghamii) produces the well-known decorative "black orchid" (pictured below), which starts off purplish and then develops light petals forming yellow-brown flowers with dots of purplish-black.
The genus Gastrodia is found in warm climates, for example, in Madagascar, tropical Asia, Oceania, Japan, and China. It consists of just 20 species, five of which are found in China, the main one being Gastrodia elata. The various gastrodia orchids grow in high humus soil in-or at the fringe of-broad leaf, coniferous, or mixed forests.
The Gastrodia elata plant, when fully developed, consists merely of an underground tuber-like an elongated potato-tapering to one end and covered with brownish cork. Like those of other tubers, the tissues contain reserve food materials (mainly starch), at the expense of which it periodically produces the flowering shoot a yard or more in length, bearing small scale leaves and yellowish-brownish flowers of the characteristic orchid type. As it ages, the stalk turns reddish, giving the plant its ancient Chinese name, chijian (red arrow).
Armillaria grows over an even larger geographical area than gastrodia, tolerating colder weather. It forms its mushrooms (sporophores) only after the tree tissues in which it grows have been killed. Thus, one can see live, apparently healthy trees without mushrooms growing on them near the orchids, even though the trees may be parasitized. The tawny, white-gilled mushrooms are common on dead stumps in woodlands and hedgerows, and are collected in Europe for use as a food.
The discovery that tianma had to grow with a fungus was an important breakthrough achieved by Chinese researchers in Yunnan and Beijing in the 1960s, as part of an analysis of how to cultivate the plant, which had become too rare to meet the demands of herb medicine use. Li Shizhen had noted in the 16th century that: "tianma is the root of chijian [it is said that Shennong named it chijian based on a terra-cotta color, a brownish-red], it is a parasitic body, of which the origin is unknown, it seems to be innate [just appears, as if from nowhere], so it was called tian, [the sky; heaven]....All the rhizomes have minute white hair-like roots packed close to each other, though they aren't linked up together, actually all of them have 'qi' linking them." These hair-like "roots" are actually the mycelium of the fungus. He also noted that "It has a root like fuling [Poria cocos, a fungus commonly used as an herb material] in the soil, without the root like fuling, the silk-shaped stem is unable to climb upward." This showed some inkling of a connection to a fungus. Tao Hongjing (425-536 A.D.), suggested that: "chijian is also type of zhi (fungus). Its stem is like an arrow shaft, red in color, the leaves grow on the top which do not only move in wind, but automatically wave without the blowing of wind." Su Ching, who wrote the Tang Bencao, stated that, "According to the Baopuzi (a Taoist book by Ge Heng), there is a formula of immortality that includes duyaozhi (which means: a spontaneously waving fungus-plant). The idea that the plant waves automatically is a peculiar one, but the plant doesn't have leaves at its top. It has small structures that are not really leaves, and do not function as leaves, since they do not need to capture sunlight. The plant may sway, however, with very little air movement, due to its top heavy nature when in flower.
A Chinese legend tells of a gastrodia collector:
A long time ago, there was an old man living in Er Xianyan who made a living by gathering herbs, especially by gathering gastrodia-he was called 'Gastrodia Man.' One year, in the Spring, the collecting did not go well. The old man searched the mountain for many days but found nothing; this upset him greatly. Then, he was seized by a sudden impulse: to find a good field and grow gastrodia himself. If he could grow the gastrodia himself, he thought, he wouldn't need to search the whole mountain, yet possibly turn up nothing, as happened this year. The old man made up his mind, and began to plant the gastrodia seeds. One year later, the gastrodia had not sprouted. He checked the field and found that all the gastrodia he put in was gone. The old man was confused and thought that the gastrodia had been stolen by someone. So, the next year, after seeding, he built a shack and watched the field all day and all night. At the time for harvesting, he figured he would get plenty of gastrodia, but when he dug up the field, he still found nothing. The old man couldn't help but sigh and figure that gastrodia must be a plant grown only by God. What he said about gastrodia soon spread about and people everywhere said that gastrodia was a gift from God, which could not be traced back to any earthly origin (hence named tian, for its source: from heaven). Since that time, people thought that gastrodia could not be grown by human efforts, so no one tried again to grow gastrodia.
This story nicely reflects the difficulty of cultivating gastrodia that has been encountered since ancient times.
Gastrodia farmers begin with small tubers harvested previously that were not suitable for use as food and medicine and plant them in ground that is prepared with fungal mycelium and wood chips. The tuber planted in the early winter (e.g., December) is appropriately harvested in the second year, also in the winter time, or in the spring of the third year; while the tuber planted in the spring is better harvested in the winter of the same year or in the spring of the next year. Before harvesting is undertaken, the nurturing fungus wood-materials are first removed, then the tubers are dug out. Both the tubers (called tianma) and the jianma (arrow gastrodia, the tuber with flowering stem) are utilized as raw materials, but any of the small white tubers and the grain-shaped tubers are kept as the seedlings for the next planting. The harvested tubers are processed promptly. After cleaning, the tubers are dumped into boiling water to be cooked according to their size. Those that weigh more than 150 grams (large tubers) are cooked for 10-15 minutes; medium tubers in the range of 100-150 grams are cooked for 7-10 minutes; and those less than 100 grams (the smallest size utilized is about 70 grams) for 5-8 minutes. These cooking times assure that the centers are softened. Then the cooked tubers are fumed with sulfur in a sulfur-fuming house for 20-30 minutes, and then dried by a gentle fire on a warm kang (a heated brick bed) at about 50-60°C. When the tubers are about 70-80% dried, they are flattened by hand, then dried further on the warm kang (at higher temperature, around 70° C) continuously until the tuber is dried completely, and then it is promptly taken off the kang to avoid scorching.
Gastrodia tubers have been appreciated in many cultures outside China, especially in Australia and New Zealand. The Maori tribe had a special place for gastrodia (called huperei or perei) in their lore. According to Maori tradition the huperei was not a plant of the earth but a creature of supernatural beings and there were many superstitions relating to it. The plant was supposed to understand the Maori language, so those who went to dig the rhizomes used the substitute name maukuuku to ensure that the huperei did not hear its name and disappear, this story imitating the problem faced by "Gastrodia Man." The Woiworung tribe in Australia found the roots to be a special food. Gastrodia elata is found in Tibet, Russia, Korea, Japan, and Taiwan; in all these places it is used as food and medicine. Although it is considered an endangered species, it has a wide growing range and is increasingly cultivated, especially in China. As a food, gastrodia is commonly cooked in soups, especially with chicken or duck, where it may be joined by another unique fungal product, cordyceps, to make a rich tonic. As medicine, it is currently used mainly for treating headache, hypertension, and neurasthenia (weakness-fatigue) and was traditionally used to treat epilepsy and other convulsions.
|The roots of Gastrodia sesamoides, the potato orchid, are rich in starch and very nourishing. They were highly prized by the Woiworung in Australia and Maori in New Zealand. The roots are hard to find because the plant has no green leaves. They could be found by watching where bandicoots (small marsupials) scratched for them.|