Only higher plants have it! Why can phytoliths reveal the history of rice cultivation?

Written by Shi Jun

Professor Yu Lupeng's team from the School of Resources and Environment of Linyi University and researchers from 13 institutions including the Institute of Geology and Geophysics of the Chinese Academy of Sciences and the Zhejiang Provincial Institute of Cultural Relics and Archaeology published an article in the international journal Science, revealing the evolutionary history of rice from wild to domesticated over the past 100,000 years. The key to unlocking this mysterious black box is actually phytoliths that are invisible to the naked eye.

What are phytoliths? Phytoliths are microscopic structures that appear in the cells and intercellular spaces of many higher plants. Their main component is silicon dioxide, which is the same as the main component of the sand we usually see. However, these silicon dioxides are filled in the intercellular spaces and inside some cells, forming some special structures.

Note that not all plants have phytoliths. Only higher plants, that is, plants with vascular bundles, have phytoliths, including ferns, gymnosperms and angiosperms, while mosses and green algae do not have phytoliths.

Why did plants evolve phytoliths?

For plants, phytoliths have two main uses: one is to enhance the mechanical strength of the plant and make its body harder; the other is to defend against animals from gnawing and put on armor.

The first function of phytoliths is to make plants hard and support the plant body. Phytoliths filled in the tissues can provide strength for plant tissues, so that they can stand proudly in the face of wind and rain. In simple terms, phytoliths are like the concrete filled between the steel frames when building a high-rise building.

Another function of phytoliths is to prevent animals from eating them. Imagine mixing a lot of sand and glass into a plate of vegetable salad. Diners will have to think about their teeth. Even if some herbivores try to eat it, it will be very difficult for their teeth, which is not good for the survival of animals.

In addition to the two main uses mentioned above, phytoliths in plants also play a part in resisting the invasion of external microorganisms; at the same time, phytoliths can also absorb and enrich some specific metal elements, such as aluminum, to prevent these elements from causing damage to plants.

I have to say that phytoliths are really versatile.

Phytoliths first appeared in the late Devonian period, when lush forests first appeared on Earth's land. Phytoliths provided the necessary support and weaponry for early lycophytes to fight against plants. Later in the evolution of plants, ferns, gymnosperms, and angiosperms inherited the ability to produce phytoliths.

Although phytoliths are weapons used by plants to fight animals, they have many special uses for us humans. First of all, phytoliths are little helpers in agricultural production. The phytoliths on the stems and leaves of rice and wheat can help them effectively resist the invasion of herbivorous insects. In this case, we have a special cooperative relationship with the little phytoliths. Of course, we can also use phytoliths to conduct a lot of paleobotany, paleoecology, and archaeology research today.

When studying ancient plants, archaeologists and paleontologists certainly hope to find the remains of flowers and fruits of plants in the strata. However, the conditions for the formation of fossils of flowers and fruits are too harsh, and even the formation of fossils of leaves is not easy. Therefore, to this day, many scientists have focused their attention on plant microfossils such as pollen and phytoliths.

After analyzing the phytoliths in the stomach fossil of Jehol Bird, researchers found that there were some phytoliths of Magnoliaceae plants in it. That is to say, Jehol Bird actually ate the leaves of Magnoliaceae plants, and its eating method was similar to that of today's Hoatzin.

Scientists have also found some phytoliths in the tartar of a dinosaur called the sauria, which are consistent with the phytolith characteristics of the grass family. This study enriches our understanding of the diet of dinosaurs. Before, we all thought that dinosaurs ate gymnosperms and ferns. But the main food of the sauria was actually grass. That is to say, in the dinosaur era, grass had become a relatively common grassland plant and was also the staple food of many dinosaurs.

How can we tell whether a rice relic is cultivated rice or wild rice in a human site? If we judge it only by the size of rice grains, we cannot get convincing evidence, because the size of rice grains is very random, and the possibility of rice grains being preserved intact is not high.

In addition to grain size, there is another criterion for judgment, that is, the structure of the rice spikelet, whether there is a broken and scattered structure on it. If this broken structure disappears, it means that these rice grains will still remain on the ear when they mature. From this perspective, we can also make a judgment-this rice has been artificially selected and domesticated. However, it is also difficult to preserve the structure of the rice spikelet intact. What should we do? We still have to look for the answer from phytoliths.

There are three different types of phytoliths in rice: dumbbell-shaped phytoliths found on stems and leaves; bimodal papillary phytoliths found on the husk of rice; and fan-shaped phytoliths found on leaves.

Among them, there is almost no difference between the dumbbell-shaped phytoliths of cultivated rice and wild rice, so they are unusable; there is a huge difference in the bimodal papillary phytoliths between cultivated rice and wild rice, but it is quite difficult to find well-preserved husks, so they are also unusable.

The breakthrough appeared in the fan-shaped phytoliths, which have many fish-scale patterns on their edges. This is a very typical feature for distinguishing wild rice from cultivated rice. Generally speaking, the number of fish-scale patterns on wild rice is less than 9, while the number of fish-scale patterns on cultivated rice is usually 8 to 14. Although there is overlap between the two, cultivated rice still has more. This feature has become a key feature for distinguishing whether rice has been domesticated.

In addition to determining whether rice has been domesticated, rice phytoliths can also help us reconstruct the timeline of the rice domestication process. In traditional research, how to determine the exact age of rice remains in human sites is a big problem facing researchers. Because over a long geological period, the movement of the strata will change the burial position of the plant body, thus affecting the judgment of researchers. At this time, the role of phytoliths appears.

The silicon dioxide in phytoliths can absorb the radiation energy of the surrounding environment and store it. After being stimulated by specific radiation, the stored energy is released in the form of light. This phenomenon is called photoluminescence. The intensity of the released light is related to the total amount of energy stored previously. By analyzing the radiation conditions that the environment around the burial site can provide to the phytoliths, researchers can derive an annual dose of energy stored by the phytoliths. By dividing the total energy of the phytoliths by the annual dose, the corresponding burial time can be obtained.

In this study, the quartz in the archaeological site has good luminescence performance and can emit more light signals when absorbing the same radiation dose, which means that researchers can accurately test the age of a single particle or a few particles of quartz. Using the single-particle technology, the researchers successfully distinguished quartz particles of different ages mixed together, effectively solving the problem caused by stratum disturbance, accurately determining the age of key layers, and thus measuring the true "age" of rice.

Using phytoliths from different rice species over the past 100,000 years to establish an evolutionary sequence can help us better understand the process of rice domestication.

The tiny phytoliths contain a huge evolutionary story, which is something we never thought of before. In the future, as the research on phytoliths deepens, it will surely bring us more new surprises.