July is the best time to enjoy the lotus, and the lotus flower has been loved by people since ancient times. As early as three thousand years ago, in the "Book of Poetry", such a verse appeared: "The mountain has a fusu, and the yin has a lotus", which is also the earliest written record of the lotus flower. The summer breeze gently blows across the lake, and a pool of lotus flowers sway in the wind, emitting a faint fragrance. From ancient times to the present, the lotus flower is not only a symbol of beauty, but also the sentiments of countless literati and inkers.
Image source: Zhou Jiangtao
Today, there are still groups of scientists who are trying to decipher the mysteries of the lotus flower and contribute wisdom to the study of the lotus flower. In the face of this beautiful flower, let's learn more about the unknown secrets of the lotus. The classification and structure of the lotus, belonging to the lotus genus Lotus, is an important aquatic economic plant. It is one of the oldest dicotyledonous plants, and together with Metasequoia, Ginkgo biloba, Chinese goosefoot and other relict plants that survived the Quaternary Ice Age, it is known as "living fossils". At present, the two surviving species of the genus Lotus in the world are: N. Asian Lotus (N. nucifera Gaertn.) and American Yellow Lotus (N. lutea Pers.）（Zheng et al., 2022）。 Asian lotus is mainly distributed in Asia and northern Australia, with a wide variety and abundant resources, and the American yellow lotus is mainly distributed in North America, the Caribbean, and the Atlantic Ocean, basically in a wild state.
Geographical distribution of Asian and American yellow lotus, image source: Reference 2. There are certain differences between the two in terms of plant type, flower type and flower color. Asian lotus plants are tall, with a variety of flower types such as single petals, heavy petals, and heavy tables, and the flower color changes between white, pink and red; The American yellow lotus plant is short and has a single yellow flower. Although Asian and American yellow lotuses are geographically spaced by the Pacific Ocean and have phenotypic differences, there is no reproductive isolation between the two and both have eight pairs of chromosomes. The lotus is mainly composed of the following parts: lotus, lotus leaf, lotus, lotus seed, lotus root and root.
Lotus structure diagram, image source: Yang Hui
Lotus: Also known as lotus, lotus, hibiscus, water hibiscus, etc., it is one of the top ten famous flowers in Chinese tradition. Lotus flowers are solitary, both sexes, and consist of calyxes, stamens, pistils, peduncles, and petals. The size, color, form and number of petals vary, forming a gorgeous and colorful flower. Lotus leaf: The lotus leaf is shield-shaped and microwave-shaped at the full margin of the leaf margin, which is the main organ of the lotus for photosynthesis. The surface of the lotus leaf has a thick waxy layer and lipid papillae that prevent the infiltration of water, so we can see that the lotus leaf is wet without water, which is the main reason for its "out of the mud without staining". Lotus: Developed from the flower tray of the lotus, it is inverted cone flat-top shaped, and after fertilization, the flower tray expands with the development of fruits and seeds, forming a lotus canopy, which initially appears yellow-green and brown after maturity. Lotus seed: The fruit of the lotus is commonly known as the "lotus seed", and the ovules develop into seeds after fertilization, and the ovary wall develops into a skin. Lotus seed leaves are rich in starch and can be eaten. In addition, lotus seeds are particularly long-lived, able to sleep for thousands of years and germinate again. Lotus root: Lotus root is the underground rhizome of the lotus, which plays the role of storing nutrients and reproduction, and is an important ingredient. When the lotus is broken, it will form a white filament, which is elastic, long, and not easy to break, which is what people often call "broken filament". Because the lotus contains tannins, it is easy to oxidize the blue-black color when cutting iron, so when cooking lotus, the use of iron cooking utensils should be reduced. Root: There are two main types of roots of the lotus, namely seed roots and adventitious roots. Seed roots are the main roots formed by the germ roots at the time of sowing, which are underdeveloped and do not play a major role. Indefinite roots grow on the nodes of the underground stem, and many lateral roots are attached to them, which play a role in fixing the plant and absorbing water and nutrients. Lotus is widely used and is used as landscape ornamental, snack food and vegetables. According to the agronomic traits and cultivation purposes, the lotus is generally divided into three categories, namely: hualien, lotus and lotus.
The main classification of lotus, image source: Xin Jia
Why does lotus heart have a bitter taste? Lotus seeds are the mature seeds of the lotus, slightly oval or spherical in shape, 1.2 to 1.8 cm long and 0.8 to 1.4 cm in diameter, consisting of four parts: the peel, the seed coat, the cotyledon and the heart of the lotus seed. Mature lotus seed coats are black-brown or tan, hard and not easy to peel off, and two yellow-white cotyledons are thick in texture with green lotus hearts in the middle.
Anatomy of the lotus seeds, image source: Xin Jia began to develop from 0 days of pollination, the lotus seeds gradually became larger in form, and the peel gradually changed from yellow to green, and then became black brown. The full development cycle of lotus seeds is about 30 days and can be divided into four developmental stages, namely the organogenesis phase (1 to 3 days after pollination, 1-3 DAP), the cell expansion phase (4-9 DAP), the nutrient accumulation phase (10-25 DAP), and the dehydration maturity phase (26-30 DAP) (Sun et al., 2020).
Lotus seed development dynamics, image source: Sun Heng
In the summer heat, we taste fresh lotus seeds and enjoy the sweetness of the taste buds, but many friends will find that the lotus seeds have a bitter taste when eaten. Why? It turns out that the feeding time of lotus seeds will greatly affect its fresh taste. Usually we choose lotus seeds 15 days after pollination, when the lotus seeds have a high water content and taste sweet and delicious. When the lotus seeds reach 18 days of development, the cotyledons accumulate higher levels of starch and have a reduced water content, so the taste is poor. In addition, the lotus seeds at this time taste bitter, which is caused by the failure to remove the clean green lotus heart. Lotus heart can synthesize a variety of alkaloids, mainly including lotus heart alkali, isocline and methyl lotus heartline. With the development of lotus seeds, lotus heart alkaloids continue to accumulate, and the bitter taste increases.
Lotus heart alkaloid chemical structure formula, image source: Xin Jia
Does the photosynthesis of the Lotus Heart require light? The heart of the lotus seed is located on the inside of the lotus seed, and its outer part is closely wrapped in the cotyledons, seed coats and peels. During the development of lotus seeds, the color of the lotus heart undergoes a transformation process from white, yellow-green to dark green, which is mainly due to the accumulation of chlorophyll. In general, the synthesis of chlorophyll in angiosperms requires light, and as one of the earliest species of angiosperms, why can lotus hearts synthesize chlorophyll in a "dark" environment? This peculiar phenomenon has aroused the interest and attention of many scholars at home and abroad. Researchers gave the lotus canopy light avoidance treatment during the development of the lotus seed, and found that the color of the lotus heart was significantly yellowed, which proved that the synthesis of chlorophyll in the lotus heart requires light.
Lotus heart shading experimental diagram, image source: Reference 4
Through genome-wide identification of the structure gene of the chlorophyll biosynthesis pathway in lotus, it was found that the chlorophyll biosynthesis pathway in lotus was consistent with that reported in other angiosperms, and the expression of most chlorophyll biosynthetic structure genes in the process of lotus heart development was significantly upregulated, which was consistent with the accumulation mode of chlorophyll. In the chlorophyll synthesis pathway, the conversion of protophyllates to chlorophyll esters catalyzed by the participation of protophyllate oxidation reductase (POR) is a critical step. For angiosperms (almost all flowering plants), light-dependent prophyllate redoxidase (LPOR) can help them convert prophyllates into chlorophyll esters, which in turn synthesize chlorophyll. Some gymnosperms (such as ginkgo biloba) and microorganisms use light-independent prophyllinate redoxtase (DPOR) to complete this reaction.
Chlorophyll synthesis pathway, image source: Xin Jia
Through the study of the nuclear genome and the chloroplast genome, the researchers found that there are two LPOR coding genes in the lotus, but no DPOR coding genes were found. This finding provides important molecular evidence for the light dependence of chlorophyll synthesis in the lotus heart.
Image source: Zhou Jiangtao
As early as the ancient times of the mainland, the lotus became a plant for us to watch, eat and sing. Nowadays, in the context of the development of agriculture with mainland characteristics, lotus cultivation has become an important source of increasing farmers' income in some areas. As an ancient species, lotus has undergone vicissitudes and seas, and its research value has far exceeded its own. In the accumulation of history, she exudes a unique charm. However, there are still many unsolved mysteries in the lotus, such as: the germination of the thousand-year-old lotus seed, the evolution of the lotus, etc., and the writing of these scientific chapters requires the joint efforts of generations of researchers. We will unlock more of the lotus's untold secrets and let this beautiful flower bloom forever. References:[1] Wang Qichao,Zhang Xingyan. Atlas of Chinese lotus varieties[M]. China Forestry Publishing House, 2005. [2] Lin Z, Zhang C, Cao D, et al., 2019. The Latest Studies on Lotus (Nelumbo nucifera)-an Emerging Horticultural Model Plant. International Journal of Molecular Sciences, 20: 3680. [3] Sun H, Li J, Song H, et al., 2020. Comprehensive analysis of AGPase genes uncovers their potential roles in starch biosynthesis in lotus seed. BMC Plant Biology, 20: 457. [4] Sun H, Song H, Deng X, et al., 2022. Transcriptome-Wide Characterization of Alkaloids and Chlorophyll Biosynthesis in Lotus Plumule. Frontiers in Plant Science, 13: 885503. [5] Zheng P, Sun H, Liu J, et al., 2022. Comparative analyses of American and Asian lotus genomes reveal insights into petal color, carpel thermogenesis and domestication. The Plant Journal, 110: 1498-1515.
This article is produced by Science Popularization China, produced by Xin Jia and Sun Heng (Wuhan Botanical Garden, Chinese Academy of Sciences), and supervised by China Science Popularization Expo