What efforts have scientists made to help you eat more sweeter sugarcane? (Part 2)

Produced by: Science Popularization China

Author: Yin Xin (Institute of Microbiology, Chinese Academy of Sciences)

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life" to interpret life phenomena and reveal biological mysteries from a unique perspective. Let us delve into the world of life and explore infinite possibilities.

In the hot summer, a glass of iced sugarcane juice brings coolness and sweetness.

In the cold wind, a bowl of brown sugar water brings warmth and nourishment.

These simple "little happiness" in daily life are all related to the crop of sugarcane. But did you know that the sweetness of sugarcane is not innate, but has been improved by scientists through repeated improvements in sugarcane germplasm.

Today, let us tell the story of scientists and sugarcane.

What is sugarcane germplasm? It is different from variety.

What is sugarcane germplasm? Many people think it is the variety of sugarcane, but there is a difference between the two.

Sugarcane that has been approved can be called a sugarcane variety, which is the type of sugarcane we can see. Before the variety is approved, all kinds of potential intermediate materials can be called germplasm, and sugarcane that has been genetically modified can also be called germplasm.

Therefore, we can believe that germplasm is the genetic basis of sugarcane and the root of determining the quality of sugarcane.

In order to select high-quality sugarcane, scientists will test it for multiple indicators and develop a series of standards, including sugar content, yield, disease resistance, insect resistance, cold resistance, drought resistance, perennial root and growth rate. For example, the national standard "Sugarcane" (GB/T 10498-2010) lists the minimum sucrose content index as 12%. Through these tests, scientists can select the most promising "excellent germplasm" and let them play a greater role in future planting.

Sugarcane has a huge genome, and hybrid breeding is often time-consuming and labor-intensive

Scientists have made many efforts to make sugarcane sweeter and more productive.

The most common method is hybrid breeding, which is a bit like "finding a partner" for sugarcane. In addition to hybridizing different varieties of sugarcane, sugarcane can also mate with "close relatives" (generally other species of the genus Saccharum in the subtribe [Saccharinae]) and even "distant relatives" (generally other species outside the genus Saccharum in the subtribe [Saccharinae] that can be hybridized with sugarcane, such as Saccharum, Rhizoma, Psoralea, Miscanthus, etc.) and inherit their advantages [6].

Among sugarcane's "close relatives" and "distant relatives", there are numerous wild germplasm resources. Under the pressure of natural selection in the wild, they tend to retain more genes for excellent traits such as disease resistance and stress resistance. Just like the good genes of parents are passed on to their children, scientists hope to cultivate sugarcane with high sugar content, high yield and stable yield through this method.

For example, some sugarcane varieties may be disease-resistant but low in sugars, while others may be high in sugars but susceptible to disease. Through cross-breeding, scientists hope to combine the best of both worlds.

However, due to the large and complex genome of sugarcane, its genome size is about 10 Gb [6], while the human genome size is about 3Gb [7]. Humans are diploid organisms that develop from fertilized eggs and have 46 chromosomes. Sperm and eggs, as human reproductive cells, have half the number of chromosomes as somatic cells, 23 chromosomes each, and are called haploid. When the sperm and egg combine to form a fertilized egg, they return to diploidy. Therefore, diploid refers to an organism whose cells contain two sets of chromosomes, one set of chromosomes from the "father" and one set of chromosomes from the "mother".

Sugarcane is much more complicated. During its evolution, the entire genome has undergone multiple "polyploidization events" (i.e., the number of chromosomes doubled) [8]. Coupled with chaotic hybridization with "relatives", the sugarcane genome is polyploid, with hundreds of chromosomes. Moreover, the "parents" of each variety are different, resulting in different genomes. Even if the "parents" are the same, the resulting gene combinations are very different, which is really "a dragon gives birth to nine sons, each one is different." Therefore, the hybridization and breeding process is not always smooth, and sometimes the new sugarcane germplasm cultivated is not as good as expected. This "hit-and-miss" process is time-consuming and laborious, and requires scientists to spend years or even decades of observation and screening.

New ideas for sugarcane improvement: exploring hyperploid gene editing technology

In order to solve these problems, scientists have begun to explore gene editing technology in recent years. Unlike the "hit and miss" of hybrid breeding, gene editing is like "precision pruning".

Gene Editing

(Photo source: veer photo gallery)

Scientists can find genes that affect sugarcane's sweetness, disease resistance and other traits by comparing and analyzing the genome, transcriptome (changes in gene expression levels) and proteome (the presence and amount of protein) of different sugarcane varieties, as well as conducting homology analysis in sugarcane's "distant relatives" such as sorghum, corn and rice, which have been studied more intensively.

For example, in susceptible varieties, there is a gene A that can be used by pathogens, causing diseases, while in resistant varieties, the A gene has lost its function and cannot be used by pathogens, thus not causing diseases. We can use gene editing technology to "cut" the A gene in susceptible varieties, making it unusable by pathogens like in resistant varieties. After such targeted transformation, excellent traits such as high sugar and high resistance can be quickly aggregated.

However, gene editing also has its difficulties and its technology is complex, especially for high-ploidy crops such as sugarcane. As mentioned earlier, sugarcane has undergone multiple "polyploidization events", resulting in many homologous genes, that is, the same gene may have multiple copies in the genome. If gene editing is required, all these homologous genes must be modified at the same time. Therefore, efficient gene editing methods are needed. For example, if the efficiency of gene editing is 50%, and the success rate of editing 2 homologous genes at the same time is 25%, then the success rate of editing 10 homologous genes at the same time is only 0.098%; if the efficiency of gene editing is increased to 80%, the success rate of editing 2 homologous genes at the same time can theoretically reach 64%, and the success rate of editing 10 homologous genes at the same time can theoretically reach 10.7%. Therefore, improving the efficiency of gene editing is the goal that scientists strive for [4].

Whether it is hybrid breeding or gene editing breeding, scientists are constantly working hard to make sugar cane sweeter and with higher yields. For example, in the field of sugar cane, scientists are constantly breeding new varieties with better taste, higher sweetness and more juice, so that people can have a better experience when chewing sugar cane. In the field of sugar cane, scientists focus on improving sugar content, yield, disease resistance, etc., to make sugar production more efficient and ensure yield and quality.

Conclusion

Science has never been far away from our lives. Sugarcane, which looks ordinary, actually condenses the efforts and wisdom of many scientists. Behind every bite of sweetness, there are countless explorations and experiments, failures and successes, as well as beautiful expectations for the future of agriculture.

Next time, when you taste a sip of brown sugar water, you may think of these silently working scientific researchers. Their efforts make every sweet bite in our daily life more delicious.

(Note: Latin parts in the text should be italicized)

References:

1.Mintz, SW (1985). Sweetness and Power: The Place of Sugar in Modern History. Penguin Books.

2.Eggleston, G., & Legendre, BL (2003). Quality management of sugarcane juice to raw sugar: Louisiana factory operations. International Sugar Journal, 105(1256), 8-18.

3.Rein, P. (2007). Cane Sugar Engineering. Bartens.

4. De Souza, AP, Grandis, A., Leite, DC, & Buckeridge, MS (2014). Sugarcane as a bioenergy source: history, performance, and perspectives for second-generation bioethanol. Bioenergy Research, 7(1), 24-35.

5. Tang Liangdong. (2024). Green and low-carbon development of sugarcane sugar industry in Guangxi: current situation, challenges and strategies. Sugarcane Sugar Industry, 53(3): 74-79.

6. Dong Guangrui, Shi Jiaxian, Hou Ailing, Zhang Jisen. (2018). Research progress on sugarcane genome. Biotechnology, 28(3): 296.

7.International Human Genome Sequencing Consortium. (2004). Finishing the euchromatic sequence of the human genome. Nature, volume 431, 931–945.

8. Jisen Zhang, Qing Zhang, Leiting Li, Haibao Tang, et al. (2018) Recent polyploidization events in three Saccharum founding species. Plant Biotechnol J, 17(1), 264-274