1.1 Wide application of astaxanthin in the beauty industry

Astaxanthin, a powerful antioxidant with remarkable skin care effects, can neutralize free radicals, slow down skin aging, improve skin elasticity and brightness, and also has anti-inflammatory and moisturizing properties. It is widely used in the beauty industry. As people pay more attention to anti-aging and skin health, more and more consumers tend to choose skin care products with natural ingredients. Astaxanthin is favored for its safety and effectiveness. As the younger generation also begins to attach importance to preventive skin care and pursue healthier skin color and texture, it further promotes the growth of market demand for astaxanthin.

In this context, new special-purpose cosmetics with astaxanthin as a functional ingredient have become a research topic in the international biochemistry field, and their application prospects are very broad. The research and development of new cosmetics in China is also changing rapidly, and the market competition is fierce. Natural and environmentally friendly cosmetic additives have become the main research direction today.

However, under the huge market demand, the production and extraction of natural astaxanthin are facing bottlenecks. Only a few large companies have the ability to produce on a large scale, and the high price of astaxanthin makes it difficult for the general public to enjoy the technical benefits of astaxanthin.

1.2 Under the huge demand gap for astaxanthin, people currently have solutions

(I) Chemical Synthesis

Astaxanthin is the end product of carotenoid synthesis, and its artificial chemical synthesis is relatively difficult, often resulting in predominantly cis-isomers. So far, only a few companies have used chemical synthesis to produce astaxanthin, but the content of astaxanthin in their products is low, and the production cost is high.

(II) Natural Extraction

1. Extraction from aquatic product processing waste

The typical method involves crushing shrimp and crab shells, followed by acid hydrolysis, and finally extraction with organic solvents such as acetone and petroleum ether. This method can achieve a yield as high as 1531ug/g. However, the calcareous components in shrimp and crab shells pose difficulties in astaxanthin extraction, and the purity of the extract needs to be further improved.

2. Extraction of astaxanthin from cultured algae

Many algae can produce astaxanthin. Taking Haematococcus pluvialis as an example, its astaxanthin content can reach 0.2% to 2.0%. However, the cultivation cycle is long, and it requires light and cell wall disruption, which is not conducive to large-scale production.

3. Extraction of astaxanthin from yeast

Currently, foreign countries mainly use Xanthophyllomyces dendrorhous as the strain for fermentation to produce astaxanthin. It does not require light and can utilize various sugars as carbon sources for rapid heterotrophic metabolism, with a short cultivation time. It has potential applications in feed additives and other fields. These advantages make Xanthophyllomyces dendrorhous a research focus. However, the wild-type strain has a relatively low astaxanthin content of only 200-300mg/kg. Significant efforts are still needed in breeding high-yielding strains, developing inexpensive culture media, and optimizing cultivation processes.

Despite the development of various methods for obtaining astaxanthin, there are still bottlenecks in the production of natural astaxanthin. Only a few large companies have achieved large-scale production of astaxanthin, resulting in technological monopolies. The mass production of astaxanthin still has a long way to go. To enable the general public to enjoy the benefits of synthetic biology, we attempt to propose our unique solution.

2.1 Focusing on Longan as the Key

Longan, a significant evergreen economic fruit tree in China, boasts a long history of cultivation, diverse varieties, and widespread distribution, along with its rich nutritional value. Traditionally, astaxanthin products have primarily targeted the Western market, limiting their consumer base. However, longan, a common fruit in Asian countries, offers a unique opportunity to bridge this gap by integrating astaxanthin into the beauty and food industries of the Asian market, thereby catering to the needs of Asian consumers in a more natural and relatable manner.

Furthermore, the genome sequence of longan was successfully sequenced in 2017, marking the first time that the complete genome database of this Sapindaceae plant has been internationally published. This achievement, coupled with years of intensive biological research, has established a relatively mature research framework. By harnessing the diverse tissues of longan, including its flesh, seeds, shells, and leaves, which are rich in secondary metabolites, particularly carotenoids, we can envision the potential for synthesizing astaxanthin within this fruit. This approach not only leverages the unique advantages of longan but also opens up new avenues for the production of astaxanthin, addressing the significant demand gap in a creative and sustainable manner.

2.2 Construction of a High-Expression System for Astaxanthin in Longan

Previous studies have demonstrated that the β-carotene ketolase (BKT) gene from the microalgae Chlamydomonas reinhardtii exhibits high catalytic activity in converting zeaxanthin to astaxanthin, achieving an astaxanthin content of up to 85% within the carotenoid pool. Co-expression of the BKT gene from Chlamydomonas reinhardtii and the β-carotene hydroxylase (BKT) gene from Haematococcus pluvialis in tomato leaves has resulted in significant accumulation of free astaxanthin.

Drawing inspiration from these findings, we employed seamless cloning technology to fuse the BKT gene extracted from Chlamydomonas reinhardtii with the BKT gene from Haematococcus pluvialis, and subsequently ligated this chimeric gene into a linearized pCAMBIA1301+CaMV35S+NOS vector, resulting in the construction of pCAMBIA1301-BHY-BKT. This construct was then introduced into longan callus tissue to validate its expression and its ability to promote astaxanthin synthesis within the fruit. Our results confirmed the feasibility of establishing a high-expression system for astaxanthin in longan, thereby paving the way for enhanced production of this valuable compound in this economically important fruit.

In summary, we have successfully established a high-expression system for astaxanthin in longan, addressing the current production challenges of this valuable compound. Our findings demonstrate the feasibility of synthesizing astaxanthin in woody plants such as longan. Going forward, we will continue to delve deeper into this system to further optimize and facilitate its industrialization.