Difference between revisions of "Team:SCAU-China/Description"

Astaxanthin (3, 3′-dihydroxy-β-carotene-4, 4′-dione) is a natural keto-carotenoids which is found in some microalgae, shrimps and salmons (Figure 1). This compound is not soluble in water, but in most of organic solvents like pyridine, ethanol and benzene. In fact, it is synthesized only in some specific species of algae, bacteria and yeasts. Animals can not synthesize astaxanthin. Astaxanthin is responsible for the pigmentation of many marine animals which acquire the red pigment from their diets. In some organisms, astaxanthin takes on a colour of brown or blue as it forms into some types of pigment-protein complexes. For humans, astaxanthin is a powerful antioxidant with broad health implications. Thus, astaxanthin has been claimed a good commercial prospect for its value in medical and health care.

Figure 1: Astaxanthin wildly exists in microalgae Haematococcus pluvialis, shrimps and salmons (from left to right), but only some microalgae can synthesize astaxanthin.

Currently, the industrial ways to produce astaxanthin are extracted from microalgae Haematococcus pluvialis, Phaffia yeast, shrimp processing waste and chemical product.However these ways aren’t safety enough and the purification is difficult. While higher plants are supposed to be an efficient and safe bioreactor to produce astaxanthin, because it has advanced protein processing system to produce complex products, such as senior terpenoids. In nature, there are many precious products were terpenoid, such as carotenoids, microbial A, paclitaxel, etc. But many complex products could not be synthesized by animal.

Although higher plants such as Zea mays are capable to synthesize zeaxanthin, which is the metabolic precursor of astaxanthin, due to their lack of β-carotene ketolase, astaxanthin still cannot be synthesized in these higher plants.

Figure 2:The biosynthesis pathway of astaxanthin formation in transgenic rice endosperm. The dotted arrows indicate pathway limitations in rice endosperm. The solid arrows indicate the existence of carotenogenic reactions. The red arrows indicate the reactions catalaysed by four exogenous transgenes Psy, CrtI, BHY and BKT.

The biosynthesis of astaxanthin from pyruvate needs more than ten enzymes. However, based on the study of Golden Rice, we deduced its biosynthesis in rice endosperm require four key enzymes (Figure 2). The PSY (phytoene synthase) catalyzes Geranylgeranyl-PP into Phytoene. The CrtI gene from Erwinia uredovora encodes phytone desaturase that could complete the catalysis process from Phytoene to Lycopene. Moreover, the β-LCY gene, encoding β-lycopene cyclases, is expression and active in rice endosperm. Therefore, when the two genes (PSY and CrtI) are drived by endosperm-specific promoters in rice, the β-Carotene (pro-vitamin A) was synthesized to produce the famous Golden Rice. From β-Carotene to astaxanthin, there are still two steps: BHY (β-carotenehy droxylase) catalyze β-Carotene to Zeaxanthin, and BKT (β-carotene ketolase) catalyzes Zeaxanthin directly to synthesize the end product astaxanthin. Because the expression of the endogenous rice gene BHY is very low, the biosynthesis of astaxanthin in rice endosperm requires at least 4 genes (PSY+CrtI+BKT+BHY, BBPC). For the combination of three genes (PSY+CrtI+BKT，BPC) maybe produce little astaxanthin (even nothing!). Therefore, we utilized a multigene vector system to assemble and transform these four genes into rice to study the metabolic synthesis of astaxanthin in endosperm.

According to the advantages listing below, we take rice (Oryza sativa) endosperm as the bioreactor for astaxanthin production.

● Rice is a low-cost, high-productivity, high-safety and commonly used model plant

● Rice is easy to plant on a large scale and has very high yield

● Rice seed is an excellent biomass container

● Genetic modification technology is quite mature in rice

● As a special nutrition storage organs, rice seed is convenient to store, extract and purify

● Astaxanthin accumulation at seed would not in terrupt the growth of the whole plant