3. Results
3.1. Selection of specific β-carotene ketolase and hydroxylase for
efficient synthesis of astaxanthin
Tomato genome harbors two BHY genes, which are involved in
zeaxanthin synthesis of leaf chloroplasts (BHY1) and of flower
chromoplast (BHY2), respectively (Galpaz et al., 2006). Therefore,
to trigger tomato to accumulate astaxanthin in fruit, besides
a suitable BKT, an additional BHY is also required. We used a
β-carotene-producing E. coli system to screen well-cooperating
BKT and BHY for astaxanthin formation. The C. reinhardtii
BKT (CrBKT) was selected because it is superior to other sources
of the enzymes for astaxanthin biosynthesis in plants (Huang
et al., 2012; Zhong et al., 2011). Several BHYs from various
organisms were co-expressed with CrBKT in β-caroteneproducing
E. coli. The BHY from H. pluvialis (HpBHY) was found
to be the best one coordinating with CrBKT for astaxanthin
formation in terms of astaxanthin conversion rate and proportion
(Fig. 2). Therefore HpBHY was selected to reinforce carotenoid
hydroxylase activity in tomato.