Seed‐specific overexpression of phytoene synthase: increase in carotenoids and other metabolic effects

Abstract

SummaryA bacterial phytoene synthase (crtB) gene was overexpressed in a seed-speci®c manner and the protein product targeted to the plastid in Brassica napus (canola). The resultant embryos from these transgenic plants were visibly orange and the mature seed contained up to a 50-fold increase in carotenoids. The predominant carotenoids accumulating in the seeds of the transgenic plants were alpha and beta-carotene. Other precursors such as phytoene were also detected. Lutein, the predominant carotenoid in control seeds, was not substantially increased in the transgenics. The total amount of carotenoids in these seeds is now equivalent to or greater than those seen in the mesocarp of oil palm. Other metabolites in the isoprenoid pathway were examined in these seeds. Sterol levels remained essentially the same, while tocopherol levels decreased signi®cantly as compared to non-transgenic controls. Chlorophyll levels were also reduced in developing transgenic seed. Additionally, the fatty acyl composition was altered with the transgenic seeds having a relatively higher percentage of the 18 : 1 (oleic acid) component and a decreased percentage of the 18 : 2 (linoleic acid) and 18 : 3 (linolenic acid) components. This dramatic increase in¯ux through the carotenoid pathway and the other metabolic effects are discussed.

Carotenoids in nature: insights from plants and beyond

“…Carotenoid accumulation has been achieved in oil seeds of canola (Brassica napus) through the overexpression of PSY, which resulted in 43-50-fold increase in total seed carotenoid content (Shewmaker et al 1999;Lindgren et al 2003). 'Golden rice 2' is a high b-carotene accumulating line overexpressing PSY from maize as well as bacterial CrtI (from Erwinia uredovora) and accumulates 23-fold more carotenoids (37 mg g -1 ) than was the case for the initial 'golden rice', which overexpressed PSY from daffodil (Beyer et al 2002;Paine et al 2005).…”

Biofortified crops to alleviate micronutrient malnutrition

“…Reconstitution of the pathway in other crops depends on the metabolic bottleneck, as illustrated by canola, tomato and potato [33][34][35][36][37]. The Grand Challenges in Global Health initiative of the Bill & Melinda Gates Foundation is funding biofortification projects on banana, cassava, and sorghum for Africa.…”

Golden Rice – five years on the road – five years to go?

“…Similarly, there seems to be no shortage of GGPP as a precursor, which might enable further increases in provitamin A content, if required. Thus, the situation seems somewhat similar to the one found with transgenic canola, which relied on the overexpression of a bacterial PSY to increase carotenoid content markedly [9]. However, the overall biochemistry might not be comparable because the rice grain is not an oil-rich seed and the need for additional desaturase activity has been demonstrated in rice.…”