Proc Natl Acad Sci U S A. 2016 Sep 27;113(39):11040-5. doi: 10.1073/pnas.1605689113.

Elevated vitamin E content improves all-trans β-carotene accumulation and stability in biofortified sorghum.

Ping Chea, Zuo-Yu Zhaoa,1, Kimberly Glassmana, David Doldea, Tiger X. Hua, Todd J. Jonesa, Silas Obukosiab, Florence Wambugub, and Marc C. Albertsena,1

aDuPont Pioneer, Johnston, IA 50131; and bAfrica Harvest Biotech Foundation International, Nairobi 00621, Kenya



Micronutrient deficiencies are common in locales where people must rely upon sorghum as their staple diet. Sorghum grain is seriously deficient in provitamin A (β-carotene) and in the bioavailability of iron and zinc. Biofortification is a process to improve crops for one or more micronutrient deficiencies. We have developed sorghum with increased β-carotene accumulation that will alleviate vitamin A deficiency among people who rely on sorghum as their dietary staple. However, subsequent β-carotene instability during grain storage negatively affects the full utilization of this essential micronutrient. We determined that oxidation is the main factor causing β-carotene degradation under ambient conditions. We further demonstrated that coexpression of homogentisate geranylgeranyl transferase (HGGT), stacked with carotenoid biosynthesis genes, can mitigate β-carotene oxidative degradation, resulting in increased β-carotene accumulation and stability. A kinetic study of β-carotene degradation showed that the half-life of β-carotene is extended from less than 4 weeks to 10 weeks on average with HGGT coexpression.




Additional Information to Biomedical Advances

As described in the PNAS publication reported by Che et. al., we have developed biofortified sorghum with stabilized all-trans β-carotene accumulation through co-expression of PSY1, CRTI, DXS and HGGT in sorghum. To identify the lead events towards the application of this nutritionally-enhanced sorghum in food production, the agronomic performance of an enhanced and stabilized β-carotene line (ABS203) was studied under confined field conditions during a single summer nursery in Johnston, Iowa. The germination rate and yield of 13 ABS203 homozygous events with their corresponding nulls and wild-type (WT) control were tested. For each event, two reps of 2-row plots were randomly distributed in the field. Twenty seeds were sowed in 4 m rows, with 40 seeds per plot. Seed germination data were collected four weeks after sowing. Sorghum plant phenotypes were recorded during plant development. For each row, seeds were harvested in 0.9 m sections in the middle of the row to avoid variation caused by row edge effect. One to five sections were harvested in each plot. The total threshed seed weight collected from each 0.9 m section was recorded.

The experiment was analyzed in a two-way treatment structure (event x segregation) with wild-type checks. The data were analyzed in a two-step process, enabling 1) comparison of one combination with any other combination, and 2) comparison of the level of an effect against any other level or the WT control. Significant differences were deemed when the probability of the difference was less than 0.05.

As shown in Figures 1 and 2, no significant correlation between the all-trans β-carotene level and germination rate (Figure 1) or between the all-trans β-carotene level and yield (Figure 2) were observed (R2<0.5 as indicated in the figures). In other words, from the data generated in this experiment, there is no yield or germination rate penalties caused by the enhanced all-trans β-carotene level. We conclude that the yield and germination differences of those 13 events with wild-type are event dependent and most likely due to the random insertion of the transgenes. We chose three events with no abnormal phenotypes and no yield or germination rate penalties to represent our top candidates from these 13 events (Figure 3). These events represent a step-change in the quantity and stability of beta-carotene in sorghum. Although we believe they could be used to alleviate vitamin A deficiency in people who consume sorghum as the dietary staple, results from these experiments have laid the groundwork for even greater accumulation and stability of beta-carotene in the future.

(The authors gratefully acknowledge Kristen Rinehart for field management and Mark Hinds for statistical analysis)


Figure 1. The correlation between seed germination rate vs. all-trans β-carotene content for thirteen ABS203 homozygous sorghum plants.

Figure 2. The correlation between grain yield vs. all-trans β-carotene content for thirteen ABS203 homozygous sorghum plants.

Figure 3. The grain yield and all-trans β-carotene content of three leading homozygous ABS203 events compared to the corresponding nulls and WT.