SummaryVitamin A deficiency remains one of the world's major public health problems despite food fortification and supplements strategies. Biofortification of staple crops with enhanced levels of pro‐vitamin A (PVA) offers a sustainable alternative strategy to both food fortification and supplementation. As a proof of concept, PVA‐biofortified transgenic Cavendish bananas were generated and field trialed in Australia with the aim of achieving a target level of 20 μg/g of dry weight (dw) β‐carotene equivalent (β‐CE) in the fruit. Expression of a Fe'i banana‐derived phytoene synthase 2a (MtPsy2a) gene resulted in the generation of lines with PVA levels exceeding the target level with one line reaching 55 μg/g dw β‐CE . Expression of the maize phytoene synthase 1 (ZmPsy1) gene, used to develop ‘Golden Rice 2’, also resulted in increased fruit PVA levels although many lines displayed undesirable phenotypes. Constitutive expression of either transgene with the maize polyubiquitin promoter increased PVA accumulation from the earliest stage of fruit development. In contrast, PVA accumulation was restricted to the late stages of fruit development when either the banana 1‐aminocyclopropane‐1‐carboxylate oxidase or the expansin 1 promoters were used to drive the same transgenes. Wild‐type plants with the longest fruit development time had also the highest fruit PVA concentrations. The results from this study suggest that early activation of the rate‐limiting enzyme in the carotenoid biosynthetic pathway and extended fruit maturation time are essential factors to achieve optimal PVA concentrations in banana fruit.

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Banana Xanthomonas wilt (BXW), caused by Xanthomonas campestris pv. musacearum, is one of the most important diseases of banana (Musa sp.) and currently considered as the biggest threat to banana production in Great Lakes region of East and Central Africa. The pathogen is highly contagious and its spread has endangered the livelihood of millions of farmers who rely on banana for food and income. The development of disease resistant banana cultivars remains a high priority since farmers are reluctant to employ labor-intensive disease control measures and there is no host plant resistance among banana cultivars. In this study, we demonstrate that BXW can be efficiently controlled using transgenic technology. Transgenic bananas expressing the plant ferredoxin-like protein (Pflp) gene under the regulation of the constitutive CaMV35S promoter were generated using embryogenic cell suspensions of banana. These transgenic lines were characterized by molecular analysis. After challenge with X. campestris pv. musacearum transgenic lines showed high resistance. About 67% of transgenic lines evaluated were completely resistant to BXW. These transgenic lines did not show any disease symptoms after artificial inoculation of in vitro plants under laboratory conditions as well as potted plants in the screen-house, whereas non-transgenic control plants showed severe symptoms resulting in complete wilting. This study confirms that expression of the Pflp gene in banana results in enhanced resistance to BXW. This transgenic technology can provide a timely solution to the BXW pandemic.

Transgenic banana ( Musa acuminata ‘Gros Michel’) integrating either of two rice chitinase genes was generated and its resistance to Black Leaf Streak disease caused by the fungus Mycosphaerella fijiensis was tested using a leaf disk bioassay. PCR screening indicated the presence of the hpt selectable marker gene in more than 90 % of the lines tested, whereas more than three quarters of the lines contained the linked rice chitinase gene resulting in a co-transformation frequency of at least 71.4 %. Further, a unique stable integration of the transgenes in each line revealed some false negative PCR results and the expected co-transformation frequency of 100 %. The transgene insert number per line ranged from 1 to 5 and single transgene insert lines (25 % of all) were identified. Considerable delay in disease development (up to 63 days post-incoculation) over a monitoring period of 108 days occurred in nine lines with extracellularly targeted chitinase out of 17 transgenic lines tested and their necrotic leaf area decreased by 73–94 % compared to the untransformed susceptible control line. Finally, correlation between symptom development and rice chitinase expression was confirmed in two lines by Western analysis. The potential of rice chitinase genes to enhance resistance against M. fijiensis in banana was demonstrated as well as the usefulness of the leaf disk bioassay for early disease screening in transgenic banana lines. Electronic supplementary material The online version of this article (doi:10.1007/s11248-012-9631-1) contains supplementary material, which is available to authorized users.

Banana is a staple crop in many regions where vitamin A deficiency is prevalent, making it a target for provitamin A biofortification. However, matrix effects may limit provitamin A bioavailability from bananas. The retinol bioefficacies of unripe and ripe bananas (study 1A), unripe high-provitamin A bananas (study 1B), and raw and cooked bananas (study 2) were determined in retinol-depleted Mongolian gerbils (n = 97/study) using positive and negative controls. After feeding a retinol-deficient diet for 6 and 4 wk in studies 1 and 2, respectively, customized diets containing 60, 30, or 15% banana were fed for 17 and 13 d, respectively. In study 1A, the hepatic retinol of the 60% ripe Cavendish group (0.52 ± 0.13 μmol retinol/liver) differed from baseline (0.65 ± 0.15 μmol retinol/liver) and was higher than the negative control group (0.39 ± 0.16 μmol retinol/liver; P < 0.0065). In study 1B, no groups differed from baseline (0.65 ± 0.15 μmol retinol/liver; P = 0.20). In study 2, the 60% raw Butobe group (0.68 ± 0.17 μmol retinol/liver) differed from the 60% cooked Butobe group (0.87 ± 0.24 μmol retinol/liver); neither group differed from baseline (0.80 ± 0.27 μmol retinol/liver; P < 0.0001). Total liver retinol was higher in the groups fed cooked bananas than in those fed raw (P = 0.0027). Body weights did not differ even though gerbils ate more green, ripe, and raw bananas than cooked, suggesting a greater indigestible component. In conclusion, thermal processing, but not ripening, improves the retinol bioefficacy of bananas. Food matrix modification affects carotenoid bioavailability from provitamin A biofortification targets.