Embryo-specific silencing of a transporter reduces phytic acid content of maize and soybean seeds

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Abstract

Phytic acid in cereal grains and oilseeds is poorly digested by monogastric animals and negatively affects animal nutrition and the environment. However, breeding programs involving mutants with less phytic acid and more inorganic phosphate (Pi) have been frustrated by undesirable agronomic characteristics associated with the phytic acid-reducing mutations. We show that maize lpa1 mutants are defective in a multidrug resistance-associated protein (MRP) ATP-binding cassette (ABC) transporter that is expressed most highly in embryos, but also in immature endosperm, germinating seed and vegetative tissues. Silencing expression of this transporter in an embryo-specific manner produced low-phytic-acid, high-Pi transgenic maize seeds that germinate normally and do not show any significant reduction in seed dry weight. This dominant transgenic approach obviates the need for incorporating recessive lpa1 mutations to create maize hybrids with reduced phytic acid. Suppressing the homologous soybean MRP gene also generated low-phytic-acid seed, suggesting that the strategy might be feasible for many crops.

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Figure 1: Seed inorganic phosphate, phytic acid and myo-inositol in the lpa1-1 allele and lpa1-mum1 knockout mutant.
Figure 2: Map-based cloning and genomic structure of the lpa1 gene.
Figure 3: Expression of the lpa1 gene.
Figure 4: Phytic acid and inorganic phosphate contents and seed dry weight of T1 transgenic seeds in which MRP4 expression is silenced.
Figure 5: Phytic acid reduction and inorganic phosphate increase in T1 transgenic seeds of soybean lines with silenced MRP expression.

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Acknowledgements

The lpa1-1 mutant was kindly provided by V. Raboy (USDA-ARS, National Small Grain Germplasm Research Facility, Aberdeen, ID). BAC clone sequencing and assembly was conducted by Pioneer Crop Genetics Research and Development sequencing laboratory. We thank Jan Hazebroek, Jessica Holland, Ryan Wally and Lisa Hagen for technical assistance, Russ Booth and Cheryl Caster for soybean transformation, and Doug Nubel, Marjorie Rudert, Shifu Zhen and Andrew Waber for assistance in the field and the greenhouse. We are grateful to Mitchell Tarczynski, Rudolf Jung and Bill Hitz for discussions.

Author information

J.S. conceived and directed the project, wrote the manuscript and identified the lpa1-mum1 mutant; H.W. performed molecular characterization of MRP4; K.S. analyzed the maize transgenic lines with supervision by J.S.; B.L. identified the TAP marker; J.M.S. constructed the soybean transformation vector and analyzed the soybean transformants; J.P.R. generated the maize transgenic lines; K.G. constructed the maize transformation vectors; H.W., J.S., R.B.M. and D.S.E. identified the lpa1 Mu insertion alleles; J.S., B.L., M.F., H.W. and K.S. performed fine mapping and cloning.

Correspondence to Jinrui Shi.

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