Phenotypic, genetic and molecular characterization of a maize low phytic acid mutant (lpa241)

Abstract

Phytic acid, myo-inositol 1,2,3,4,5,6-hexakisphosphate, is the major storage compound of phosphorous (P) in plants, predominantly accumulating in seeds (up to 4-5% of dry weight) and pollen. In cereals, phytic acid is deposited in embryo and aleurone grain tissues as a mixed "phytate" salt of potassium and magnesium, although phytates contain other mineral cations such as iron and zinc. During germination, phytates are broken down by the action of phytases, releasing their P, minerals and myo-inositol which become available to the growing seedling. Phytic acid represents an anti-nutritional factor for animals, and isolation of maize low phytic acid ( lpa) mutants provides a novel approach to study its biochemical pathway and to tackle the nutritional problems associated with it. Following chemical mutagenesis of pollen, we have isolated a viable recessive mutant named lpa 241 showing about 90% reduction of phytic acid and about a tenfold increase in seed-free phosphate content. Although germination rate was decreased by about 30% compared to wild-type, developement of mutant plants was apparentely unaffected. The results of the genetic, biochemical and molecular characterization experiments carried out by SSR mapping, MDD-HPLC and RT-PCR are consistent with a mutation affecting the MIPS1S gene, coding for the first enzyme of the phytic acid biosynthetic pathway.

A paramutation phenomenon is involved in the genetics of maize low phytic acid1-241 (lpa1-241) trait

“…As independently found for both lpa1-1 and lpa1-241 alleles, mips1S gene expression was reduced in developing seeds (Pilu et al, 2003;Shukla et al, 2004), but in both alleles no molecular lesions in the gene coding region were found (Shukla et al, 2004;Pilu et al, 2005).…”

“…They produce seeds with reduced phytic acid content and a proportionally higher level of free phosphate, whereas the total amount of seed P is not significantly altered (Raboy et al, 2000, Pilu et al, 2003Shi et al, 2005). The high inorganic phosphate phenotype (HIP) associated with the lpa1 phenotype can be quickly determined using Chen's assay (Chen et al, 1956), so lpa mutant seeds can easily be screened.…”

“…Although biochemical, mapping and gene expression data suggest the mips1s gene as a candidate for lpa1 mutation in maize (Raboy et al, 2000;Pilu et al, 2003;Shukla et al, 2004;Pilu et al, 2005), recent transposon mutagenesis experiments found that a novel gene designated ZmMRP4 (accession number EF86878), coding a multidrug resistance-associated-protein (MRP) mapping near the mips1s sequence, is the actual responsible gene for lpa1 mutation (Shi et al, 2007). MRP proteins represent a subfamily of ATP-binding cassette transmembrane transporters widespread in all eukaryotes, which in plants are involved in several functions, such as xenobiotic detoxification, organic ions transport, oxidative stress tolerance (Swarbreck et al, 2003) and even transpiration control (Klein et al, 2006).…”

“…The lpa1-241 mutant was originally isolated from the M 2 progeny of chemically (ethyl methane sulphonate) mutagenized populations (Pilu et al, 2003). Plants heterozygous for lpa1-241 in the ACR inbred line were used as donors in crosses with the B73 inbred line for five generations and selfed.…”

Nutritionally Enhanced Staple Food Crops

“…The total seed phosphorus in these mutants was unaffected, while reduction in phytic acid resulted in corresponding increases in inorganic P in the seed. However, lp mutants, in general, had reduced germination and seedling development and yielded low compared to WT (Meis et al 2003;Pilu et al 2003;Oltmans et al 2005;Shi et al 2007), with some exception in barley (Bregitzer and Raboy 2006), soybean (Yuan et al 2007), and common bean (Campion et al 2009). Badone et al (2010) isolated a low phytic acid mutant in maize, lp1-241, with a reduction of up to 90% of phytic acid and strong pleiotropic effect on the whole plant including higher level of anthocyanins as compared to wild type either in the embryo ($3.8-fold) or in the aleurone layer ($0.3-fold).…”