Dynamics of inositol phosphate pools (tris-, tetrakis- and pentakisphosphate) in relation to the rate of phytate synthesis during seed development in common bean (Phaseolus vulgaris)

Genetic Control of Seed Phytate Accumulation and the Development of Low-Phytate Crops: A Review and Perspective

Breeding low phytic acid (lpa) crops is a strategy that has potential to both improve the nutritional quality of food and feed and contribute to the sustainability of agriculture. Here, we review the lipid-independent and -dependent pathways of phytate synthesis and their regulatory mechanisms in plants. We compare the genetic variation of the phytate concentration and distribution in seeds between dicot and monocot species as well as the associated temporal and spatial expression patterns of the genes involved in phytate synthesis and transport. Quantitative trait loci or significant single nucleotide polymorphisms for the seed phytate concentration have been identified in different plant species by linkage and association mapping, and some genes have been cloned from lpa mutants. We summarize the effects of various lpa mutations on important agronomic traits in crop plants and propose SULTR3;3 and SULTR3;4 as optimal target genes for lpa crop breeding.

Protein, Phytate and Minerals in Grains of Commercial Cowpea Genotypes

The objective of this study was to investigate and characterize cowpea (Vigna unguiculata) genotypes for total grain protein content, storage protein fractions (globulin, albumin, prolamin, basic and acid glutelins), and phytate and minerals contents. Eighteen cowpea genotypes were selected. Total grain protein content varied from 21.4% to 29.2%, for BRS Marataoã and Paulistinha genotypes, respectively. The variation in the concentration of each protein fraction was significant (P<0.05) only for glutelins (basic and acid). The genotypes studied exhibited great similarity in the PAGE electrophoretic profile of the grain protein fractions and also in the mineral content. BRS Paraguaçu genotype exhibited higher Zn content than thegenotypes that have been previously recommended for this characteristic. The lowest phytate grain content was observed in four of the 18 genotypes studied, which also exhibited high protein contents. Although the results did not converge to the selection of a few genotypes, some specific differences were detected that which may be further explored. Considering total grain protein, mineral and phytate contents, the genotype Paulistinha revealed a better balance unveiling high grain total protein content, low grain phytate content and more homogeneous mineral composition.

Phytic Acid and Transporters: What Can We Learn from low phytic acid Mutants

Phytic acid has two main roles in plant tissues: Storage of phosphorus and regulation of different cellular processes. From a nutritional point of view, it is considered an antinutritional compound because, being a cation chelator, its presence reduces mineral bioavailability from the diet. In recent decades, the development of low phytic acid (lpa) mutants has been an important goal for nutritional seed quality improvement, mainly in cereals and legumes. Different lpa mutations affect phytic acid biosynthetic genes. However, other lpa mutations isolated so far, affect genes coding for three classes of transporters: A specific group of ABCC type vacuolar transporters, putative sulfate transporters, and phosphate transporters. In the present review, we summarize advances in the characterization of these transporters in cereals and legumes. Particularly, we describe genes, proteins, and mutants for these different transporters, and we report data of in silico analysis aimed at identifying the putative orthologs in some other cereal and legume species. Finally, we comment on the advantage of using such types of mutants for crop biofortification and on their possible utility to unravel links between phosphorus and sulfur metabolism (phosphate and sulfate homeostasis crosstalk).

Phytic acid transport in Phaseolus vulgaris: A new low phytic acid mutant in the PvMRP1 gene and study of the PvMRPs promoters in two different plant systems

Phytic acid (InsP₆) is the main storage form of phosphate in seeds. In the plant it plays an important role in response to environmental stress and hormonal changes. InsP₆ is a strong chelator of cations, reducing the bioavailability of essential minerals in the diet. Only a common bean low phytic acid (lpa1) mutant, affected in the PvMRP1 gene, coding for a putative tonoplastic phytic acid transporter, was described so far. This mutant is devoid of negative pleiotropic effects normally characterising lpa mutants. With the aim of isolating new common bean lpa mutants, an ethyl methane sulfonate mutagenized population was screened, resulting in the identification of an additional lpa1 allele. Other putative lpa lines were also isolated. The PvMRP2 gene is probably able to complement the phenotype of mutants affected in the PvMRP1 gene in tissues other than the seed. Only the PvMRP1 gene is expressed at appreciable levels in cotyledons. Arabidopsis thaliana and Medicago truncatula transgenic plants harbouring 1.5 kb portions of the intergenic 5' sequences of both PvMRP genes, fused upstream of the GUS reporter, were generated. GUS activity in different organs suggests a refined, species-specific mechanisms of regulation of gene expression for these two PvMRP genes.

Characterization and molecular modeling of Inositol 1,3,4 tris phosphate 5/6 kinase-2 from Glycine max (L) Merr.: comprehending its evolutionary conservancy at functional level

Soybean genome encodes a family of four inositol 1,3,4 trisphosphate 5/6 kinases which belong to the ATP-GRASP group of proteins. Inositol 1,3,4 trisphosphate kinase-2 (GmItpk2), catalyzing the ATP-dependent phosphorylation of Inositol 1,3,4 trisphosphate (IP3) to Inositol 1,3,4,5 tetra phosphate or Inositol 1,3,4,6 tetra phosphate, is a key enzyme diverting the flux of inositol phosphate pool towards phytate biosynthesis. Although considerable research on characterizing genes involved in phytate biosynthesis is accomplished at genomic and transcript level, characterization of the proteins is yet to be explored. In the present study, we report the isolation and expression of single copy Itpk2 (948 bp) from Glycine max cv Pusa-16 predicted to encode 315 amino acid protein with an isoelectric point of 5.9. Sequence analysis revealed that GmITPK2 shared highest similarity (80%) with Phaseolus vulgaris. The predicted 3D model confirmed 12 α helices and 14 β barrel sheets with ATP-binding site close to β sheet present towards the C-terminus of the protein molecule. Spatio-temporal transcript profiling signified GmItpk2 to be seed specific, with higher transcript levels in the early stage of seed development. The present study using various molecular and bio-computational tools could, therefore, help in improving our understanding of this key enzyme and prove to be a potential target towards generating low phytate trait in nutritionally rich crop like soybean.

Seed Biofortification and Phytic Acid Reduction: A Conflict of Interest for the Plant?

Most of the phosphorus in seeds is accumulated in the form of phytic acid (myo-inositol-1,2,3,4,5,6-hexakisphosphate, InsP₆). This molecule is a strong chelator of cations important for nutrition, such as iron, zinc, magnesium, and calcium. For this reason, InsP₆ is considered an antinutritional factor. In recent years, efforts to biofortify seeds through the generation of low phytic acid (lpa) mutants have been noteworthy. Moreover, genes involved in the biosynthesis and accumulation of this molecule have been isolated and characterized in different species. Beyond its role in phosphorus storage, phytic acid is a very important signaling molecule involved in different regulatory processes during plant development and responses to different stimuli. Consequently, many lpa mutants show different negative pleitotropic effects. The strength of these pleiotropic effects depends on the specific mutated gene, possible functional redundancy, the nature of the mutation, and the spatio-temporal expression of the gene. Breeding programs or transgenic approaches aimed at development of new lpa mutants must take into consideration these different aspects in order to maximize the utility of these mutants.

A defective ABC transporter of the MRP family, responsible for the bean lpa1 mutation, affects the regulation of the phytic acid pathway, reduces seed myo-inositol and alters ABA sensitivity

• We previously identified the lpa1 (low phytic acid) 280-10 line that carries a mutation conferring a 90% reduction in phytic acid (InsP(6) ) content. In contrast to other lpa mutants, lpa1(280-10) does not display negative pleiotropic effects. In the present paper, we have identified the mutated gene and analysed its impact on the phytic acid pathway. • Here, we mapped the lpa1(280-10) mutation by bulk analysis on a segregating F(2) population, an then, by comparison with the soybean genome, we identified and sequenced a candidate gene. The InsP(6) pathway was analysed by gene expression and quantification of metabolites. • The mutated Pvmrp1(280-10) cosegregates with the lpa1(280-10) mutation, and the expression level of several genes of the InsP(6) pathway are reduced in the lpa1(280-10) mutant as well as the inositol and raffinosaccharide content. PvMrp2, a very similar paralogue of PvMrp1 was also mapped and sequenced. • The lpa1 mutation in beans is likely the result of a defective Mrp1 gene (orthologous to the lpa genes AtMRP5 and ZmMRP4), while its Mrp2 paralog is not able to complement the mutant phenotype in the seed. This mutation appears to down-regulate the InsP(6) pathway at the transcriptional level, as well as altering inositol-related metabolism and affecting ABA sensitivity.