Accelerating breeding of pearl millet (Pennisetum glaucum): a focus on grain quality traits

Pearl millet is a crop with the potential to meet real world challenges and achieve sustainable agricultural systems. Its high amount of macro and micronutrients, low vulnerability to pathogens, tolerance to drought and salinity, and short life cycle, make this crop ideal as staple crop for growing populations (Basavaraj, 2010-J SAT Agric Res 8:1). However, the nutritional quality of pearl millet is hampered by the high content of anti-nutrient molecules such as phytic acid (PA) and C-glucosylflavones (CGFs), which limit the bio-availability of minerals and cause goitrogenic effects, respectively. PA is the major source of phosphorous in seeds, but it forms mineral complexes with Fe, Zn, Ca, K and Mg on the gut, causing problems such as poor mineral bioavailability and eutrophication of waterways (Bouwman et al, 2013-PNAS 52:20882). The development of low PA (lpa) mutants of cereals crops and legumes could help to alleviate this situations. Nonetheless, lpa plants often show poor seed germination that compromise the crop yield, decreasing its agronomic value making them undesirable for farmers. (Redekar et al, 2015-BMC Gen 16:1074). With the aim to produce association studies by integrating phenotypic and GBS data, we evaluated the natural genetic variability of the levels of PA in two pearl millet populations. A total of 153 inbred lines derived from African landraces, has been screened to quantify the content of PA in flour extracts and the most contrasting lines (reduced and high PA - RPA and HPA respectively; and reduced and high CGFs - RCGF and HCGF) have been identified and further analyzed. These inbred lines were used for the evaluation of the expression profiles of genes involved in PA biosynthesis during the seed development. A total of 11 orthologous genes of the PA and one of the CGFs biosynthesis pathways were identified in the pearl millet genome and their expression was investigated through different seed developmental stages. For the expression analysis three stages of seed development were established: early, middle and late development. Total RNA was isolated and gene expression was assayed by Real Time PCR. Actually, a major MIPS (myo-inositol phosphate synthase - Pgl_GLEAN10001337) increase of expression was detected during the middle developmental stage on the HPA phenotype. A similar peak of expression was previously reported for other species as common bean, rice and soybean. Since this expression peak was not detected on the RPA inbred line, we hypothesize that the diminished MIPS expression level may be correlated with the decrease of PA content. This outcome is very interesting for our work, since MIPS catalyze the rate-limiting step in the synthesis of all inositol-containing compounds (Rasmussen et al, 2010-Biochem Soc T 38:689). For the CGFs biosynthesis, a slightly CGT (C-Glucosyl Transferase - Pgl_GLEAN10014648) increase of expression was detected on early seed development and its contribution for the observed differences in CGFs content is under evaluation. Our results contribute to increase the understanding of the glycosyl flavones biosynthesis pathway which is not yet fully clarified. This work is supported by CERES-NewPearl project, jointly funded by Fondazione Cariplo and Agropolis Fondation.