2013 Annual Report
1a.Objectives (from AD-416):
A multidisciplinary team involving USDA, Embrapa, JIRCAS, IRRI and Moi University will work on the identification and characterization of genes associated with maize P efficiency (tolerance to low P). The objectives for this research project include:
1. Pup-1 candidate gene identification in maize
2. QTL/gene mapping for P use efficiency in maize
3. Inheritance studies on maize root architecture under high and low P
4. Validation of maize Pup-1 candidate genes and if necessary, novel P efficiency QTL (if maize Pup1 homologues are not functional in P efficiency)
1b.Approach (from AD-416):
A multidisciplinary team involving USDA, Embrapa, JIRCAS, IRRI and Moi University will work on the identification and characterization of genes associated with maize P efficiency (tolerance to low P). Bioinformatics will be used to identify homologues of the rice Pup-1 (P uptake efficiency) gene in maize and a set of markers for these genes will be developed. An Embrapa inbred line panel that was developed for breeding for P efficiency be phenotyped in the field for this trait measured as grain yield under contrasting P conditions and in the greenhouse/lab for root architecture traits; also the Buckler association panel will be phenotyped for P efficiency and root architecture traits in the green house/lab at USDA-ARS. Finally, a maize RIL population from the cross of a highly P efficient tropical maize line (L3) with a P inefficient line (L22) will be phenotyped to identify QTLs for P acquisition, internal P efficiency and root architecture traits. Also, candidate genes for p efficiency will be mapped on the same RIL population, in order to verify the co-segregation of Pup-1 homologs with QTLs for different P-efficiency traits. Finally, association analysis using Embrapa’s elite inbred lines panel and the Buckler maize association panel will be carried out to validate candidate genes.
In FY 2013, we built upon work by our collaborators where they used a bioinformatics-based search of maize homologs of the rice using OsPSTOL1 as the query. This gene is responsible for the rice P efficiency (which is the ability to maintain yields under low P soil conditions) QTL, Pup1 (P uptake 1). This analysis identified four predicted genes in the maize genome sharing more than 65% of sequence identity with OsPSTOL1, which are our initial candidate genes for analysis. These genes are located on chromosomes 3, 4 and 8. Genetic markers (SNPs or single nucleotide polymorphisms) diagnostic for these 4 genes between the parents of a maize genetic mapping population were identified and converted to a type of genetic marker that was used to map their physical position on the maize genome. Subsequently, working with collaborators in Embrapa (Brazil), unique maize lines called testcross hybrids were evaluated in the field under low and high P over two growing seasons for traits related to P efficiency. The traits evaluated were: grain yield; anther silk interval; plant height; ear height; and phenotypic indices for P accumulation and partitioning. These datasets are currently being analyzed. Also, a genetic map was made for our maize genetic mapping population using different genetic markers including the DNA sequence of ZmPSTOL1 candidate genes as markers. This different maize mapping population was also evaluated in the field under low and high P for the same P efficiency related traits described above. QTLs for phosphorus use efficiency (PUE), phosphorus acquisition efficiency (PAE) and phosphorus internal utilization efficiency (PUE) were identified from the field phenotyping. The same maize mapping population was also used to quantify the root growth of the entire root systems on each plant using our RootReader2D platform. The maize seedlings were grown in paper pouches with low P nutrient solution, digital photography was used to capture 2D digital images of each root system, and root system traits were analyzed using RootReader 2D software. QTLs for root traits, total plant dry weight, root:shoot ratio, P content and P utilization efficiency evaluated in nutrient solution under low P were also mapped in this population. One region of the maize genome looks very intriguing with regards to maize P efficiency genes. This region is on chromosome 8, flanked by the maize candidate genes ZmPSTOL1 and Zm1PSTOL1. In this region, QTLs were identified for volume of fine roots, total root dry weight and P acquisition efficiency. The co-localization of QTLs for these traits with the ZmPSTOL1 gene suggest that this gene may be involved with the development of fine roots that positively affects plant growth and P acquisition efficiency under low P availability in nutrient solution.