2013 Annual Report
1a.Objectives (from AD-416):
A research group led by the ADODR at ARS and collaborators from Embrapa (Brazil), JIRCAS (Japan), IRRI (Phillipines) and Moi University (Kenya) have come together to work on maize adaptation to acid soils that limit maize yields on acid soils that are widespread throughout the world and particularly in sub-Saharan Africa. The group will work to identify genes that confer tolerance to the abiotic stresses that occur on acid soils, particularly aluminum toxicity and phosphorous deficiency, and this molecular information will be used within a molecular breeding program to improve maize for agriculture on acid soils.
1b.Approach (from AD-416):
The research team is using a multidisciplinary approach to identify maize genes that confer tolerance to aluminum (A1) toxicity and phosphorous (P) deficiency that limit maize yields on acid soils. Molecular, genomic, genetic and physiological approaches will be used to identify the genes and associated physiological mechanisms for tolerance to A1 toxicity and P deficiency. The specific tolerance traits to be studied will be the ability for roots to grow in A1 toxic soils, the ability to modify the whole root architecture to acquire P from the acid soil, and the ability to maintain higher yields on low P and/or A1 toxic soils. A comparative genomics approach will be used to look for maize genes that are similar in sequence and function to sorghum A1 tolerance and rice P efficiency (tolerance to P deficiency) genes that our group already has discovered. Once these A1 tolerance and P efficiency genes in maize have been identified and verified, we will identify molecular markers that are diagnostic for the best versions of these genes for use in maize molecular breeding programs for acid soil tolerance in Africa.
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.