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Research Project: Characterizing Circadian Regulatory Networks in Grain Crops to Establish their Role in Development and Abiotic Responses

Location: Plant Gene Expression Center

2013 Annual Report

The circadian clock in crop plants controls important performance traits including growth, timing of flowering and stress and pathogen responses by coordinating daily and seasonal changes in physiology. The long term goal of this project is to define at the genetic and molecular levels the circadian system in grain crops, including corn, and its impact on agronomic traits. This project will use genomic, genetic, and molecular methods to identify and characterize the circadian system in corn, utilizing resources and tools in corn and other model plant systems as appropriate. The circadian system genes identified will provide gene targets for enhancing crop performance and adaptation to global climate change. The objectives of the project are: Objective 1: Identify and characterize genes required for circadian rhythms in grain crops. Objective 2: Identify and characterize genes required for circadian clock-regulated developmental processes in grain crops. Objective 3: Analyze the contribution of the circadian system to drought responses in grain crops.

The genes required for circadian rhythms in maize remain uncharacterized. The goal of Objective 1 is to identify and/or construct mutants in candidate genes to define the genes that participate in the core circadian oscillator. Subsequent analysis of mutants will establish the function of their gene products to understand the molecular nature of the maize circadian oscillator. The hypotheses to be tested are: Mutations in circadian clock genes will alter circadian clock-driven transcription; Additional mutant alleles in clock genes can be identified and constructed using publicly available germplasm collections; and, Regional mutagenesis with the Ds transposon will create additional gi2 knockout alleles. Work in model plants demonstrates that the circadian system is deeply imbedded in regulatory networks that control growth and developmental processes. Whether such a regulatory system exists in maize remains an open question. The goal of Objective 2 is to investigate whether circadian regulation is an important contributor to maize growth and development by studying circadian clock mutants. The hypotheses to be tested are: Maize circadian clock genes are involved in regulation of maize flowering time; gi functions within the genetic networks known to control maize flowering time; The gi and tocl1 genes underlie known flowering time QTL; gi activity contributes to the timing of the juvenile to adult transition; and, Maize clock genes participate in regulation of growth. Specific core circadian oscillator genes play important roles in the responses of model plants to drought stress, in part through regulation of phytohormone signaling. The goal of Objective 3 is test whether drought stress and phytohormone responses in maize depend on the activity of circadian clock genes. The hypotheses to be tested are: The tocl1 gene is involved in maize drought responses; and, The tocl1 gene contributes to ABA responses in maize.

Progress Report
This report documents progress for Project Number 5335-21000-039-00D, which started in March 2013 and continues research from Project Number 5335-21000-026-00D, entitled "Genomic and Genetic Analysis of the Cereal Circadian System and Crop Production." Since inception progress has been made for Objectives 1 and 2. Crosses to produce double mutants have been made and the progeny of the crosses are currently growing in the field. The first pollinations for outcrosses between the existing gi1 mutant line and the B73 inbred have been made and the progeny are growing in the field. Preliminary experiments have been done to establish the parameters, both experimentally and phenotypically, for analysis of mutant seedling analysis. The plants needed to bulk the mutant and inbred material for Objectives 1-3 are currently growing in the field.

1. The role of the paralogous gigantea1 (gi1) and gi2 genes in the maize circadian clock and development. The gi1 and gi2 genes are highly similar and may serve overlapping functions in the maize circadian system. The double gi1 gi2 mutant is being constructed from the single mutants to test for redundancy in function. Individual gi1 and gi2 mutant plants were crossed to make the population needed to screen for lines with both mutations. Not only will the double mutant line provide insight into the relative contribution of each gene to circadian rhythms and development, it will also enable testing of the effect a complete loss of gi function has on the maize circadian clock.

2. Seedling development in maize. What role does the circadian clock play in seedling establishment? In model systems, circadian clock regulation of light and hormone signaling is critical to the growth trajectory of seedlings. Whether the maize circadian clock similarly controls seedling development is an open question. Morphometric traits of normal and gi mutant seedlings were measured over a range of lighting conditions. These findings will be used to establish protocols for testing whether normal maize seedling development requires circadian clock function.