NON-TECHNICAL SUMMARY: This project will focus on determining the role of cytokinin synthesis and metabolism in the regulation of maize kernel development in stress and non-stress conditions.

OBJECTIVES: To understand the mechanisms that regulate photosynthate partitioning into paths for biosynthesis and use of sucrose, starch, and sugar alcohols.

APPROACH: Temperature shift experiments, transgenic maize with altered gnees for cytokinin synthesis will be used to alter kernel development.

KEYWORDS: gene expression; plant genetics; photosynthesis; metabolic regulation; heat stress; cytokinins; oxidases; enzyme activity; kernels; seed development; corn; stress tolerance; photosynthate; partitioning; metabolic pathways; alcohols; sucrose; starches; carbohydrate metabolism; biosynthesis; transgenic plants

PROGRESS: 2006/01 TO 2006/12
A second study on the effects of high temperature during endosperm cell division on protein accumulation was published in Crop Science xx:2006. In the first study, we (Manjardino, Smith and Jones, Crop Science 45: 2005) confirmed that a 2- or 4-day heat stress (35 degrees C) caused a 20 to 48% reduction in total protein content. Specifically, zein content was reduced by an average of 53%, but zein composition was only mildly affected where the concentration of glutelins and albumins plus globulins were negatively affected. The second study focused on the mechanisms by which heart stress affect early zein accumulation. This study showed that both the 27 kD and cluster 1 zeins of the sub-family 4 (ZSF4C1) zein mRNA steady-state levels are significantly delayed by 4 days of high temperature and the transcription rate of both zein proteins were reduced. Cytokinin oxidase (Ckx) is the principal enzyme involved in cytokinin catabolism. The core of research continues to focus on characterization of the developmental regulation and temporal and tissue specific expression of cytokinin level and Ckx during maize kernel development. We continue to use molecular tools to characterize and to manipulate cytokinin levels in planta. Transgenic plants over expressing the bacterial form of the cytokinin synthesis gene (isopentyl transferase-ipt) have been developed and moved into a homogenous background. Gene dose experiments continue to support the finding that only one dose of the ipt gene is sufficient to result in the highest level of ipt expression compared to non-transgenic control and that increased gene dose has no additive affect suggesting control at the transcriptional and/or translational levels. The ipt gene expression experiments were complete, but results are confounded by genomic DNA. A genomic DNA digestion step has been added and primers designed at intron/exon junctions to distinguish PCR products resulting from cDNA and residual genomic dna in order to optimize gene expression studies.

IMPACT: 2006/01 TO 2006/12
Our data continues to suggest that regulation of Ckx expression or overexpression of cytokinin synthesis genes in specific kernel component tissues may be a viable molecular approach to increasing endogenous cytokinin levels during kernel development and thus to stabilizing grain yield of maize against the periodic occurrence of heat stress or more long-term global climate change.

PUBLICATIONS (not previously reported): 2006/01 TO 2006/12
1. Monjardino, P., Smith A.G., and Jones, R.J. 2006. Zein transcription and endoreduplication in maize endosperm are differentially affected by heat stress. Crop Sci. 46:2581-2589.
2. Liu, A., Malzer, G.L., Rehm, G.W., and Jones, R.J. 2006. Degradation of 14C-Zinc ammonium acetate in soils as influenced by soil type, soil sterilization and carriers. J. Plant Nutr. 29:1003-1019.
3. Liu, A., Malzer, G.L., Rehm, G.W., and Jones, R.J. 2006. Fate of ammonium acetate in soils and its uptake by corn. J. Plant Nutr. 29:797-708.

PROJECT CONTACT:
Name: Jones, R. J.
Phone: 612-625-1267
Fax: 612-625-1268
Email: jones012@umn.edu