The Steber Laboratory is part of the Wheat Health, Genetics, and Quality Research Unit, United States Department of Agriculture, Agricultural Research Service, and is housed in and affiliated with the Department of Crop and Soil Sciences, at Washington State University in Pullman, Washington. The lab is also affiliated with the Molecular Plant Sciences Program at Washington State University. The principal investigator is Camille Steber, and the lab was started in 1998.
The Steber Lab examines the molecular mechanisms controling seed development, dormancy, and germination in plants. The practical goal is to reduce problems with low Falling Numbers in wheat grain. The Hagberg-Perten falling numbers method is used by the wheat industry to detect elevated grain alpha-amylase. Farmers world-wide receive steep discounts for wheat with low Falling Numbers/high alpha-amylase because it is associated with poor end-product quality such as cakes that fall and sticky bread or noodles. There are two main causes of low falling numbers: 1) preharvest sprouting, the initiation of mature grain (caryopsis) germination on the mother plant when cool moist conditions occur before harvest or 2) late maturity alpha-amylase (LMA), the inappropriate expression of alpha-amylase during grain maturation in response to cold or high temperature stress. In both cases genetic susceptibility leads to the problem in response to environmental triggers. An understanding of the genetics controlling plant responses to these environmental triggers is critical to research progress.
The best way to prevent problems with low Falling Numbers (FN) in farmers' fields is to improve genectic resistance to the underlying causes of preharvest sprouting and LMA. This is being done by: 1) characterizing the FN of released varieties grown by Washington State University variety trials to allow farmers, seedsmen, and breeders to make informed decisions; 2) collaborating to develop alternatives to FN testing that are faster, more accurate, and capable of better standardization; 3) identifying genetic sources of preharvest sprouting tolerance through mapping in existing released varieties, and mapping and cloning of genes that increase grain dormancy through increased ABA hormone sensitivity; 4) identifying genetics sources of LMA resistance in Pacific Northwest and US germplasm; 5) developing molecular markers to speed breeding for preharvest sprouting and LMA tolerance; 6) investigating how the plant hormones ABA and GA control grain germination and the expression of alpha-amylase.
Seed dormancy and germination are controlled by the balance between the abscisic acid (ABA) and gibberellin (GA) hormone signalling pathways. ABA induces seed dormancy during embryo maturation, whereas GA stimulates seed germination and is associated with dormancy-breaking treatments such as cold stratification and dry after-ripening. Moreover, GA stimulates and ABA inhibits the expression of alpha-amylase in wheat and barley grain. The roles of ABA and GA in LMA will also be investigated. Research has focused on understanding the mechanisms of GA signaling, and the relative roles of ABA and GA in the control of seed dormancy, dormancy loss, and germination. In addition to stimulating seed germination, GA also stimulates stem elongation, the transition to flowering, and fertility. Because GA stimulates seedling and stem elongation, GA synthesis and sensitivity are also important in determining wheat plant height as well as seedling emergence and stand establishment. The GA-insensitive Reduced height (Rht) mutants of wheat are semi-dominant, semi-dwarf mutants widely used to prevent plants from falling over and lodging.
Preharvest sprouting is associated with lack of grain dormancy and sensitivity to the plant hormone abscisic acid (ABA) in wheat. ABA stimulates dormancy as well as adaptive responses to drought, cold and salt stress. Our research found that higher PHS tolerance is associated with higher ABA sensitivity and decreased GA sensitivity. Previous work identified wheat mutants exhibiting ABA hypersensitive and insensive phenotypes during wheat germination. A mutation in the MKK3 gene was identified as a strong candidate for the ERA8 (enhanced response to ABA8) mutation in wheat.
Past research from the lab has made contributions to improving wheat drought tolerance, to identifying wheat variants with increased resistance to the herbicide glyphosate, to improving wheat Rhizoctonia root rot tolerance, and developed a floral dip method for wheat transformation.
Dr. Steber's lab uses Arabidopsis as a model system to study gibberellin (GA) hormone signaling and the mechanisms by which GA stimulates seed germination. This research included the cloning of the SLY1 gene and foundational research defining the mechanisms of GA hormone signaling. GA stimulation of seed germination requires the down-regulation of negative regulators of seed germination called the DELLA domain proteins (the Rht genes in wheat). DELLAs are down-regulated both by protein destruction via the ubquitin-proteasome pathway and by direct interaction with the GA receptors. DELLA proteolysis occurs in response to GA biosynthesis and requires three functionally redundant GA receptors, GIBBERELLIN INSENSITIVE DWARF1 (GID1a, GID1b and GID1c), and the SLEEPY1 (SLY1) F-box subunit of an SCF E3 ubiquitin ligase. In sly1 mutants in which DELLA proteins remain stable after GA application, the DELLA repressor can also be down-regulated either by after-ripening or by overexpression of the GA receptor, GID1. This requires GA biosynthesis and the presence of a functional DELLA domain. Both the DELLA motif and GA are required for the protein interaction of DELLA protein and GID1. Ongoing research is aimed at further elucidating the non-proteolytic mechanisms for DELLA regulation and its functional role in seed dormancy loss and germination. Potential proteins involved in proteolysis-independent GA signaling have been identified by yeast two-hybrid screen.