HORMONE AND STRESS-REGULATED GENE EXPRESSION IN PLANTS
We are interested in the mechanisms regulating the developmental transition from embryogenesis to seed germination, a process governed by two phytohormones, gibberellins (GA) and abscisic acid (ABA). In addition, since plant embryos are very resistant to environmental stresses such as drought and cold, we also study the role of stress-induced proteins in mediating stress tolerance in plants.
Hormone signaling and regulation of gene expression in aleurone cells of cereal grains
Fig 1. Cover of Plant Cell (January issue 2003) highlighting the use of cereal aleurone system in investigating GA/ABA regulation.
In this project we study the effect of GA and ABA on the expression of genes encoding alpha-amylases, proteases and nucleases in germinating cereal grains. We follow both biochemical and genetic approaches to investigate the structure and function of these genes and their protein products. At least three interacting signaling pathways are involved in the GA/ABA actions and the antagonism between them (Fig 1). We are particularly interested in the action of three regulatory proteins, a repressor SLENDER (SLR/SLN), a transcription factor GAMYB, and a protein kinase PKABA1. Both gain-of-function (i.e., over-expression) and loss-of-function (i.e., RNAi and dominant negative mutants) approaches have been followed in order to ascertain the role of any regulator molecules in the interacting signaling pathways. Two new research directions have been initiated to identify new signaling intermediates. First, we have been screening the rice T-DNA insertion mutant population (TRIM) for both knockout and activation tagged mutants with altered hormone sensitivities. Second, libraries of small molecules have been screened for those capable affecting GA/ABA actions. Several hormone modulators have already been isolated, and some of them also affect seed germination in both rice and Arabidopsis. Furthermore, Arabidopsis mutants resisting to these chemical have been isolated. Cloning and characterization of genes encoding proteins interacting with these hormone modulators are currently underway.
Regulation and function of stress-induced genes
We are particularly interested in the function of drought, salinity and cold stress induced proteins in cereals and Arabidopsis. One of these proteins, HVA1, contains long stretches of amphipathic alpha-helical structure, and its over- expression in transgenic plants leads to elevated levels of stress tolerance (Fig 2). Another stress/ABA-induced protein, HVA22, has apparent homologs in many diverse eukaryotes, but not in any prokaryotes. Investigating the role of the HVA22 homolog in yeast suggest that this protein be involved in vesicular transport. Studies of HVA22 in Arabidopsis reveal that this protein likely functions as an autophagy suppressor preventing premature self-destruction under mild stress conditions. Observations in cereal aleurone cells also show that HVA22 suppresses the GA-induced central digestive vacuole formation, an important aspect of programmed cell death. Following a gene expression profiling approach by microarray analysis, we have identified several root specific stress-induced proteins with unique structural and functional features. The biochemical mode of action of these stress proteins is currently under investigation.
Fig 2. Interacting signaling pathways regulating seed germination and stress tolerance in cereal aleurone cells.
Biotechnology in enhancing stress tolerance
In addition to basic research, we are also interested in developing biotechnology applications. Two patents on the use of HVA1 and stress/ABA inducible promoters to generate stress tolerant plants have been granted. We have also been developing tissue/cell type specific and ABA/stress induced promoters as molecular switches in transgenic plants. Efforts are underway to establish collaborations with industrial interests in further developing this technology. We expect to secure intellectual properties for these novel promoters, and then collaborate with other researchers in utilizing them to drive the expression of beneficial genes in specific cell types under proper conditions.
Gomez-Cadenas, A., Zentella, R., Walker-Simmons, M. K., and Ho, T.-H. D. 2001. Gibberellin/Abscisic Acid Antagonism in Barley Aleurone Cells. Site of action of the protein kinase PKABA1 in relation to gibberellin signaling molecules. Plant Cell 13: 667-79.
Lu, C.-A., Ho, T.-H. D., Ho, S.-L., and Yu, S.-M. 2002. Three Novel MYB Proteins with One DNA Binding Repeat Mediate Sugar and Hormone Regulation of alpha-Amylase Gene Expression. Plant Cell 14: 1963-1980.
Zentella, R., Yamauchi, D., and Ho, T.-H. D. 2002. Molecular Dissection of the Gibberellin/Abscisic Acid Signaling Pathways by Transiently Expressed RNA Interference in Barley Aleurone Cells. Plant Cell 14: 2289-2301.
Chen, C.-N., Chu, C.-C., Zentella, R., Pan, S.-M., and Ho, T.-H. D. 2002. At HVA22 gene family in Arabidopsis: Phylogenetic relationship, ABA and stress regulation and tissue-specific expression. Plant Mol. Biol. 49: 631-642.
Brands, A., and Ho, T.-H. D. 2002. Function of a Plant Stress-Induced Gene, HVA22. Synthetic Enhancement Screen with Its Yeast Homolog Reveals Its Role in Vesicular Traffic1. Plant Physiol. 130: 1121-1131.
Casaretto, J., and Ho, T.-H. D. 2003. The Transcription Factors HvABI5 and HvVP1 Are Required for the Abscisic Acid Induction of Gene Expression in Barley Aleurone Cells. Plant Cell 15: 271-284.
Ho, T.-H. D., and Wu, R. 2004. Genetic engineering for enhancing plant productivity and stress tolerance. In: Nguyen, H., and Blum, A. (Eds.). Integration of plant physiology and biotechnology for plant breeding. p. 489-502.
Shen, J. Q., Casaretto, J. A., Zheng, P., and Ho, T.-H. D. 2004. Functional definition of ABA response complexes: the promoter untis necessay and sufficient for ABA induction of gene expression in barley (Hordeum vulgare L). Plant Mol. Biol. 54: 111-124.
Casaretto, J. A., and Ho, T.-H. D. 2005. Transcriptional regulation by abscisic acid in barley (Hordeum vulgare L.) seeds involves autoregulation of the transcription factor HvABI5. Plant Mol. Biol. 57: 21-34.
Yu, S.-M., Ko, S.-S., Hong, C.-Y., Sun, H.-J, Hsing, Y.-I. and Ho, T.-H.D. 2007. Global functional analyses of rice promoters by genomics approaches. Plant Mol. Biol. 65: 417-425.
