Dr. Huazhong Shi
Title: Associate Professor
Education: Ph.D., Wuhan University, China, 1995; Research Associate, University of Arizona, 1999-2001, University of California, 2001-2003, Purdue University, 2003-2004
Research Area: Biochemistry
Office: Chemistry 413-A
Principal Research Interests
- Gene regulation in response to environmental stresses in plants
- Molecular mechanisms of plant salt tolerance
- Sulfonation of small molecules and plant stress response
Gene regulation in plant stress response
Fine regulation of gene expression is required for normal growth, development and adaptation to environmental stress conditions in plants. Many genes are repressed at normal growth conditions while activated by stresses. In order to identify regulator proteins mediating gene repression and activation, the Shi lab established a forward genetic screening for mutations affecting gene expression in response to stress conditions. The forward genetic approach utilized a stress-inducible promoter fused with the firefly luciferase reporter gene and a highly sensitive CCD camera to detect bioluminescence generated in small plant seedlings expressing the luciferase gene (Figure 1). By using this high throughput mutant screening system, the Shi lab has identified a number of mutants, designated shiny (shi in short) mutants, showing elevated expression of luciferase gene in response to abiotic stresses. Eight SHI genes have been cloned using map-based cloning. Two of the SHI genes encode proteins forming a complex to regulate stress-inducible gene expression through modulating RNA polymerase II CTD phosphorylation. Another three SHI proteins are mRNA splicing factors that are presumably components of repressor complex modulating stress-inducible gene transcription. Two of the eight SHI genes encode proteins functioning in vesicle trafficking. The Shi lab is employing all available means to study the functions of these SHI genes in order to gain deep understanding of stress-inducible gene repression and activation. In addition, the Shi lab initiated a forward genetic screening for mutations altering heat stress responsive gene expression. Positional cloning of the mutant genes is currently underway.
Na+ transport and plant salt tolerance
Plants possess three cellular mechanisms to reduce Na+ toxicity, i.e. restricted Na+ entry, Na+ exclusion, and Na+ compartmentation into the vacuole. These three cellular mechanisms are executed by three membrane transporters named AtHKT1, SOS1 and AtNHX1 in Arabidopsis (Figure 2). Through a genetic screening for suppressors of Na+ hypersensitive mutant sos1, the Shi lab identified loss-of-function mutations in AtHKT1 and gain-of-function mutations in AtNHX1 that can suppress the salt sensitivity of sos1 mutant. Single, double and triple mutants with mutations in these three important salt tolerance determinants have been created. The functions of these transporters and their coordination in Na+ uptake, long-distance transport, and redistribution in plants have been studied. Structure-function analysis of the dominant gain-of-function mutations in AtNHX1 and molecular design of superactive AtNHX1 transporters based on such analysis will be our future focus. The ultimate goal of this project is to create salt tolerant crops capable of growing in marginal lands with high salinity to maximize land usage and to secure world food supply.
Sulfonation of small molecules and its function in plant stress response
Sulfonation of small molecules is an enzymatic process in living organisms catalyzed by sulfotransferases to transfer a sulfonate group from the universal donor 3’-phosphoadenosine 5’-phosphosulfate (PAPS) to the hydroxyl group of various molecules (Figure 3). Sulfonation changes the physiochemical properties and the biological activity of molecules, thus influencing the physiology of organisms. In human, sulfonation of small molecules plays important roles in detoxification of toxic compounds and modulation of steroid hormones. In Arabidopsis, 18 cytosolic sulfotransferases (SOTs) have been identified based on sequence similarity. The Shi lab carried out a genetic and biochemical analysis of the SOT12 and found that SOT12 can sulfonate the plant hormone salicylic acid (Figure 3) and xenobiotic compounds. The role of SOT12 in both biotic and abiotic stress response and tolerance is being studied. The Shi lab is also interested in the functions of other SOTs, in particular, their roles in stress response.