Research
I. Mechanisms of Sugar Signaling and Regulation in Plants
In plants, sugars not only serve as metabolic resources and structural constituents of cells but also have hormone-like regulatory activities. Sugars modulate nearly all fundamental processes throughout the entire life cycle of plants. Sugar signal crosstalks or interacts with hormone, environmental stimulus and metabolic signals through an interconnected network. The identity of essential components in the sugar signaling pathway, the mechanism underlying sugar regulated transcription, and how they link to the complex signaling network are largely unknown.
In cereals, £\-amylases are essential enzymes for hydrolysis of seed storage starch for providing non-photosynthetic carbon sources during germination and seedling development. During germination, expression of £\-amylase genes is induced by hormone gibberellin (GA) and sugar depletion and repressed by hormone abscisic acid (ABA) and sugar, which serving as a model system for studying mechanisms of sugar signaling/regulation and sugar-hormone cross signaling in plants. Our current research foci are as follows:
1. Components in the sugar signaling pathway
Our recent studies indicated that, in addition to hexokianses (HXKs) and sucrose non-fermenting factor 1 (SNF1) having similar functions in both rice and yeast, other factors, e.g., the calcineurin B-like (CBL) protein interacting protein kinase (CIPK) and MYBS transcription factors, were also involved in sugar signaling in rice. By screening the T-DNA tagged rice mutant library, more than two dozens of signaling and transcription factors that potentially could be involved in sugar signaling were also identified. These regulatory factors are being characterized.
2. Mechanisms of sugar and GA cross signaling
Our recent studies indicated that expression of £\-amylase genes during cereal grain germination and seedling growth is repressed by sugars in embryos and activated by GA in endosperms through the sugar response complex (SRC) and GA response complex (GARC) of £\-amylase promoters, respectively. We also found that the interaction between transcription factor MYBGA and the GA response element (GARE) in GARC interferes sugar repression of SRC. This mechanism prevents the sugar feedback repression of £\-amylases in endosperms and ensures high amounts of sugars are produced during vigorous seedling growth. The detailed mechanisms of sugar and GA cross signaling are being investigated.
II. Rice Functional Genomics
1. Generation a T-DNA tagged rice mutant library
T-DNA insertional mutagenesis is an effective approach for high throughput rice functional genomics analysis. By using a multifunctional T-DNA containing enhancers next to the left border and a promoterless GUS gene next to the right border, we have generated a T-DNA tagged rice mutant library containing 60,000 gene trap and gene activation/knockout mutant lines. Over 20,000 flanking sequence tags (FSTs) have been obtained and opened to the public through a website( http://trim.sinica.edu.tw/). T-DNA was preferentially ( ~80 %) integrated into the genic region of rice genome. The rice mutant library and EST database offer promising features as valuable resources for rice gene functional analyses using forward and reverse genetic approaches.
2. Screening of rice mutants
Several mutants with altered responses to GA, sugars, salt, drought, low temperature and diseases and phenotypes of important agronomic traits have been identified. Many tissue specific and stress inducible genes/promoters have also been identified through the GUS staining screening. Some of these rice mutants are being characterized.
III. Plant Molecular Farming
1. Production of industrial anzymes and pharmaceuticals
Several plant based protein expression systems for production of industrial enzymes (e.g., amylopullulanase and phytase) and pharmaceutical proteins (e.g., vaccine and monoclonal antibody) have been established.
2. Bioconversion of lignocellulosic biomass to ethanol
Similar plant molecular farming platform technologies are also applied for bioconversion of lignocellulosic agriculture bioproducts into ethanol. Screening of rice composts for microbes with high cellulolytic activities and screening of rice mutant library for mutants with altered cell wall structures are carried out in order to generate ideal ethanol source crops capable of in planta enzyme digestion.
