Glutathione S-transferases (GSTs) play roles in both normal cellular metabolism as well as in the detoxification of a wide variety of xenobiotic compounds, and they have been intensively studied with regard to herbicide detoxification in plants. A newly discovered plant GST subclass has been implicated in numerous stress responses, including those arising from pathogen attack, oxidative stress, and heavy-metal toxicity. In addition, plant GSTs play a role in the cellular response to auxins and during the normal metabolism of plant secondary products like anthocyanins and cinnamic acid. This review presents the current knowledge about the functions of GSTs in regard to both herbicides and endogenous substrates. The catalytic mechanism of GST activity as well as the fate of glutathione S-conjugates are reviewed. Finally, a summary of what is known about the gene structure and regulation of plant GSTs is presented.

Glutathione S-transferases (GSTs) are enzymes that detoxify heterocyclic compounds (xenobiotics) by covalently linking glutathione to the substrate, forming a glutathione S-conjugate. A glutathione pump in the vacuolar membrane of barley actively sequesters herbicide-glutathione S-conjugates; glutathionation allows recognition and entry of the conjugates into vacuoles. The protein encoded by the Bronze-2 gene in maize performs the last genetically defined step in anthocyanin biosynthesis, resulting in the deposition of red and purple pigments in the vacuoles of maize tissues. We show here that Bz2 encodes a GST with activity in maize, transformed Arabidopsis thaliana plants and Escherichia coli. We demonstrate that anthocyanins extracted from maize protoplasts expressing BZ2 are conjugated with glutathione, and that vanadate, a known inhibitor of the glutathione pump in plant vacuolar membranes, inhibits the accumulation of anthocyanins in the vacuole. These results provide a biochemical function for BZ2, and suggest a common mechanism for the ability of plants to sequester structurally similar but functionally diverse molecules in the vacuole.

The Bronze2 (Bz2) gene in maize (Zea mays) encodes a glutathione S-transferase that performs the last genetically defined step in anthocyanin biosynthesis-tagging anthocyanin precursors with glutathione, allowing for recognition and entry of anthocyanins into the vacuole. Here we show that Bz2 gene expression is highly induced by heavy metals such as cadmium. Treatment of maize seedlings with cadmium results in a 20-fold increase i n 822 message accumulation and a 50-fold increase in the presence of the unspliced, intron-containing transcript. The increase in message levels during cadmium stress appears to result, at least in part, from activation of an alternative mRNA start site approximately 200 nucleotides upstream of the normal start site; this site is not used in unstressed or heat-stressed tissues. The effect of cadmium on the RNA splicing of Bz2 seems to be specific: splicing of other introncontaining maize genes, including a maize actin gene under the control of the cadmium-inducible 822 promoter, is unaffected by cadmium stress. Conversely, 822 intron splicing is not affected by other stress conditions that induce Bz2 gene expression, such as abscisic acid, auxin, or cold stress. Surprisingly, the increase in Bz2 mRNA during cadmium stress does not result in an increase in Bz2 glutathione Stransferase activity. We propose that an alternative protein may be encoded by 822 that has a role during responses t o heavy metals.Specific enzymatic and regulatory roles have been assigned for most genes of the anthocyanin pathway in maize (Zea mays) as a result of genetic and biochemical analysis. Bz2 is one of the last structural genes required for the production of anthocyanin pigmentation in maize (Coe et al., 1988;Holton and Cornish, 1995). In bz2 mutants cytoplasmic synthesized anthocyanin precursors are unable to reach their final destination in the vacuole. This inappropriate cytoplasmic accumulation of the precursor pigment cyanidin-3-glucoside causes tissues to develop a bronze color as a result of oxidation and condensation reactions that are poorly understood (Stafford, 1990). These oxidized secondary metabolites in the cytoplasm of bz2 mutants are

Just-in-Time Teaching (JiTT) is a teaching and learning approach that combines the best features of traditional in-class instruction with the communication and resource potential available via the Web. We describe here how JiTT can be used to teach biology to undergraduate and graduate level students, both science majors as well as nonscience majors. A key characteristic of JiTT is the creation of a feedback loop between the classroom and the Web using Internet "Warm Up" assignments that are due prior to class time. By examining student responses to Warm Up exercises before class, faculty members can determine the level of understanding, prior knowledge, and misconceptions that students bring to class. Classroom time can then be spent addressing these misconceptions while discussing course content. In class Cooperative Learning exercises reinforce course content in an informal group setting. Other features of JiTT, such as "What is Biology Good For," make clear the relevance of specific concepts in biology in society and increase student motivation. Assessment results have been positive, including decreased attrition rates, increases in student attitudes, interactivity, study habits, and cognitive gains in classrooms using JiTT.

We have isolated two genes from Zea mays encoding proteins of 82 and 81 kD that are highly homologous to the Drosophila 83-kD heat shock protein gene and have analyzed the structure and pattern of expression of these two genes during heat shock and development. Southern blot analysis and hybrid select translations indicate that the highly homologous hsp82 and hsp81 genes are members of a small multigene family composed of at least two and perhaps three or more gene family members. The deduced amino acid sequence of these proteins based on the nucleotide sequence of the coding regions shows 64-88% amino acid homology to other hsp90 family genes from human, yeast, Drosophila, and Arabidopsis. The promoter regions of both the hsp82 and hsp81 genes contain several heat shock elements (HSEs), which are putative binding sites for heat shock transcription factor (HSF) commonly found in the promoters of other heat shock genes. Gene-specific oligonucleotide probes were synthesized and used to examine the mRNA expression patterns of the hsp81 and hsp82 genes during heat shock, embryogenesis, and pollen development. The hsp81 gene is only mildly heat inducible in leaf tissue, but is strongly expressed in the absence of heat shock during the pre-meiotic and meiotic prophase stages of pollen development and in embryos, as well as in heat-shocked embryos and tassels. The hsp82 gene shows strong heat inducibility at heat-shock temperatures (37-42 degrees C) and in heat shocked embryos and tassels but is only weakly expressed in the absence of heat shock. Promoter-GUS reporter gene fusions made and analyzed by transient expression assays in Black Mexican Sweet (BMS) Maize protoplasts also indicate that the hsp82 and hsp81 are regulated differentially. The hsp82 promoter confers strong heat-inducible expression of the GUS reporter gene in heat-treated cells (60- to 80-fold over control levels), whereas the hsp81 promoter is only weakly heat inducible (5- to 10-fold over control levels).