CEPCEB Members

Daniel R. Gallie Professor Department of Biochemistry Boyce Hall 3432 University of California Riverside, California 92521-0129 Phone: (951) 827-7298 Fax: (951) 827-4434   Areas of Expertise Cellular and Viral Translational Regulatory Mechanisms Programmed Cell Death in Plants Role of Ethylene, Abscisic Acid, and Cytokinin during Maize Growth and Development and in Response to Abiotic Stress Role of Ascorbic Acid during Plant Growth and in Environmental Stress Response Programs

Background Research Interests Mechanisms Regulating the Translation of Cellular mRNAs Viral Translational Mechanisms  Analyzing the Impact of Heat Stress on Plant Gene Expression Programmed Cell Death in Plants The Role of Ethylene, Abscisic Acid, and Cytokinin in Regulating Plant Growth and Responses to Environmental Stress The Function of Ascorbic Acid in Regulating Plant Growth and Responses to Environmental Stress Current Lab Personnel Selected Publications (Bibliography page)

Background

I received my B.S. in Chemistry from the University of Michigan and my Ph.D. from the University of California at Davis. I studied plasmid inheritance in Agrobacterium tumefaciens under the direction of Dr. Clarence Kado. During postdoctoral studies at the John Innes Centre in Norwich, England, I investigated the translational regulatory role of the 5'-leader (called W) from tobacco mosaic virus that was one of the first descriptions of a translational enhancer. During additional postdoctoral studies with Dr. Virginia Walbot at Stanford University, I continued investigating the role of the 5' and 3'-untranslated regions of mRNAs in regulating translation.

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Research Interests

Mechanisms Regulating the Translation of Cellular mRNAs

Since the elucidation of the basic translation process in the 1950's and 1960's, translation was thought to occur in a linear fashion. Not until the late 1980's was genetic evidence from yeast studies presented suggesting that the poly(A) tail at the 3' terminus of messenger RNAs may be required for translation initiation which occurs at the 5' terminus. The model presented at that time invoked an interaction between the 60S ribosomal subunit and the protein that binds to the poly(A) tail, i.e., the poly(A) binding protein (PABP). Over the last decade, my laboratory has made substantial contributions to the understanding of how a messenger RNA is translated into protein using plant, animal, and yeast systems I demonstrated that the poly(A) tail was functionally dependent on the 5' cap structure, to enhance translation in plants, animals, and yeast. This suggested that PABP interacted with a protein bound to the 5' cap structure, not with the 60S ribosomal subunit. Our comparative approach has made it possible to demonstrate which aspects of translation in plants are unique and which are conserved with animals, yeast, or both.

My research has shown that an mRNA adopts a circular conformation during translation that is mediated by a physical and functional interaction between the ends of the mRNA. In typical mRNAs, this circularization is mediated by the complex, eukaryotic initiation factor (eIF) 4F bound at the 5’ cap structure and PABP bound to the 3’ poly(A) tail. Not only did my research show that the large subunit of eIF4F, i.e., eIF4G, is responsible for physically interacting with PABP, but a second initiation factor, eIF4B, also interacts with PABP, a finding that was subsequently confirmed in animal cells. One functional consequence of this interaction is to increase the binding affinity of PABP for the poly(A) tail. The interaction between PABP and factors bound to the 5' cap structure has been verified by other labs and is now the dominant model explaining the interaction between the termini of an mRNA.

Figure 1. Circularization of mRNAs During Translation

The interaction between the termini of the mRNA may act as a selection mechanism such that only full-length mRNAs are recruited for translation. The interaction may also be important to optimize the re-initiation of a ribosome onto the mRNA. This may also prove to be an optimization of a process that might have occurred prior to the evolution of poly(A) tails. Related to this, we have shown that the efficiency of translation initiation increases with the length of the 3' untranslated region, data suggesting that a longer 3' untranslated region may increase the likelihood that a terminating ribosome may be re-recruited at the 5' end of the message by maintaining the association of the ribosome with that particular mRNA for a longer period of time.

Since this important discovery, my research has focused in two areas: (1) mapping the functional domains within eIFs and PABP that are responsible for the assembly of what is referred to as the “surveillance complex” that includes eIF4F, eIF4B, and PABP and is involved in determining the competence of an mRNA for translation and (2) investigating the regulation that controls the assembly of the surveillance complex.

Figure 2. Interaction map of the initiation surveillance complex

My research has shown that plant eIF4B differs from animal eIF4B in that the former contains one additional RNA binding domain not reported for other eukaryotic homologs and that each domain requires dimerization for RNA binding activity. A functional interaction of eIF4B with eIFiso4G and eIF4A was shown for the first time to involve a physical interaction with conserved sequence elements adjacent to the RNA binding domains. The eIFiso4G-binding domain lies immediately C-proximal to the N-proximal RNA binding domain. Two binding sites for PABP and eIF4A were mapped to conserved domains on either side of the C-terminal RNA binding domain which represent two 41 amino acid repeats conserved among plant eIF4B proteins, and to a lesser extent, in human eIF4B. These results support the notion that eIF4B functions by organizing multiple components of the translation initiation machinery and RNA.

Figure 3. Interaction Domains for PABP, eIF4A, eIF3, RNA, and eIFiso4G in eIF4B. Comparison of the organization of the RNA and protein binding domains in wheat and human eIF4B.

PABP is differentially phosphorylated in plants and its phosphorylation state determines its affinity and type of binding to poly(A) RNA where phosphorylated PABP binds poly(A) RNA cooperatively but with a 10-fold lower affinity than hypophosphorylated PABP which binds non-cooperatively. The highest degree of cooperative binding is observed between phosphorylated and hypophosphorylated PABP, indicating that an interaction between PABP of differing phosphorylation states is favored. This suggests that the assembly of PABP molecules on a poly(A) tail is likely to be heterogeneous in its phosphorylation state. My research has shown that the phosphorylation state of PABP also determines the strength and specificity of its interaction with eIF4B, eIF4G, and eIFiso4G. eIF4G promotes cooperative RNA binding of hypophosphorylated PABP but not phosphorylated PABP, suggesting that eIF4G specifically interacts with hypophosphorylated PABP. In contrast, eIFiso4G promotes RNA binding of hypophosphorylated and phosphorylated PABP. eIF4B increases RNA binding of phosphorylated PABP to a greater extent than hypophosphorylated PABP. The interaction domains for eIF4G and eIFiso4G in plant PABP differ in number and location. Whereas the eIF4G and eIF4B interaction domains are located within the RRM1 of PABP, eIFiso4G interacts at two sites, i.e., one within RRM1-2 that overlaps the eIF4B binding site and a second within RRM3-4. Competition between eIFiso4G and eIF4B to bind PABP supported the notion that their binding sites in PABP overlap and that these factors are required to interact with separate molecules of PABP. The role of PABP phosphorylation in determining the specificity of its interaction with eIF4G, eIFiso4G, and eIF4B, in conjunction with the overlapping eIF4G, eIFiso4G, and eIF4B interaction domains in PABP, suggests that two molecules of PABP are required to interact with eIF4G and eIF4B (or eIFiso4G and eIF4B) simultaneously. Such a possibility would involve the interaction of four molecules (i.e., eIF4B, eIF4G, and two molecules of PABP). As PABP self-interacts during its cooperative binding to poly(A) RNA and eIF4B binds eIFiso4G, the interaction among four molecules may provide greater stability to the complex than would interactions among three molecules (i.e., eIF4B, eIF4G, and one molecule of PABP). Thus, the overlapping nature of the eIF4G, eIFiso4G, and eIF4B interaction domains in PABP may require eIF4B and eIF4G (or eIF4B and eIFiso4G) to interact with distinct molecules of PABP and thereby increase the stability of the interaction between the termini of an mRNA.

Figure 4. Comparison of the organization of the eIF4G, eIFiso4G, and eIF4B interaction domains in wheat PABP (Ta PAB) with the eIF4G and eIF4B interaction domains in human (Hs PAB) and yeast (Sc PAB) PABP. RRMs and the conserved C-terminal domains are indicated by the shaded boxes.

Figure 5. Comparison of eIFiso4G and eIF4B interaction domains of wheat PABP (Ta PAB) with the corresponding sequence of tobacco PAB3 (Nt PAB3), Arabidopsis PAB2 (At PAB2), human PAB (Hs PAB), and Saccharomyces cerevisiae PAB (Sc PAB). Conservation of identical residues relative to wheat PABP is indicated by shading. The RNP1 and RNP2 motifs conserved among PAB proteins are indicated by asterisks. Helices (H) and -sheets (S) as determined for human PAB are indicated. RRM1 and RRM2 domains are indicated by brackets and the eIFiso4G and eIF4B interaction domains indicated by lines. The portion of PABP illustrated in each case is indicated by residue numbers before and after each sequence.

My laboratory has also shown that the 3' terminal stem-loop structure from cell-cycle regulated histone mRNAs functions to increase the efficiency of translation initiation in animal cells. Interestingly, the 3' terminal stem-loop structure did not function to enhance translation in higher plants which do not have histone mRNAs that terminate in a stem-loop structure. This work demonstrates that whether it be the 3' terminal stem-loop structure from cell-cycle regulated histone mRNAs or a poly(A) tail, both remain dependent on the 5' cap structure for their function as enhancers of translation.

Figure 6. Proposed physical and function model of the association of SLBP with the eIF4F/eIF3/PABP complex. SLBP is shown bound to the 3'-terminal stem-loop of a cell cycle-regulated histone mRNA. Association of SLBP with the eIF4F/eIF3/PABP complex requires eIF4G and perhaps eIF3. eIF4E is not required for the physical association of SLBP with the complex but is required for SLBP function in that it is necessary for the binding of the complex to the 5' cap.

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Viral Translational Mechanisms  

Although most mRNAs have a 5' cap structure and a poly(A) tail, there are some exceptions. Some viral mRNAs, such as tobacco etch virus (TEV) are not capped and other such as tobacco mosaic virus (TMV) terminate in a tRNA-like structure instead of a poly(A) tail. As mentioned above, the cell-cycle regulated histone mRNAs in animals and in lower plants terminate in a stem-loop structure instead of a poly(A) tail. These exceptions pose the interesting question of how they achieve efficient translation in the absence of a cap or a poly(A) tail when both are necessary to promote efficient translation initiation. I have shown how viral mRNAs, e.g., tobacco mosaic virus, tobacco etch virus, and alfalfa mosaic virus, recruit the same translational machinery as cellular mRNAs but do so in novel ways. With these studies, I have shown that, as with typical mRNAs in which circularization is mediated by the 5’ cap structure and the 3’ poly(A) tail, the translation of viral mRNAs also involves a functional interaction between the termini of the mRNA even when these viral mRNAs naturally lack a 5’ cap structure or poly(A) tail.

My research has shown that the 5'-leader sequence (called W) of tobacco mosaic virus (TMV) functions as a translational enhancer exhibiting functional overlap with the 5'-cap and the poly(A) tail but not with the native TMV 3'-UTR which contains an independent translational enhancer. This was one of the first and most thoroughly investigated examples translational enhancers of which many examples have subsequently been identified. W is recognized by HSP101, a member of the HSP101/HSP104/ClpB family of heat stress proteins, which is sufficient to mediate the translational enhancement associated with W. Therefore, HSP101 and W comprise a two-component translational regulatory mechanism. Our genetic analysis has suggested that the translational activity of HSP101 requires eIF4G and eIF3. Consistent with the role of HSP101 in mediating the translational function of W, the enhancement afforded by W increases following a heat stress which also elevates expression of HSP101. eIF4F (much more so than an isoform of eIF4F, called eIFiso4F) was specifically required for the activity of W, suggesting that it is functionally similar to a 5'-cap and a poly(A) tail in that it serves to recruit eIF4F in order to enhance translation from an mRNA. This was the first identification of a protein required for specific translational enhancement of capped mRNAs, the first report of a translational regulatory function for any heat stress protein, and the first functional distinction between the two eIF4G proteins present in eukaryotes.

Figure 7. Role of HSP101 in the Translation of TMV. The 5'-leader of TMV binds HSP101which, in turn, recruits eIF4G and eIF3 to promote co-translational disassembly of the virion particle. HSP101 is shown bound to W, the TMV 5'-leader and recruits eIF4G and eIF3 to the viral mRNA. Following translation initiation, the 80S ribosome co-translationally strips the viral coat protein from the genomic mRNA.

I have shown that the 143 nt tobacco etch virus (TEV) 5'-leader is sufficient to confer cap-independent translation. The TEV 5'-leader functionally interacts with the poly(A) tail to promote optimal cap-independent translation and requires eIF4G and PABP. A 5'-proximal, 45 nt RNA pseudoknot-containing domain within the TEV 5' leader is required to promote cap-independent translation. Mutations that disrupted the pseudoknot reduced cap-independent translation, including mutations to loop 3 that exhibit complementarity to a conserved region in eukaryotic 18S rRNAs. Changes to the loop that maintained its potential for base-pairing with 18S rRNA had only a small effect, supporting the possibility that base-pairing between the pseudoknot and the 18S rRNA may be involved in promoting cap-independent translation. We can conclude from these studies that when an alternative to a 5' cap or a poly(A) tail has evolved in an mRNA, a requirement for an interaction between the terminal regulatory elements of the mRNA appears to be a common prerequisite for optimizing translation.

Figure 8.The predicted structure of the TEV 5'-leader. A, the entire TEV 5'-leader (nt 1–143) is displayed with the predicted structure of the three pseudoknots, i.e. PK1, PK2, and PK3. The stems (e.g. S1 or S2) and loops (e.g. L1, L2, or L3) for each pseudoknot are indicated. B, the TEV 5'-leader containing an alternative structure, i.e. SL1 and SL2, in place of PK2 is shown. This alternative structure is also consistent with the enzymatic and chemical probing data. The AUG at the 3' terminus represents the initiation codon of the TEV polyprotein-coding region.

I have shown that the 3'-untranslated region of TMV is functionally equivalent to a poly(A) tail. A single RNA pseudoknot structure contained within the TMV 3'-untranslated region was responsible for the translational regulation and this RNA pseudoknot structure was conserved in many viruses related to TMV. The 3'-untranslated region of other viruses that do not terminate in a poly(A) tail also enhanced translation, data suggesting that this may be a common theme in many plant RNA viruses.

The three plus-strand genomic RNAs of Alfalfa mosaic virus (AMV) and the subgenomic messenger for viral coat protein (CP) contain a 5'-cap structure but no 3'-poly(A) tail. Binding of the CP to the 3'-end of AMV RNAs is required for efficient translation of the viral RNAs and to initiate infection in plant cells. My laboratory showed that the CP is required for translation of AMV RNA 3. The stimulation of translation by the CP was cap-dependent and could be reproduced in yeast cells. AMV CP interacted specifically with eIF4F and eIFiso4F suggesting that the role of the CP in translation of viral RNAs mimics the role of PABP in translation of cellular mRNAs.

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Analyzing the Impact of Heat Stress on Plant Gene Expression

Thermal stress is an environmental factor that many plants must contend with on a daily basis during the summer. Heat stress has profound effects on protein synthesis and affects transcriptional activity as well as translation. I have a long term interest in how heat stress impacts the translational process which is an area that has received little attention to date. My laboratory had previously demonstrated that the leader sequence of an mRNA can play an important role in overcoming the translation repression that normally occurs following a heat stress. Following a heat stress, the communication between the 5' cap structure and the poly(A) tail is disrupted, primarily through the loss in the function of the cap. Heat stress causes dramatic changes in the phosphorylation status of two translation initiation factors, i.e., eIF4B and eIF4A, both of which are associated with the 5' cap structure and that recovery from heat stress was greatly affected when the expression of the heat stress proteins was blocked. In addition to the heat stress -mediated repression of translation, the stability of mRNAs increases following a heat stress and correlates with the severity of the stress. This was observed in plants but not in animals, suggesting an important difference in the way that plants respond to heat stress compared to animals. The increase in mRNA stability could be a result of a heat-mediated repression in the mRNA-degradatory machinery and the activity of all detectable RNase activities decreased following a heat stress and correlated with the severity of the stress.

In addition to examining the function of the heat stress protein, HSP101, in maize and rice, I identified just how this heat stress protein assembles into the hexameric complexes that it forms in plants and yeast. I identified three regions critical for its self-assembly and, most interestingly, demonstrated that one of the two ATP-binding sites required for self-assembly regulates the interaction of one of the interaction domains. This was the first identification of the regions required for self-assembly for any species.

Figure 9. The schematic structure of Hsp101. The five domains and the conserved signature sequence elements are indicated above the schematic. The consensus sequence for each signature sequence element is indicated below the schematic. The two Walker A ATP-binding sites, the predicted coiled-coil regions, and the interaction domains identified with this work are also indicated.

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Programmed Cell Death in Plants

I have a long standing interest in seed quality and have investigated the programmed cell death that occurs during endosperm development of cereals. Development of the maize endosperm, the tissue in the seed that serves a storage function for the embryo, requires transport of sucrose from the leaves to the ear which is then converted into starch. Sucrose transport terminates during endosperm development and the organ eventually dies as part of its normal development. My laboratory has shown that maize endosperm undergoes programmed cell death late in its development so that, with the exception of the aleurone layer, the tissue is dead by the time the grain matures. The progression of endosperm programmed cell death (PCD) is accompanied by an increase in nuclease activity and the internucleosomal degradation of nuclear DNA, hallmarks of apoptosis in animals. Ethylene and abscisic acid regulate the onset of endosperm cell death in cereal endosperm where ethylene promotes the cell death program and abscisic acid opposes the cell death program. The progression of the cell death program in developing maize endosperm follows a highly organized pattern. Ethylene is produced in two discrete peaks during the development of cereal endosperm and it participates in regulating PCD in this tissue. The application of exogenous ethylene throughout seed development resulted in earlier and more extensive cell death in developing maize and was accompanied by more extensive DNA fragmentation whereas treatment of developing ears of maize with 2-aminoethoxyvinyl glycine (an inhibitor of ethylene biosynthesis) or 1-methylcyclopropene (an inhibitor of ethylene perception) reduced cell death and DNA fragmentation during endosperm development. It is likely that initiation of cell death is controlled by the timing and location of ethylene receptor synthesis or action which would provide a basis for the selective cell death that occurs within a developing seed. Mutations affecting starch biosynthesis such as shrunken1 (sh1) and shrunken2 (sh2) exhibited a premature onset and an accelerated execution of the PCD program during endosperm development which correlated with significantly higher levels of ethylene production in the mutant kernels.

I also demonstrated that wheat endosperm undergoes a programmed cell death during its development. It shares features with the maize program but differs in some aspects of its execution. Cell death initiated and progressed stochastically in wheat endosperm in contrast to maize where cell death initiates within the upper central endosperm and expands outward. Following a peak of ethylene production during early development, wheat endosperm DNA underwent internucleosomal fragmentation that was detectable from mid to late development. The developmental onset and progression of DNA degradation was regulated by the level of ethylene production or perception. These observations suggest that programmed cell death of the endosperm and regulation of this program by ethylene is not unique to maize but that differences in the execution of the program appear to exist among cereals.

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The Role of Ethylene, Abscisic Acid, and Cytokinin in Regulating Plant Growth and Responses to Environmental Stress

My laboratory has investigated the role of ethylene, abscisic acid, and cytokinin in regulating plant growth and regulating the response to environmental stress. Maize has served as my main focus because of its importance to the world food supply and the fact that considerably less is known about the function of hormones such as ethylene in cereals than is known in dicot species. Additional work is being performed in Arabidopsis. I reported the isolation of the maize gene families for ACC synthase, ACC oxidase, the ethylene receptor, and EIN2 and EIL, which act downstream of the ethylene receptor. I showed that ACC oxidase is expressed primarily in the endosperm, and only at low levels in the developing embryo late in its development. ACC synthase is expressed throughout endosperm development but, in contrast to ACC oxidase, it is transiently expressed to a significantly higher level in the developing embryo at a time that corresponds with the onset of endosperm cell death. Only two ethylene receptor gene families were identified in maize, in contrast to the five types previously identified in Arabidopsis. Members of both ethylene receptor families were expressed to substantially higher levels in the developing embryo than in the endosperm, as were members of the EIN2 and EIL gene families. These results suggest that the endosperm and embryo both contribute to the synthesis of ethylene, and they provide a basis for understanding why the developing endosperm is especially sensitive to ethylene-induced cell death while the embryo is protected.

Figure 10. Comparison of maize proteins involved in ethylene biosynthesis and perception with those from Arabidopsis. Conserved residues in ACC synthases ( A) and ACC oxidases ( B) are indicated by a letter above each vertical black line. PPD, pyridoxal phosphate binding domain. The proposed coiled-coiled region in ACC oxidases is indicated by the large black box. C The N-terminal, hydrophobic, transmembrane domains in ethylene receptors are indicated by gray boxes. Cys-4 and Cys-6, which form a disulfide linkage between monomers are indicated by the Cs at the left end of the scheme. The five consensus motifs (H, N, G1, F, and G2) within the histidine protein kinase domain as described by Hua et al. (1998) are indicated, and the aspartate and lysine residues conserved in the receiver domain of ETR1 are indicated by a letter above each vertical black line. The serine-rich domain (S) is also indicated. The proposed coiled-coiled region is indicated by the large black box. D N-terminal transmembrane domains in EIN2 are indicated by gray boxes. The proposed coiled-coiled region is indicated by the black box. E The N-terminal acidic domain (AD), the five basic domains (BD1-BD5), the proline-rich domain (PRD), and the glutamine-rich domain (Q-rich) near the C-terminus are indicated.

I also isolated ACC synthase mutants of maize, affected in the first step in ethylene biosynthesis. Loss of ACC synthase expression resulted in delayed leaf senescence under normal growth conditions and inhibited drought-induced senescence. The mutant leaves continued to be photosynthetically active under both conditions indicating that leaf function was maintained. These observations suggest that ethylene may serve to regulate leaf performance throughout its lifespan as well as to determine the onset of natural senescence and mediate drought-induced senescence. This is the first time that this machinery had been described for any cereal and the first demonstration that it plays a significant role in responding to drought conditions for any plant species. A patent “Staygreen Maize” U.S. Patent Application No 7,230,161 was recently issued and this technology is currently undergoing field trials.

Figure 11. ACC synthase knockout mutant - Staygreen (a) Leaf 1 from three representative B73 wild-type (i.e. ZmACS6/ZmACS6), heterozygous (i.e. ZmACS6/Zmacs6), and homozygous (i.e. Zmacs6/Zmacs6) plants at 50 days after pollination. Ethylene production determined from seedling leaves for each genotype is indicated below and the percentage of ethylene in mutant leaves relative to wild-type leaves (defined as 100%) is indicated. (b) Dark-induced senescence in ZmACS6/ZmACS6, ZmACS6/Zmacs6, and Zmacs6/Zmacs6 leaves following 2 weeks of light deprivation.

Within the maize spikelet, the basic repeating unit of the inflorescence, the spikelet meristem gives rise to an upper and lower floret, the latter of which normally aborts. I demonstrated that this abortion is controlled by cytokinin and overproduction of this hormone results in the rescue of the floret from abortion, resulting in two functional florets per spikelet. The pistil in each floret was fertile but the spikelet produced just one kernel composed of a fused endosperm with two viable embryos. Such grain had substantially increased protein and oil content. They also had reduced carbohydrate levels, resulting in “low-carb” corn. This outcome has tremendous practical benefit in increasing the value of corn to US farmers, in addressing protein-energy malnutrition affecting many of the 800 million people in the world suffering from a lack of high quality foods.

Figure 12. The Plant Journal Cover. Double Embryo Maize Kernel

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The Function of Ascorbic Acid in Regulating Plant Growth and Responses to Environmental Stress

Figure 13. Reducing DHAR Expression Improves Drought-Tolerance in Plants  (A) Images of representative expanded leaves were taken at 30 and 120 min after their detachment. (B) Water loss from the detached leaves held at room temperature was followed by determining leaf weight every 5 min for 40 min. Four leaves from separate plants were used. The rate of water loss as loss in leaf weight was plotted against time, and the average and standard deviation were reported.

My laboratory has investigated the function of the enzyme, dehydroascorbate reductase (DHAR). Increasing the expression of DHAR, the enzyme responsible for recycling vitamin C (ascorbic acid) once it has been used, can be used as an effective strategy to increase the level of ascorbic acid in crops, thus improving their nutritional value as a simple means to increase vitamin C in the daily diet. This technology has significant potential value to humankind and a patent: “Dehydroascorbate Reductase (‘DHAR’) Genes from Triticum Aestivum and Their Use to Modulate Ascorbic Acid Levels in Plants” U.S. Patent Application No. 6,903,246 has been issued. In subsequent work, my laboratory has show that decreasing the expression of DHAR in guard cells of leaves can be used as an effective strategy to decrease the level of ascorbic acid in crops, thus improving their tolerance to drought. This technology will benefit farmers that use irrigation to water their crops to conserve water which is important in a state like California where rapid population growth continues to increase the demand on this scare resource as well as those farmers who grow crops in arid areas, such as exists in many third world countries. We also demonstrated that increasing the level of ascorbic acid increases the tolerance of plants to ozone, a significant component of smog that limits crop yields. This is important in a state like California where rapid population growth has resulted in the encroachment of development into agricultural areas that has increased the level of ozone in areas like the Central Valley. My laboratory has also shown that DHAR is important in regulating overall plant growth and that the level of ascorbic acid also regulates the activity of DNase and RNase, those enzymes responsible for degrading DNA and RNA during leaf senescence. Thus, by combining work in areas of protein synthesis, hormonal regulation of maize flower development, hormonal regulation of endosperm programmed cell death, the role of ethylene in regulating environmental stress responses in maize, the role of ascorbic acid in plant growth and tolerance to environmental stress, I am working towards an integrated understanding of those factors that affect crop (i.e., specifically maize) yield so that technologies, of which some have already been patented, can be developed to improve crop performance .

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Current Lab Personnel (Coming!)

Selected Publications (Bibliography page)

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