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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2011)

The biological role of ethylene, H2C=CH2, was first identified as a plant hormone responsible for leaf drop from the observation that plants relatively close to gas lamps lost their leaves. Later, it was then known as an important gaseous hormone for climacteric fruit ripening. Further research revealed that ethylene not only regulates entry into several types of plant developmental cell death and senescence programs, but also mediates plant responses to biotic and abiotic stress. For example, ethylene has been implicated in promoting kernel abortion under shading stress in maize. Ethylene production is controlled by the nutritional and stress status of a plant. Despite the broad range of ethylene's effects on development, the primary steps in ethylene action are assumed to be similar in all cases: They all involve the binding of ethylene to a receptor, followed by activation of one or more signal transduction pathways leading to the cellular response. Ultimately, ethylene exerts its effect primarily through alterations in the pattern of gene expression.

Although the hormonal control of root growth and development has been extensively studied, relatively little is known about the role that ethylene plays in cereal root development. To understand how the ethylene biosynthetic machinery is spatially regulated in maize roots and how changes in its expression alter root growth, the expression of ACC synthase (encoded by ZmACS2, ZmACS6, and ZmACS7 in maize) was observed in the root cap and in cortical cells whereas the expression of ACC oxidase (encoded by ZmACO15, ZmACO20, ZmACO31, and ZmACO35 in maize) was detected in the root cap, protophloem sieve elements, and the companion cells associated with metaphloem sieve elements. The results suggest that expression of ZmACS6 is important in regulating growth of maize roots in response to physical resistance.

To date, many studies on ethylene insensitivity have focused on the function of the Arabidopsis dominant-negative mutant ethylene receptor gene (etr1-1) in Arabidopsis or other species. To understand more about the effect of ethylene on cereal crops, maize dominant-negative ethylene receptors (e.g. Zmetr2 and Zmers1) were generated by altering a conserved cysteine residue in one of the transmembrane domains in the N-terminal region of the receptor. Taking the advantage of its short generation time, Zmetr2 and Zmers1 were first studied in Arabidopsis. The results suggest that Cys65 in maize ZmERS1 and ZmETR2 plays the same role that it does for Arabidopsis receptors. Moreover, the results demonstrate that the mutant maize ethylene receptors are functionally dependent on subfamily 1 ethylene receptors in Arabidopsis to exert their dominance, indicating substantial functional conservation between maize and Arabidopsis ethylene receptors despite their sequence divergence.

The etr1-1 mutant confers a state of ethylene insensitivity constitutively during the lifetime of a plant. Therefore, the effect of etr1-1 on plant growth and development is limited in that it does not reveal what roles ethylene might play in specific cell types or developmental stages separate from its global influence on the plant. To study the role of ethylene in specific organs or at certain developmental stages in maize, expression of the maize dominant-negative ethylene receptor, Zmetr2, was driven by organ-specific promoters, i.e., from PEPC (Phosphoenolpyruvate carboxylase) and RbcS (Rubisco small subunit), for leaf-specific expression, and SH1 (Shunken1), for kernel-specific expression in transgenic maize. Such an approach allows the examination of the effects of creating a state of ethylene insensitivity in specific organs to determine the role of ethylene under normal growth conditions or conditions of stress.

    Cover page: Ethylene Insensitivity in Maize: Analysis of Ethylene Receptors and the Ethylene Response in Maize
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2013)
    Abstract Background The programmed cell death 4 (PDCD4) protein is induced in animals during apoptosis and functions to inhibit translation and tumor promoter-induced neoplastic transformation. PDCD4 is composed of two MA3 domains that share similarity with the single MA3 domain present in the eukaryotic translation initiation factor (eIF) 4G, which serves as a scaffold protein to assemble several initiation factors needed for the recruitment of the 40S ribosomal subunit to an mRNA. Although eIF4A is an ATP-dependent RNA helicase that binds the MA3 domain of eIF4G to promote translation initiation, binding of eIF4A to the MA3 domains of PDCD4 inhibits protein synthesis. Genes encoding PDCD4 are present in many lower eukaryotes and in plants, but PDCD4 in higher plants is unique in that it contains four MA3 domains and has been implicated in ethylene signaling and abiotic stress responses. Here, we examine the evolution of PDCD4 in plants. Results In older algal lineages, PDCD4 contains two MA3 domains similar to the homolog in animals. By the appearance of early land plants, however, PDCD4 is composed of four MA3 domains which likely is the result of a duplication of the two MA3 domain form of the protein. Evidence from fresh water algae, from which land plants evolved, suggests that the duplication event occurred prior to the colonization of land. PDCD4 in more recently evolved chlorophytes also contains four MA3 domains but this may have resulted from an independent duplication event. Expansion and divergence of the PDCD4 gene family occurred during land plant evolution with the appearance of a distinct gene member following the evolution of basal angiosperms. Conclusions The appearance of a unique form of PDCD4 in plants correlates with the appearance of components of the ethylene signaling pathway, suggesting that it may represent the adaptation of an existing protein involved in programmed cell death to one that functions in abiotic stress responses through hormone signaling.
      Cover page: The unique evolution of the programmed cell death 4 protein in plants
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2010)
      Ethylene, a regulator of plant growth and development, is perceived by specific receptors that act as negative regulators of the ethylene response. Five ethylene receptors, i.e., ETR1, ERS1, EIN4, ETR2, and ERS2, are present in Arabidopsis and dominant negative mutants of each that confer ethylene insensitivity have been reported. In contrast, maize contains just two types of ethylene receptors: ZmERS1, encoded by ZmERS1a and ZmERS1b, and ZmETR2, encoded by ZmETR2a and ZmETR2b. In this study, we introduced a Cys to Tyr mutation in the transmembrane domain of ZmERS1b and ZmETR2b that is present in the etr1-1 dominant negative mutant and expressed each protein in Arabidopsis. Mutant Zmers1b and Zmetr2b receptors conferred ethylene insensitivity and Arabidopsis expressing Zmers1b or Zmetr2b were larger and exhibited a delay in leaf senescence characteristic of ethylene insensitive Arabidopsis mutants. Zmers1b and Zmetr2b were dominant and functioned equally well in a hemizygous or homozygous state. Expression of the Zmers1b N-terminal transmembrane domain was sufficient to exert dominance over endogenous Arabidopsis ethylene receptors whereas the Zmetr2b N-terminal domain failed to do so. Neither Zmers1b nor Zmetr2b functioned in the absence of subfamily 1 ethylene receptors, i.e., ETR1 and ERS1. These results suggest that Cys65 in maize ZmERS1b and ZmETR2b plays the same role that it does in Arabidopsis receptors. Moreover, the results demonstrate that the mutant maize ethylene receptors are functionally dependent on subfamily 1 ethylene receptors in Arabidopsis, indicating substantial functional conservation between maize and Arabidopsis ethylene receptors despite their sequence divergence.
        Cover page: Analysis of the functional conservation of ethylene receptors between maize and Arabidopsis
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2010)
        Arabidopsis thaliana knockout lines for the plant-specific eukaryotic translation initiation factors eIFiso4G1 (i4g1) and eIFiso4G2 (i4g2) genes have been obtained. To address the potential for functional redundancy of these genes, homozygous double mutant lines were generated by crossing individual knockout lines. Both single and double mutant plants were analyzed for changes in gross morphology, development, and responses to selected environmental stressors. Single gene knockouts appear to have minimal effect on morphology, germination rate, growth rate, flowering time, or fertility. However, double mutant i4g1/i4g2 knockout plants show reduced germination rates, slow growth rates, moderate chlorosis, impaired fertility and reduced long term seed viability. Double mutant plants also exhibit altered responses to dehydration, salinity, and heat stress. The i4g2 and i4g1/i4g2 double mutant has reduced amounts of chlorophyll a and b suggesting a role in the expression of chloroplast proteins. General protein synthesis did not appear to be affected as the levels of gross protein expression did not appear to change in the mutants. The lack of a phenotype for either of the single mutants suggests there is considerable functional overlap. However, the strong phenotypes observed for the double mutant indicates that the individual gene products may have specialized roles in the expression of proteins involved in plant growth and development.
          Cover page: Deletion of the eIFiso4G subunit of the Arabidopsis eIFiso4F translation initiation complex impairs health and viability
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