Small GTP-binding proteins of the Rac family are important signaling switches that regulate plant growth and development due to their ability to shuttle between the inactive GDP-bound and the active GTP-bound form. The ratio between the two forms is tightly regulated in the cell. The Arabidopsis genome encodes eleven Rac proteins designated Arac1-11. Based on the C-terminus, Aracs can be divided into typeI and typeII. TypeI Aracs associate with the membrane by prenylation while typeII Aracs associate with the membrane by palmitoylation. Differences in the effector-binding region and the C-terminal hypervariable region place Arac7 in a separate phylogenetic group than the other typeII Aracs. The work described in this dissertation examines the expression patterns, functions and regulation of the Arac7 protein and provides evidence that supports a distinct role for Arac7 within type II Aracs. Arac7 expression is shown to repress auxin-induced gene expression and to enhance ABA-stimulated gene expression in Arabidopsis. Plants overexpressing Arac7 are less responsive to auxin but show increased responses to ABA while the opposite is observed in plants with decreased levels of Arac7 mRNA. Arac7 expression is high in the lateral root primordia and can be traced back to the first pericicle divisions that give rise to these primordia. This, together with the increased number of lateral roots of plants overexpressing Arac7, is consistent with a role in lateral root formation for this small GTPase. Interestingly, transcription from the Arac7 promoter is stimulated by auxin but repressed by ABA. Together with the observed functions on auxin and ABA signaling, the regulated expression of Arac7 provides a feedback mechanism to ensure that cells maintain sensitivity to signals. This work also shows that Arac7 is constitutively associated with the plasma membrane where it partitions into detergent-resistant membrane domains (DRMs). Moreover, at the plasma membrane, the majority of Arac7 partitions into high molecular weight complexes. These properties contrast with those observed for the typeI Arac5 which is distributed between the cytosol and non-DRMs regions of the plasma membrane and exists predominantly in low molecular weight forms. Together, these observations suggest different functions and regulatory mechanisms for typeI and typeII Aracs.

Pollen dimorphism has been intensively studied in many species of Angiosperms but not in the Onagraceae. Some species of Oenothera have been reported to present differences in the sizes and starch contents of their pollen grains but most of the research done in the ultrastructure of pollen an microsporogenesis has been performed in species that do not present any pollen polymorphism. In both O. picensis and O. villaricae, each haploid set of chromosomes is united into a ring called a Renner complex. These Renner complexes are each transmitted intact from one generation to the next and, since Oenothera picensis and Oenothera villaricae form fully fertile hybrids when crossed in either direction, it is possible to construct several different combinations of Renner complexes and cytoplasmic backgrounds. In this work, I show the existence of pollen polymorphism in Oenothera picensis, Oenothera villaricae and their hybrids, report on the origin of the polymorphism during microsporogenesis and provide evidence that the different pollen sizes are representing two different genome. I present the hypothesis and evidence that this polymorphism is a morphological and physiological expression of the different genetic characteristics of the pollen grains. Finally, I will indicate how these differences may be related to the phenomenon of non-random fertilization associated with these species of Oenothera.

Pollen tube growth, the process that transports the sperm cell to the ovule, is fundamental for plant sexual reproduction. The actin cytoskeleton is essential to the process of pollen tube elongation. To further understand how actin is regulated in the pollen tube, I characterized two actin regulatory proteins: profilin and plant-villin (ABP-135) from Lilium longiflorum . Profilin, a small actin monomer binding protein, is abundant in pollen. By a combination of rapid fixation, immunological staining and live cell analysis, I show that profilin is a soluble protein evenly distributed in the cytosol. After estimating its intracellular concentration, I elevated its concentration by microinjection to study the effect on cytoplasmic streaming and cell growth. Increasing profilin concentration by 25% resulted in half-maximal growth inhibition, while a 60% increase was necessary to half-maximally inhibit cytoplasmic streaming. My results suggest that actin polymerization is essential for tube growth, that profilin is an actin-monomer sequestering protein, capable of regulating actin's polymerization state, and that the participation of actin on growth is separable from that on cytoplasmic streaming. I characterized a second actin-binding protein, originally identified as an F-actin bundling factor, which co-localizes with actin cables in the pollen tubes. It is composed of several isoforms, and is widely distributed in different plant organs. By cloning its cDNA, I identified this protein as the plant homologue of villin; these are calcium dependent severing, capping, nucleating and bundling proteins. The actin cytoskeleton may be regulated via this plant villin, which could respond directly to the calcium gradient present at the tip of the pollen tube.

No description available.

Possible roles for wall-based enzyme activity in aging of daylily petals are presented. We are asking if daylily senescence is controlled in part by reactions associated with. We suggest that membrane changes leading to cell death may be induced in part by lipoxygrenase activity and by ROS that are increasing because of reduced effectiveness of certain protective enzymes. The flowers are insensitive to ethylene, but exogenous ABA prematurely upregulates events that occur during natural senescence, such as loss of differential membrane permeability, increases in lipid peroxidation and the induction of proteinase and RNase activities. The possibility is discussed that ABA is a constituent of the signal transduction chain leading to programmed cell death of daylily petals. Six daylily genes, whose levels increase during senescence, were isolated from a daylily cDNA library. Southern blot analysis showed that all six DSA genes, designated as DSA3, 4, 5, 6, 12 and 15, are members of multigene families. The GenBank database homology search suggested, that DSA3 belongs to the cytochrome P450 superfamily, DSA4 is an aspartic proteinase, DSA5 may be a water stress protein, DSA6 is a putative S-1 type nuclease, DSA12 is very similar to impedance induced protein, and DSA15 is a fatty acid elongase. The accumulation of dsag mRNAs, with the exception of DSA4, is accelerated 3.2 to 43 times in ABA treated prematurely senescing petals. Levels of DSA mRNAs in leaves are very low, less than 4% of the maximum detected in petals, and there is no significant change during senescence. Except for the gene for a putative impedance protein (DSA12), DSAs are also expressed at low levels in daylily roots. (Abstract shortened by UMI.)