Calmodulin is a small Ca2+-binding protein that acts to transduce second messenger signals into a wide array of cellular responses. Plant calmodulins share many structural and functional features with their homologs from animals and yeast, but the expression of multiple protein isoforms appears to be a distinctive feature of higher plants. Calmodulin acts by binding to short peptide sequences within target proteins, thereby inducing structural changes, which alters their activities in response to changes in intracellular Ca2+ concentration. The spectrum of plant calmodulin-binding proteins shares some overlap with that found in animals, but a growing number of calmodulin-regulated proteins in plants appear to be unique. Ca2+-binding and enzymatic activation properties of calmodulin are discussed emphasizing the functional linkages between these processes and the diverse pathways that are dependent on Ca2+ signaling.
The DEFORMED ROOTS AND LEAVES1 ( DRL1 ) gene is single copy in the Arabidopsis genome, and based on overall amino acid similarity and conservation of functional domains, the DRL1 protein is homologous with yeast TOT4/KTI12. TOT4/ KTI12 associates with Elongator, a multisubunit complex that binds the RNA polymerase II transcription elongation complex. Recessive mutations at the DRL1 locus caused defective organ formation indicative of disorganized shoot, inflorescence, flower, and root meristems. DRL1 is a putative ATP/GTP binding protein; in addition, calmodulin binding activity was demonstrated in vitro for the C terminus of the DRL1 protein. Phenotypic and genetic data position DRL1 relative to regulatory loci for leaf development, in which it acts early. We identified Arabidopsis homologs for the six Elongator components and hypothesize that DRL1 regulates transcription elongation through a putative plant Elongator. Upregulation of the AN-GUSTIFOLIA transcript in the strong drl1-2 allele supports this model.
Complementary DNA (cDNA) clones encoding calmodulin isoforms were isolated from an Arabidopsis leaf XgtlO library by screening with cloned barley calmodulin cDNA probes. Two cDNAs, one a 626-base pair partial-length clone (ACaM-1) and one a 1400-base pair full-length clone (ACaM-2), encode calmodulin polypeptides that differ by four conservative amino acid substitutions. None of the amino acid sequence differences occur within the four Ca2+-binding domains of the proteins. Whereas the deduced amino acid sequences of the two Arabidopsis calmodulin isoforms share 97% identity, the nucleotide sequences encoding the two isoforms share 87% sequence identity. Most of these nucleotide sequence differences (80%) occur in codon wobble positions. ACaM-1 and ACaM-2 both hybridize with a distinct set of restriction fragments of Arabidopsis total DNA, indicating that they were derived from transcripts of separate genes; these genes are single-or very low-copy in the Arabidopsis genome. Both cDNAs hybridize to messenger RNA (mRNA) species of 0.8 kilobases that are expressed to a greater extent in developing siliques compared with leaves, flowers, and stems. Northern blot and polymerase chain reaction assays both indicate that ACaM-1 mRNA is more highly expressed than ACaM-2 mRNA in developing siliques. The steady-state levels of both isoform mRNAs increase as a result of touch stimulation; the kinetics and extent of increase are comparable for the two mRNAs.
Ribulosebisphosphate carboxylase/oxygenase activase is a recently discovered enzyme that catalyzes the activation of ribulose-1,5-bisphosphate carboxylase/oxygenase ["rubisco"; ribulose-bisphosphate carboxylase; 3-phospho-Dglycerate carboxy-lyase (dimerizing), EC 4.1.1.39] in vivo. Clones of rubisco activase cDNA were isolated immunologically from spinach (Spinacea oleracea L.) and Arabidopsis thaliana libraries. Sequence analysis of the spinach and Arabidopsis cDNAs identified consensus nucleotide binding sites, consistent with an ATP requirement for rubisco activase activity. A derived amino acid sequence common to chloroplast transit peptides was also identified. After synthesis of rubisco activase in vitro, the transit peptide was cleaved and the protein was transported into isolated chloroplasts. Analysis of spinach and Arabidopsis nuclear DNA by hybridization indicated a single rubisco activase gene in each species. Leaves of spinach and Arabidopsis wild type contained a single 1.9-kilobase rubisco activase mRNA. In an Arabidopsis mutant lacking rubisco activase protein, mRNA species of 1.7 and 2.1 kilobases were observed under conditions of high-stringency hybridization with a wild-type cDNA probe. This observation indicates that the lesion in the mutant arises from an error in mRNA processing.
The effect of different feeding behaviours of 1st and 4th instar Trichoplusia ni on photosynthesis of Arabidopsis thaliana var. Columbia was characterized using spatially resolved measurements of fluorescence and leaf temperature, as well as leaf gas exchange,. First instars made small holes with a large perimeter-to-area ratio and avoided veins, while 4th instars made large holes with a low perimeter-to-area ratio and consumed veins. Herbivory by 1st instars reduced photosynthesis more strongly in the remaining leaf tissue than that by 4th instars. Photosystem II operating efficiency (PhiPSII) was correlated with the rate of CO2 exchange, and reductions in PhiPSII in areas around the missing tissues contributed to a 15.6% reduction in CO2 assimilation on the first day following removal of 1st instars. The corresponding increases in non-photochemical quenching and greater rates of non-stomatal water loss from these regions, as well as the partial reversal of low PhiPSII by increasing the ambient CO2 concentration, suggests that localized water stress and reduced stomatal conductance contributed to the inhibition of photosynthesis. Damage by 1st but not 4th instars reduced the maximum quantum efficiency of photosystem II photochemistry (Fv/Fm) by 4-8%. While herbivory by both 1st and 4th instars increased dark respiration rates, the rates were too low to have contributed to the observed reductions in CO2 exchange. The small holes produced by 1st instars may have isolated patches of tissue from the vascular system thereby contributing to localized water stress. Since neither 1st nor 4th instar herbivory had a detectable effect on the expression of the Rubisco small subunit gene, the observed differences cannot be attributed to changes in expression of this gene. The mode of feeding by different instars of T. ni determined the photosynthetic response to herbivory, which appeared to be mediated by the level of water stress associated with herbivore damage.
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