other hand, catabolism, such as nucleic acid breakdown and proteolysis, becomes active through induction of a number of hydrolytic enzymes (Matile, 1992; Noodé n, 1988; Summary Smart, 1994;Thiman, 1980;Thomas and Stoddart, 1980). Leaf senescence, although a deteriorative cellular process, Four mutants that show the delayed leaf senescence phenotype were isolated from Arabidopsis thaliana.is assumed to be an evolutionarily acquired, active genetic trait that makes an important contribution to fitness of Genetic analyses revealed that they are all monogenic recessive mutations and fall into three complementation plants, for example by remobilizing nutrients from vegetative tissues to reproductive organs (Matile, 1992; Noodé n, groups, identifying three genetic loci controlling leaf senescence in Arabidopsis. Mutations in these loci cause delay in Thiman, 1980;Thomas and Stoddart, 1980). Elucidating the genetic mechanism of leaf senescence is essential all senescence parameters examined, including chlorophyll content, photochemical efficiency of photosystem II, to understanding the senescence phenomenon itself and also for practical purposes such as improvement of plant relative amount of the large subunit of Rubisco, and RNase and peroxidase activity. Delay of the senescence productivity, pre-or post-harvest storage, and stress tolerance. However, despite the biological and practical import-symptoms was observed during both age-dependent in planta senescence and dark-induced artificial senescence ance of leaf senescence, the genetic mechanism controlling the leaf senescence process remains poorly understood. in all of the mutant plants. The results indicate that the three genes defined by the mutations are key geneticWe therefore undertook a systematic genetic screening to identify the genes that control leaf senescence, using elements controlling functional leaf senescence and provide decisive genetic evidence that leaf senescence is aArabidopsis thaliana as a model system. genetically programmed phenomenon controlled by several monogenic loci in Arabidopsis. The results further Results suggest that the three genes function at a common step of age-dependent and dark-induced senescence processes.Isolation of Arabidopsis mutants with delayed leaf

MOR1 is a member of the MAP215 family of microtubule-associated proteins and is required to establish interphase cortical microtubule arrays in plant cells.1 Here we show that MOR1 binds microtubules in vivo, localising to both cortical microtubules and to areas of overlapping microtubules in the phragmoplast. We demonstrate an essential role for MOR1 in cytokinesis through genetic complementation of the cytokinesis defective gem1-1 mutation with MOR1. Phenotypic analysis of gem1-1 and a T-DNA insertion allele gem1-2 confirm that MOR1/GEM1 is essential for regular patterns of cytokinesis. Both mutations, gem1-1 and gem1-2, cause the truncation of MOR1/GEM1. Moreover, the C-terminal domain absent in both mutants binds microtubules in vitro. These data demonstrate that MOR1/GEM1 plays an essential role in the cytokinetic phragmoplast. TextIn general microtubules form four distinct microtubule assemblies sequentially through the plant cell cycle. The interphase cortical array is involved in cell expansion. The preprophase band of microtubules delineates the plane of cell division. The spindle separates daughter chromosomes, and the phragmoplast, which forms in late anaphase, guides golgi-derived vesicles to a site where they will fuse to form the new cell plate that separates the daughter cells. This pattern of alternating arrays is reflected in every cell division in the plant, with the exception of meiotic cell divisions and subsequent gametophytic and endosperm mitoses, all of which lack a preprophase band. As in animal cells microtubule associated proteins must govern the organisation of these microtubule arrays. Three classes of plant structural MAP have been identitied two of which are unique to plants, MAP-652 and MAP1903,4, and the third is a homologue of Xenopus MAP215, named MOR11,5.The MAP215 family also includes human Ch-TOGp6, Dictyostelium discoideum DdCP2247, Drosophila melanogaster Msps8, Caenorhabditis elegans Zyg-99,

Toc75 (translocon at the outer envelope membrane of chloroplasts, 75 kD) is the protein translocation channel at the outer envelope membrane of plastids and was first identified in pea (Pisum sativum) using biochemical approaches. The Arabidopsis (Arabidopsis thaliana) genome contains three Toc75-related sequences, termed atTOC75-I, atTOC75-III, and atTOC75-IV, which we studied using a range of molecular, genetic, and biochemical techniques. Expression of atTOC75-III is strongly regulated and at its highest level in young, rapidly expanding tissues. By contrast, atTOC75-IV is expressed uniformly throughout development and at a much lower level than atTOC75-III. The third sequence, atTOC75-I, is a pseudogene that is not expressed due to a gypsy/Ty3 transposon insertion in exon 1, and numerous nonsense, frame-shift, and splicejunction mutations. The expressed genes, atTOC75-III and atTOC75-IV, both encode integral envelope membrane proteins. Unlike atToc75-III, the smaller atToc75-IV protein is not processed upon targeting to the envelope, and its insertion does not require ATP at high concentrations. The atTOC75-III gene is essential for viability, since homozygous atToc75-III knockout mutants (termed toc75-III) could not be identified, and aborted seeds were observed at a frequency of approximately 25% in the siliques of self-pollinated toc75-III heterozygotes. Homozygous toc75-III embryos were found to abort at the two-cell stage. Homozygous atToc75-IV knockout plants (termed toc75-IV) displayed no obvious visible phenotypes. However, structural abnormalities were observed in the etioplasts of toc75-IV seedlings and atTOC75-IV overexpressing lines, and toc75-IV plants were less efficient at deetiolation than wild type. These results suggest some role for atToc75-IV during growth in the dark.The majority of plastid proteins are translated on cytosolic ribosomes and subsequently imported into plastids (Keegstra and Cline, 1999;Chen et al., 2000;Hiltbrunner et al., 2001a;Jarvis and Soll, 2001;Jarvis and Robinson, 2004). An amino-terminal transit peptide directs each of these proteins specifically to the plastid. Upon arrival in the stroma, the transit peptide is cleaved and the mature protein is either folded into its final conformation or targeted to another compartment of the plastid (Keegstra and Cline, 1999;Jarvis and Robinson, 2004). Preproteins are translocated through the plastid double membrane envelope by two membrane-bound protein complexes: the translocon at the outer envelope membrane of chloroplasts (Toc), and the translocon at the inner envelope membrane of chloroplasts (Tic).Components of the Toc complex include Toc34, Toc75, and Toc159, which were first identified in pea (Pisum sativum) using biochemical approaches (Hirsch et al., 1994;Perry and Keegstra, 1994;Schnell et al., 1994;Seedorf et al., 1995;Tranel et al., 1995). These three proteins make up the core Toc complex, which was recently characterized with respect to structure and component stoichiometry (Schleiff et al., 2003b). Toc34 and Toc1...

a small crucifer Arabidopsis thaliana has emerged as the Vitaly Citovsky* plant of choice for most genetic and developmental studies.