Flowering plants possess a unique reproductive strategy, involving double fertilization by twin sperm cells. Unlike animal germ lines, the male germ cell lineage in plants only forms after meiosis and involves asymmetric division of haploid microspores, to produce a large, non-germline vegetative cell and a germ cell that undergoes one further division to produce the twin sperm cells. Although this switch in cell cycle control is critical for sperm cell production and delivery, the underlying molecular mechanisms are unknown. Here we identify a novel F-box protein of Arabidopsis thaliana, designated FBL17 (F-box-like 17), that enables this switch by targeting the degradation of cyclin-dependent kinase A;1 inhibitors specifically in male germ cells. We show that FBL17 is transiently expressed in the male germ line after asymmetric division and forms an SKP1-Cullin1-F-box protein (SCF) E3 ubiquitin ligase complex (SCF(FBL17)) that targets the cyclin-dependent kinase inhibitors KRP6 and KRP7 for proteasome-dependent degradation. Accordingly, the loss of FBL17 function leads to the stabilization of KRP6 and inhibition of germ cell cycle progression. Our results identify SCF(FBL17) as an essential male germ cell proliferation complex that promotes twin sperm cell production and double fertilization in flowering plants.

We have characterized the structure and expression of a senescence-associated gene (sen1) of Arabidopsis thaliana. The protein-coding region of the gene consists of 5 exons encoding 182 amino acids. The encoded peptide shows noticeable similarity to the bacterial sulfide dehydrogenase and 81% identity to the peptide encoded by the radish din1 gene. The 5'-upstream region contains sequence motifs resembling the heat-shock- and ABA-responsive elements and the TCA motif conserved among stress-inducible genes. Examination of the expression patterns of the sen1 gene under various senescing conditions along with measurements of photochemical efficiency and of chlorophyll content revealed that the sen1 gene expression is associated with Arabidopsis leaf senescence. During the normal growth phase, the gene is strongly induced in leaves at 25 days after germination when inflorescence stems are 2-3 cm high, and then the mRNA level is maintained at a comparable level in naturally senescing leaves. In addition, dark-induced senescence of detached leaves or of leaves in planta resulted in a high-level induction of the gene. Expression of the sen1 gene was also strongly induced in leaves subjected to senescence by 0.1mM abscisic acid or 1 mM ethephon treatment. The induced expression of the gene by dark treatment was not significantly repressed by treatment with 0.1 mM cytokinin or 50 mM CaCl2 which delayed loss of chlorophyll but not that of photochemical efficiency.

Senescence is a sequence of biochemical and physiological events that constitute the final stage of development. The identification of genes that alter senescence has practical value and is helpful in revealing pathways that influence senescence. However, the genetic mechanisms of senescence are largely unknown. The leaf of the oresara9 (ore9) mutant of Arabidopsis exhibits increased longevity during age-dependent natural senescence by delaying the onset of various senescence symptoms. It also displays delayed senescence symptoms during hormone-modulated senescence. Map-based cloning of ORE9 identified a 693-amino acid polypeptide containing an F-box motif and 18 leucinerich repeats. The F-box motif of ORE9 interacts with ASK1 (Arabidopsis Skp1-like 1), a component of the plant SCF complex. These results suggest that ORE9 functions to limit leaf longevity by removing, through ubiquitin-dependent proteolysis, target proteins that are required to delay the leaf senescence program in Arabidopsis.

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