SummaryAlthough the developmental programs of plants and animals differ, key regulatory components of their cell cycle have been conserved. Particular attention has been paid to the role of the complexes between highly conserved cyclin and cyclin-dependent kinases in regulating progression through the cell cycle. The recent demonstration that roscovitine is a potent and selective inhibitor of the animal cyclin-dependent kinases cdc2 (CDK1), CDK2 and CDK5 prompted an investigation into its effects on progression through the plant cell cycle. Roscovitine induced arrests both in late G1 and late G2 phase in BY-2 tobacco cell suspensions. Both blocks were fully reversible when roscovitine was used at concentrations similar to those used in the animal system. Stationary-phase cells subcultured in the presence of roscovitine were arrested at a 2C DNA content. This arrest was more efficient without exogenous addition of plant growth regulator. Roscovitine induced a block in G1 earlier than that induced by aphidicolin. S-phase synchronized cells treated with roscovitine were arrested at a 4C DNA content at the G2/ M transition. The expression analysis of a mitotic cyclin (NTCYCl) indicated that the roscovitine-induced G2 block probably occurs in late G2. Finally, cells in metaphase were insensitive to roscovitine. The purified CDK/cyclin kinase activities of late G1 and early M arrested cells were inhibited in vitro by roscovitine. The implications of these

Membrane-associated and soluble lipoxygenases from green tomato (Lycopersicon esculentum Mill. cv Ailsa Craig) fruit have been identified. Microsomal lipoxygenase was localized partly in the plasma membrane and tonoplast fractions. The possibilities of glycosyl-phosphatidylinositol or transmembrane polypeptide anchors in the membrane were ruled out by differential solubilization and temperature-induced phase separation in Triton X-114. High performance liquid chromatography of reaction products combined with polarography showed that tomato lipoxygenase is capable of specific oxygenation of fatty acids esterified in phospholipids. This possibility of direct action on membrane phospholipids strengthened the hypothesis of a role for lipoxygenase in plant senescence and membrane turnover. Membrane-associated lipoxygenase is polymorphic, with two forms differing by their isoelectric points (pis) (around 4.2 and 5.1). The p i of the soluble lipoxygenase corresponds to the minor microsomal enzyme, with a p i of 5.1. The charge-differing isoforms were separated and analyzed by western blotting using anti-soybean lipoxygenase antibodies. A single polypeptide with an apparent molecular weight of 92,000 was identified in each case for the soluble and microsomal enzymes. It is suggested that a charge modification of the soluble lipoxygenase allows its association with the membrane.Plant lipoxygenases (EC 1.13.1 1.12) are non-heme ironcontaining enzymes that catalyze the hydroperoxidation of polyunsaturated fatty acids containing a cis,cis-1,4-pentadiene system. Much of the characterization of plant lipoxygenases has focused on the soybean enzymes. Three isozymes, differing with respect to their PIS, each consisting of a single polypeptide of approximate mol wt 95,000, have been identified. Sequences of full-length cDNAs have been determined quite recently for soybean and some other plant enzymes, allowing identification of marked similarities between the corresponding derived amino acid sequences. A large region of sequence similarity contains a hydrophobic portion, although most plant lipoxygenases are soluble (Siedow, 1991, for review).A function has yet to be established conclusively for any plant lipoxygenase, and putative roles in growth, develop-

A membrane-associated lipoxygenase and a soluble lipoxygenase have been identified in camation (Dianthus caryophyllus L. cv Reve) petals. Treatments of microsomal membranes by nonionic or zwitterionic detergents indicated that lipoxygenase is tightly bound to the membranes. By phase separation in Triton X-114, microsomal lipoxygenase can be identified in part as an integral membrane protein. Soluble lipoxygenase had an optimum pH range of 4.9 to 5.8, whereas microsomal lipoxygenase exhibited maximum activity at pH 6.1. Both soluble and membraneassociated lipoxygenases produced carbonyl compounds and hydroperoxides simultaneously, in the presence of oxygen. The membranous enzyme was fully inhibited by 0.1 millimolar n-propyl gallate, nordihydroguaiaretic acid, or salicylhydroxamic acid, but the effect of the three inhibitors on the soluble enzyme was much lower. The soluble lipoxygenase is polymorphic and three isoforms greatly differing by their isoelectric points were identified. Lipoxygenase activity in flowers was maximal at the beginning of withering, both in the microsomal and the soluble fractions. Substantial variations in the ratio of the two forms of lipoxygenase were noted at different sampling dates. Our results allowed us to formulate the hypothesis of a strong association of one soluble form with defined membrane constituents.Lipoxygenases (EC 1. 13.11.12), commonly found in higher plants, catalyze the hydroperoxidation of polyunsaturated lipids to various primary and secondary oxidation products. Among characterized isoforms, similarities at the molecular level, high homologies, and cross-reactivities have been reported, suggesting a common evolutionary pathway of lipoxygenases in plants and animals (7). However, different isoforms of lipoxygenases can be distinguished on the basis of their reaction pH optima, protein isoelectric points, positional specificity of catalysis, or aerobic production of secondary products.Fat-oxidizing factors, like lipoxygenase, have been shown to be involved in processes linked to aging in various animal tissues and in processed vegetable products (14). Current theories on the physiological roles of plant lipoxygenases suggest involvement in such degradative processes as senescence, wounding, and infection, all of which include membrane breakdown (6, 16). Linoleic and linolenic acids, which are the principal substrates for lipoxygenases, are also major constituents of the phospholipid plant membrane component (14). Many plant lipoxygenases clearly show little reactivity toward esterified fatty acids, such as triglycerides and phospholipids (6). The release of free fatty acids catalyzed by phospholipases thus appears to be a probable prerequisite to the action of most plant lipoxygenases. Until recently, very little was known about the subcellular localization of the enzyme. Nonspecific absorption of lipoxygenase to membranous fractions was supposed to explain the difficulties encountered in the localization studies (16). However, the main cytosolic location of...

The nuclear DNA content of 28 taxa of Musa was assessed by flow cytometry, using line PxPC6 of Petunia hybrida as an internal standard. The 2C DNA value of Musa balbisiana (BB genome) was 1.16 pg, whereas Musa acuminata (AA genome) had an average 2C DNA value of 1.27 pg, with a difference of 11% between its subspecies. The two haploid (IC) genomes, A and B, comprising most of the edible bananas, are therefore of similar size, 0.63 pg (610 million bp) and 0.58 pg (560 million bp), respectively. The genome of diploid Musa is thus threefold that of Arabidopsis thaliana. The genome sizes in a set of triploid Musa cultivars or clones were quite different, with 2C DNA values ranging from 1.61 to 2.23 pg. Likewise, the genome sizes of tetraploid cultivars ranged from 1.94 to 2.37 pg (2C). Apparently, tetraploids (for instance, accession I.C.2) can have a genome size that falls within the range of triploid genome sizes, and vice versa (as in the case of accession Simili Radjah). The 2C values estimated for organs such as leaf, leaf sheath, rhizome, and flower were consistent, whereas root material gave atypical results, owing to browning. The genomic base composition of these Musa taxa had a median value of 40.8% GC (SD = 0.43%).