The Mediterranean Sea is a two-basin system, with the boundary zone restricted to the Strait of Sicily and the narrow Strait of Messina. Two main population groups are recognized in the Mediterranean endemic seagrass Posidonia oceanica, corresponding to the Western and the Eastern basins. To address the nature of the East-West cleavage in P. oceanica, the main aims of this study were: (i) to define the genetic structure within the potential contact zone (i.e. the Strait of Sicily) and clarify the extent of gene flow between the two population groups, and (ii) to investigate the role of present water circulation patterns vs. past evolutionary events on the observed genetic pattern. To achieve these goals, we utilized SSR markers and we simulated, with respect to current regime, the possible present-day dispersal pattern of Posidonia floating fruits using 28-day numerical Lagrangian trajectories. The results obtained confirm the presence of the two main population groups, without any indices of reproductive isolation, with the break zone located at the level of the Southern tip of Calabria. The populations in the Strait of Sicily showed higher affinity with Western than with Eastern populations. This pattern of genetic structure probably reflects historical avenues of recolonization from relict glacial areas and past vicariance events, but seems to persist as a result of the low connectivity among populations via marine currents, as suggested by our dispersal simulation analysis.

Leaves are specialized organs characterized by defined developmental destiny and determinate growth. The overexpression ofKnotted1-like homeobox genes in different species has been shown to alter leaf shape and development, but a definite role for this class of genes remains to be established. Transgenics that overexpress Knotted1-like genes present some traits that are characteristic of altered cytokinin physiology. Here we show that lettuce (Lactuca sativa) leaves that overexpressKNAT1, an Arabidopsis kn1-like gene, acquire characteristics of indeterminate growth typical of the shoot and that this cell fate change is associated with the accumulation of specific types of cytokinins. The possibility that the phenotypic effects of KNAT1 overexpression may arise primarily from the modulation of local ratios of different cytokinins is discussed.

S L" M M A R YQuiescent centre cells, which are known to have their cycle extended mostly in G, and to divide rarely, are stimulated to undergo DNA synthesis in root meristems of Allium cepa L. by treatment with ascorhic acid. The effect of ascorbic acid is dramatic: 21 "" of all the cells in the quiescent centre remain in the G, phase while 79",, enter S phase. From a mean value of 2204 X l()''//m'' in water-grown roots, the \olume of the quiescent centre drops to a mean value ot 167 x lO' /(m' following treatment with ascorbic acid. These results indicate that ascorbic acid stimulates not only the activity of the quiescent centre cells but also cell proliferation in the entire root meristem. The rate of cell progression througli tlie cycle may be related to cellular ascorbic acid content.

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Administration of 1 mM dehydroascorbate (DHA) results in a rapid and large increase in cellular ascorbate (AA) content in both Lupinus albus L. and Allium cepa L. root tips. Uptake of DHA from the medium occurs at a high rate within 10-12 h of incubation, and is slowed down thereafter. In the first few h, DHA reduction to AA is apparently correlated to GSH depletion and slightly higher DHA reductase activity. DHA incubation also seems to induce new GSH synthesis. Longer DHA incubation (24 h) affects root growth by inhibiting cell proliferation. At this stage, an apparently generalised oxidation of SH-containing proteins is observed in DHA-treated roots. Treatment with 1 mM L-galactono-gamma-lactone, the last precursor of AA biosynthesis, results in an increase in AA content similar to that obtained with DHA, but stimulates growth and affects the redox state of SH-containing proteins in the opposite way. A possible multi-step mechanism of DHA reduction/removal is suggested and the hypothesis that DHA inhibits cell cycle progression by affecting the redox state of SH-containing proteins is discussed.

The stability on the expression level of putative reference genes (RGs) for reverse transcriptase-quantitative PCR (RT-qPCR) was tested in the seagrass Posidonia oceanica for plants collected in different environmental conditions (at different depths and in different salinity and pH). Besides adult plants, seedlings were also used for pH and salinity experiments. The putative RGs encompassed the most frequently used in other species to normalize RT-qPCR in a range of experimental conditions. Assignment of the best RGs has been performed with three Excelbased applets, BestKeeper, geNorm and NormFinder, and the results showed that best RGs may change depending on the experimental conditions tested, plant age and on the software utilized. Preferred sets of RGs are proposed for each of the conditions and a general procedure concerning their selection is discussed. In addition, expression levels of four genes involved in plant response to light, salinity and pH variation have also been evaluated.

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