Since the tetraploidization of the Arabidopsis thaliana ancestor 30-35 million years ago (Mya), a wave of chromosomal rearrangements have modified its genome architecture. The dynamics of this process is unknown, as it has so far been impossible to date individual rearrangement events. In this paper, we present evidence demonstrating that the majority of rearrangements occurred before the Arabidopsis-Brassica split 20-24 Mya, and that the segmental architecture of the A. thaliana genome is predominantly conserved in Brassica. This finding is based on the conservation of four rearrangement breakpoints analysed by fluorescence in situ hybridization (FISH) and RFLP mapping of three A. thaliana chromosomal regions. For this purpose, 95 Arabidopsis bacterial artificial chromosomes (BACs) spanning a total of 8.25 Mb and 81 genetic loci for 36 marker genes were studied in the Brassica oleracea genome. All the regions under study were triplicated in the B. oleracea genome, confirming the hypothesis of Brassica ancestral genome triplication. However, whilst one of the breakpoints was conserved at one locus, it was not at the two others. Further comparison of their organization may indicate that the evolution of the hexaploid Brassica progenitor proceeded by several events, separated in time. Genetic mapping and reprobing with rDNA allowed assignment of the regions to particular Brassica chromosomes. Based on this study of regional organization and evolution, a new insight into polyploidization/diploidization cycles is proposed.

Spring drought can adversely affect the productivity of barley (Hordeum vulgare L.) by reducing the yield. Because seed osmopriming can enhance crop productivity, we examined the potential of CaCl 2 treatment to improve drought tolerance in spring barley. Initially, we applied the priming procedure (5, 50, and 500 mM) to caryopses and assessed its effectiveness using a routine germination test, followed by measuring the level of divalent cations. Since drought adaptation is a complex phenomenon, we tested a comprehensive set of physiological parameters including (1) relative water content (RWC), (2) gas exchange parameters, and (3) photosynthetic pigments concentration in leaves of 3-week-old plants developed from the seeds subjected to osmopriming, followed by exposure to increasing water shortage. The plants were sampled at two selected time points, determined by soil moisture retention (pF = 3.6 and 4.2). The effect of CaCl 2 pretreatment was characterized in three distinct spring barley varieties, which differed in their response to drought stress (drought-tolerant Sebastian and Cam/B1/C1 and drought-susceptible Georgie), to assess potential interactions between osmopriming and genetically determined drought tolerance. Our results clearly demonstrate that CaCl 2 priming improves drought tolerance in stress-tolerant as well as drought-susceptible barley cultivars. Furthermore, we show that the beneficial effects of calcium preconditioning interact significantly with genetically determined drought tolerance.

Expressed sequence tags (ESTs) from the Arabidopsis thaliana sequencing project were used to construct a genetic RFLP map for Brassica oleracea. Of the 110 A. thaliana ESTs tested, 95 were found to be informative RFLP probes in map construction. In total, 212 new loci corresponding to the 95 ESTs were added to the existing genetic map of B. oleracea. The enriched map covers all nine basic linkage groups and confirms that the chromosomes of B. oleracea and A. thaliana are similar in linear organization. However, varying levels of sequence conservation between the chromosomes of B. oleracea and A. thaliana were detected in different regions of the genomes. Long conserved regions encompassing entire chromosome arms in both genomes were identified; these are probably shared by descent. On the other hand, extensive rearrangements were observed in numerous chromosome regions, producing a mosaic of A. thaliana -like segments in the genome of Brassica. The presence of extensive chromosome duplication in A. thaliana was taken into consideration in the construction of the comparative maps of B. oleracea and A. thaliana.