Foundation characteristics of edible Musa triploids revealed from allelic distribution of SSR markers.

BACKGROUND AND AIMS: The production of triploid banana and plantain (Musa spp.) cultivars with improved characteristics (e.g. greater disease resistance or higher yield), while still preserving the main features of current popular cultivars (e.g. taste and cooking quality), remains a major challenge for Musa breeders. In this regard, breeders require a sound knowledge of the lineage of the current sterile triploid cultivars, to select diploid parents that are able to transmit desirable traits, together with a breeding strategy ensuring final triploidization and sterility. Highly polymorphic single sequence repeats (SSRs) are valuable markers for investigating phylogenetic relationships. METHODS: Here, the allelic distribution of each of 22 SSR loci across 561 Musa accessions is analysed. KEY RESULTS AND CONCLUSIONS: We determine the closest diploid progenitors of the triploid 'Cavendish' and 'Gros Michel' subgroups, valuable information for breeding programmes. Nevertheless, in establishing the likely monoclonal origin of the main edible triploid banana subgroups (i.e. 'Cavendish', 'Plantain' and 'Mutika-Lujugira'), we postulated that the huge phenotypic diversity observed within these subgroups did not result from gamete recombination, but rather from epigenetic regulations. This emphasizes the need to investigate the regulatory mechanisms of genome expression on a unique model in the plant kingdom. We also propose experimental standards to compare additional and independent genotyping data for reference.

A new image of plantain diversity assessed by SSR, AFLP and MSAP markers.

Using both SSR and AFLP markers, the genetic diversity of 30 plantains constituting a representative sample of the phenotypic diversity was assessed. The results confirmed a very narrow genetic base of this cultivar group. SSR and AFLP data support the hypothesis that these cultivars may have arisen from vegetative multiplication of a single seed. MSAP were used to survey cytosine methylation status at CCGG sites in order to obtain an alternative source of diversity data. A higher degree of polymorphism was revealed allowing the classification of the samples into three clusters. No correlation was observed between the phenotypic classification and methylation diversity. Implications for breeding programs are discussed.

Common nodABC genes in Nod locus 1 of Azorhizobium caulinodans: nucleotide sequence and plant-inducible expression.

Azorhizobium caulinodans strain ORS571 induces nitrogen-fixing nodules on roots and stem-located root primordia of Sesbania rostrata. Two essential Nod loci have been previously identified in the bacterial genome, one of which (Nod locus 1) shows weak homology with the common nodC gene of Rhizobium meliloti. Here we present the nucleotide sequence of this region and show that it contains three contiguous open reading frames (ORFA, ORFB and ORFC) that are related to the nodABC genes of Rhizobium and Bradyrhizobium species. ORFC is followed by a fourth (ORF4) and probably a fifth (ORF5) open reading frame. ORF4 may be analogous to the nodI gene of R. leguminosarum, whereas ORF5 could be similar to the rhizobial nodF genes. Coordinated expression of this set of five genes seems likely from the sequence organization. There is no typical nod promoter consensus sequence (nod box) in the region upstream of the first gene (ORFA) and there is no nodD-like gene. LacZ fusions constructed with ORFA, ORFB, ORFC, and ORF4 showed inducible beta-galactosidase expression in the presence of S. rostrata seedlings as well as around stem-located root primordia. Among a series of phenolic compounds tested, the flavanone naringenin was the most efficient inducer of the expression of this ORS571 nod gene cluster.