RESUMO
Rhizomatosae is a taxonomic section of the South American genus Arachis, whose diagnostic character is the presence of rhizomes in all its species. This section is of particular evolutionary interest because it has three polyploid (A. pseudovillosa, A. nitida and A. glabrata, 2n = 4x = 40) and only one diploid (A. burkartii, 2n = 2x = 20) species. The phylogenetic relationships of these species as well as the polyploidy nature and the origin of the tetraploids are still controversial. The present study provides an exhaustive analysis of the karyotypes of all rhizomatous species and six closely related diploid species of the sections Erectoides and Procumbentes by cytogenetic mapping of DAPI/CMA heterochromatin bands and 5S and 18-26S rDNA loci. Chromosome banding showed variation in the DAPI heterochromatin distribution pattern, which, together with the number and distribution of rDNA loci, allowed the characterization of all species studied here. The bulk of chromosomal markers suggest that the three rhizomatous tetraploid species constitute a natural group and may have at least one common diploid ancestor. The cytogenetic data of the diploid species analyzed evidenced that the only rhizomatous diploid species-A. burkartii-has a karyotype pattern different from those of the rhizomatous tetraploids, showing that it is not likely the genome donor of the tetraploids and the non-monophyletic nature of the section Rhizomatosae. Thus, the tetraploid species should be excluded from the R genome, which should remain exclusively for A. burkartii. Instead, the karyotype features of these tetraploids are compatible with those of different species of the sections Erectoides and Procumbentes (E genome species), suggesting the hypothesis of multiple origins of these tetraploids. In addition, the polyploid nature and the group of diploid species closer to the tetraploids are discussed.
Assuntos
Arachis/genética , Genoma de Planta/genética , Heterocromatina/genética , Arachis/citologia , Evolução Biológica , Mapeamento Cromossômico , Cromossomos de Plantas/genética , DNA de Plantas/genética , DNA Ribossômico/genética , Diploide , Cariótipo , Cariotipagem , Filogenia , Poliploidia , TetraploidiaRESUMO
Nod factors are among the best-studied molecules implicated in the signal exchange that leads to legume-rhizobia symbiosis. The role of these molecules in symbiosis development has been primarily studied in legumes invaded through infection threads. In these plants, Nod factors generate several responses required for nodulation, including the induction of cortical cell division to form the nodule primordium. Arachis hypogaea L. (peanut) exhibits a specific mode of rhizobial infection and nodule morphogenetic programme in which infection threads are never formed. The role of Nod factors in this particular mechanism is unknown. In this work, a peanut symbiont mutant strain unable to produce Nod factors was obtained and characterised. The strain Bradyrhizobium (Arachis) sp. SEMIA 6144 V2 is altered in the nodC gene, which encodes an N-acetylglucosaminyl transferase involved in the first step of the Nod factor biosynthetic pathway. Further research revealed that, although its ability to colonise peanut roots was unaffected, it is not capable of inducing the division of cortical cells. The results obtained indicate that rhizobial Nod factors are essential for the induction of cortical cell division that leads to nodule primordium formation.
Assuntos
Arachis/crescimento & desenvolvimento , Arachis/microbiologia , Bradyrhizobium/fisiologia , Lipopolissacarídeos/metabolismo , Arachis/citologia , Arachis/genética , Proteínas de Bactérias/biossíntese , Proteínas de Bactérias/genética , Proteínas de Bactérias/metabolismo , Divisão Celular/genética , Divisão Celular/fisiologia , Morfogênese/genética , Morfogênese/fisiologia , Nodulação/genética , Nodulação/fisiologia , Raízes de Plantas/genética , Raízes de Plantas/crescimento & desenvolvimento , Raízes de Plantas/microbiologia , Nódulos Radiculares de Plantas/crescimento & desenvolvimento , Transdução de Sinais/genética , Transdução de Sinais/fisiologia , SimbioseRESUMO
BACKGROUND AND AIMS: Polyploidy is a dominant feature of flowering-plant genomes, including those of many important crop species. Arachis is a largely diploid genus with just four polyploid species. Two of them are economically important: the cultivated peanut and A. glabrata, a tropical forage crop. Even though it is usually accepted that polyploids within papilionoid legumes have arisen via hybridization and further chromosome doubling, it has been recently suggested that peanut arose through bilateral sexual polyploidization. In this paper, the polyploid nature of the recent, spontaneously originated triploid cytotype of the tropical lucerne, A. pintoi, was analysed, and thereby the mechanism by which polyploids may arise in the genus. METHODS: Chromosome morphology of 2x and 3x A. pintoi was determined by the Feulgens technique and the rDNA sites were mapped by FISH. To investigate whether polyploidization occurred by means of unreduced gametes, a detailed analysis of the microsporogenesis and pollen grains was made. KEY RESULTS: The 2x and 3x plants presented 9m + 1sm and a satellited chromosome type 2 in each haploid genome. Physical mapping revealed a cluster of 18S-26S rDNA, proximally located on chromosome 6, and two 5S rDNA loci on chromosomes 3 and 5. Diploid plants presented 10II in meiosis while trivalents were observed in all triploids, with a maximum of 10III by cell. Diploid A. pintoi produced normal tetrads, but also triads, dyads and monads. Two types of pollen grains were detected: (1) normal-sized with a prolate shape and (2) large ones with a tetrahedral morphology. CONCLUSIONS: Karyotype and meiotic analysis demonstrate that the 3x clone of A. pintoi arose by autopolyploidy. The occurrence of unreduced gametes strongly supports unilateral sexual polyploidization as the most probable mechanism that could have led to the origin of the triploid cytotype. This mechanism of polyploidization would probably be one of the most important mechanisms involved in the origin of economically important species of Arachis, either by triploid bridge or bilateral sexual polyploidization.
Assuntos
Arachis/genética , Evolução Biológica , Genoma de Planta/genética , Meiose , Triploidia , Arachis/citologia , Arachis/fisiologia , Pareamento Cromossômico , Cromossomos de Plantas/genética , DNA Ribossômico/genética , Células Germinativas Vegetais/fisiologia , Hibridização Genética , Hibridização in Situ Fluorescente , Cariotipagem , Meiose/genética , Microscopia Eletrônica de Varredura , Pólen/fisiologia , Pólen/ultraestrutura , Poliploidia , Corantes de RosanilinaRESUMO
The potential role of the fungi, isolated from the peanut rhizosphere, in the production of extracellular and intracellular acid and alkaline phosphatase, was evaluated in vitro. Acid and alkaline extracellular phosphatases showed the highest activities, and the Penicillium species were the most efficient in their production. The correlation analysis showed that extracellular acid and intracellular acid phosphatase produced by Aspergillus niger A. terreus, Penicillium sp. y P. brevicompactum were negatively correlated; while the extracellular and intracellular phosphatase activities, were positively correlated. The extracellular acid phosphatase activities produced in vitro by majority of fungi assayed, were not correlated with the acid phosphatase activity present in the peanut soil rhizosphere. Nevertheless, the extracellular alkaline phosphatase activities produced in vitro, were negatively correlated with the extracellular alkaline phosphatase activities present in the rhizosphere. The ability of phosphatase production by fungi isolated from peanut rhizosphere suggests they have great potential to contribute to the P mineralization in this zone.