Changes in chlorophyll fluorescence parameters and oxidative stress responses of bush bean genotypes for selecting contrasting acclimation strategies under water stress

Drought resistance of bean landraces was compared in order to select genotypes with either high morphological or high biochemical-physiological plasticity. The lines in the former group exhibited fast reduction in fresh and dry mass, decreased the water potential in primary leaves after irrigation withdrawal and the biomass mobilized from the senescent primary leaves was allocated into the roots. These genotypes had high frequency of primary leaf abscission under water stress. The genotypes with plasticity at the biochemical level maintained high water potential and photochemical efficiency, i.e. effective quantum yield, high photochemical (qP) and low non-photochemical (NPQ) quenching in primary leaves under drought stress. While superoxide dismutase activity was not influenced by the drought and the genotype, catalase activity increased significantly in the primary leaves of the genotypes with efficient biochemical adaptation. Lines with high morphological plasticity exhibited higher quaiacol peroxidase activity under drought. Proline may accumulate in both cases, thus it may be a symptom of protein degradation or a successful osmotic adaptation. On the basis of contrasting responses, the genetic material cannot be screened for a large-scale breeding program by a single physiological parameter but by a set of the methods presented in this work.

A receptor kinase gene regulating symbiotic nodule development

Leguminous plants are able to establish a nitrogen-fixing symbiosis with soil bacteria generally known as rhizobia. Metabolites exuded by the plant root activate the production of a rhizobial signal molecule, the Nod factor, which is essential for symbiotic nodule development. This lipo-chitooligosaccharide signal is active at femtomolar concentrations, and its structure is correlated with host specificity of symbiosis, suggesting the involvement of a cognate perception system in the plant host. Here we describe the cloning of a gene from Medicago sativa that is essential for Nod-factor perception in alfalfa, and by genetic analogy, in the related legumes Medicago truncatula and Pisum sativum. The identified 'nodulation receptor kinase', NORK, is predicted to function in the Nod-factor perception/transduction system (the NORK system) that initiates a signal cascade leading to nodulation. The family of 'NORK extracellular-sequence-like' (NSL) genes is broadly distributed in the plant kingdom, although their biological function has not been previously ascribed. We suggest that during the evolution of symbiosis an ancestral NSL system was co-opted for transduction of an external ligand, the rhizobial Nod factor, leading to development of the symbiotic root nodule.

A receptor kinase gene regulating symbiotic nodule development

Leguminous plants are able to establish a nitrogen-fixing symbiosis with soil bacteria generally known as rhizobia. Metabolites exuded by the plant root activate the production of a rhizobial signal molecule, the Nod factor, which is essential for symbiotic nodule development. This lipo-chitooligosaccharide signal is active at femtomolar concentrations, and its structure is correlated with host specificity of symbiosis, suggesting the involvement of a cognate perception system in the plant host. Here we describe the cloning of a gene from Medicago sativa that is essential for Nod-factor perception in alfalfa, and by genetic analogy, in the related legumes Medicago truncatula and Pisum sativum. The identified 'nodulation receptor kinase', NORK, is predicted to function in the Nod-factor perception/transduction system (the NORK system) that initiates a signal cascade leading to nodulation. The family of 'NORK extracellular-sequence-like' (NSL) genes is broadly distributed in the plant kingdom, although their biological function has not been previously ascribed. We suggest that during the evolution of symbiosis an ancestral NSL system was co-opted for transduction of an external ligand, the rhizobial Nod factor, leading to development of the symbiotic root nodule.

Genetic mapping of the non-nodulation phenotype of the mutant MN-1008 in tetraploid alfalfa (Medicago sativa)

Abstract. Roots of the non-nodulating Medicago sativa mutant MN-1008 neither undergo root-hair curling, cortical cell division nor any of the early molecular events that accompany nodule initiation and development following rhizobial infection or treatment with Nod factor. These observations suggested that the mutation(s) impaired a pivotal function in Nod factor perception or in the signal transduction pathway. In this paper we show that the genetic lesion conditioning the recessive non-nodulation phenotype in the tetraploid alfalfa mutant MN-1008 can be localized to a single region on LG5 of the M. sativa genetic map. This conclusion is based on genetic analyses conducted at the tetraploid level, involving both segregation analysis and genetic mapping of the trait with respect to molecular DNA markers. The genetic mapping of the Nod(-) phenotype was performed in a segregating tetraploid F2 population, taking advantage of the availability of an advanced genetic map for diploid alfalfa. Two tightly linked flanking markers have been identified which will facilitate the physical mapping and cloning of the gene(s) that underlie(s) the non-nodulation phenotype.