Actinorhizal nitrogen fixing nodules: infection process, molecular biology and genomics

Actinorhizal hosts are non-leguminous perennial plants belonging to 8 angiosperm families. They are capable of forming root nodules as a result of infection by a nitrogen-fixing actinomycete called Frankia. Actinorhizal nodules consist of multiple lobes, each of which represents a modified lateral root with infected cells in the expanded cortex. This article summarizes the most recent knowledge about this original symbiotic process. The infection process is described both at cytological and molecular levels. The use of transgenic Casuarinaceae for studying in actinorhizal nodules the regulation of several symbiotic promoters from legumes is also discussed. With progress in plant genome sequencing, comparative genomics in legumes and actinorhizal plants should contribute to the understanding of the evolutionary history of nitrogen-fixing symbioses. Key words : Nitrogen-fixation, actinorhizal nodules, Frankia, Casuarina, symbiotic gene. African Journal of Biotechnology Vol. 2 (12), pp. 528-538, December 200

Symbiotic Performance of Diverse Frankia Strains on Salt-Stressed Casuarina glauca and Casuarina equisetifolia Plants

Symbiotic nitrogen-fixing associations between Casuarina trees and the actinobacteria Frankia are widely used in agroforestry in particular for salinized land reclamation. The aim of this study was to analyze the effects of salinity on the establishment of the actinorhizal symbiosis between C. glauca and two contrasting Frankia strains (salt sensitive; CcI3 vs. salt tolerant; CeD) and the role of these isolates in the salt tolerance of C. glauca and C. equisetifolia plants. We show that the number of root nodules decreased with increasing salinity levels in both plants inoculated with CcI3 and CeD. Nodule formation did not occur in seedlings inoculated with CcI3 and CeD, at NaCl concentrations above 100 and 200 mM, respectively. Salinity also affected the early deformation of plant root hairs and reduced their number and size. In addition, expression of symbiotic marker Cg12 gene, which codes for a subtilase, was reduced at 50 mM NaCl. These data suggest that the reduction of nodulation in C. glauca under salt stress is in part due to inhibition of early mechanisms of infection. We also show that prior inoculation of C. glauca and C. equisetifolia with Frankia strains CcI3 and CeD significantly improved plant height, dry biomass, chlorophyll and proline contents at all levels of salinity tested, depending on the Casuarina-Frankia association. There was no correlation between in vitro salt tolerance of Frankia strains and efficiency in planta under salt-stressed conditions. Our results strongly indicate that increased N nutrition, photosynthesis potential and proline accumulation are important factors responsible for salt tolerance of nodulated C. glauca and C. equisetifolia

Actinorhizal Signaling Molecules: Frankia Root Hair Deforming Factor Shares Properties With NIN Inducing Factor

Actinorhizal plants are able to establish a symbiotic relationship with Frankia bacteria leading to the formation of root nodules. The symbiotic interaction starts with the exchange of symbiotic signals in the soil between the plant and the bacteria. This molecular dialog involves signaling molecules that are responsible for the specific recognition of the plant host and its endosymbiont. Here we studied two factors potentially involved in signaling between Frankia casuarinae and its actinorhizal host Casuarina glauca: (1) the Root Hair Deforming Factor (CgRHDF) detected using a test based on the characteristic deformation of C. glauca root hairs inoculated with F. casuarinae and (2) a NIN activating factor (CgNINA) which is able to activate the expression of CgNIN, a symbiotic gene expressed during preinfection stages of root hair development. We showed that CgRHDF and CgNINA corresponded to small thermoresistant molecules. Both factors were also hydrophilic and resistant to a chitinase digestion indicating structural differences from rhizobial Nod factors (NFs) or mycorrhizal Myc-LCOs. We also investigated the presence of CgNINA and CgRHDF in 16 Frankia strains representative of Frankia diversity. High levels of root hair deformation (RHD) and activation of ProCgNIN were detected for Casuarina-infective strains from clade Ic and closely related strains from clade Ia unable to nodulate C. glauca. Lower levels were present for distantly related strains belonging to clade III. No CgRHDF or CgNINA could be detected for Frankia coriariae (Clade II) or for uninfective strains from clade IV

Effets du gaz carbonique sur la biosynthese autocatalytique d'ethylene des avocats (Persea americana, Mill)

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Functional Characterization of the Versatile MYB Gene Family Uncovered Their Important Roles in Plant Development and Responses to Drought and Waterlogging in Sesame

The MYB gene family constitutes one of the largest transcription factors (TFs) modulating various biological processes in plants. Although genome-wide analysis of this gene family has been carried out in some species, only three MYB members have been functionally characterized heretofore in sesame (Sesamum indicum L.). Here, we identified a relatively high number (287) of sesame MYB genes (SIMYBs) with an uncommon overrepresentation of the 1R-subfamily. A total of 95% of SIMYBs was mapped unevenly onto the 16 linkage groups of the sesame genome with 55 SIMYBs tandemly duplicated. In addition, molecular characterization, gene structure, and evolutionary relationships of SIMYBs were established. Based on the close relationship between sesame and Arabidopsis thaliana, we uncovered that the functions of SIMYBs are highly diverse. A total of 65% of SIMYBs were commonly detected in five tissues, suggesting that they represent key TFs modulating sesame growth and development. Moreover, we found that SIMYBs regulate sesame responses to drought and waterlogging, which highlights the potential of SIMYBs towards improving stress tolerance in sesame. This work presents a comprehensive picture of the MYB gene family in sesame and paves the way for further functional validation of the members of this versatile gene family

Analysis of the expression pattern conferred by the PsEnod12B promoter from the early nodulin gene of Pisum sativum in transgenic actinorhizal trees of the Casuarinaceae family

To investigate the similarities between nitrogen-fixing symbioses in Rhizobium-legumes and Frankia-actinorhizal plants, we studied the gus expression pattern conferred by the early nodulin promoter PsEnod12B from Pisum sativum in two transgenic Casuarinaceae trees, Allocasuarina verticillata and Casuarina glauca. Respectively 6 and 13 transgenic lines of C. glauca and A. verticillata were obtained following genetic transformation with the disarmed strain of Agrobacterium tumefaciens C58C1 (pGV2260; pBIN-PsEnod12B-gus). In non-symbiotic tissues, the gus gene was expressed exclusively in the vascular system of the aerial parts of the plants. In mature nodules, PsEnod12B drove some gus expression in large Frankia-infected cortical cells. This specificity was different from that observed in pea nodules where PsEnod12B was found expressed in the infection zone. No reporter gene activity was visible at the early stages of the symbiotic process with Frankia, including infected root hairs and prenodules. Whereas some transcription factors of Allocasuarina and Casuarina are able to interact with the PsEnod12B promoter, the pattern of expression is modified in nodules of these non-legume heterologous hosts

Tolerance to environmental stress by the nitrogen-fixing actinobacterium <em>Frankia</em> and its role in actinorhizal plants adaptation

International audienceEnvironmental stresses are caused by human activities or natural events. Several of them including salinity, heavy metals, and extreme temperature affect both soil characteristics and plant growth and productivity. Actinorhizal plants are pioneer species that are able to grow in poor soils and improve soil fertility. They are widely used in agroforestry for different purposes including reclamation of degraded and contaminated lands. This capacity is mainly due to the plants forming a nitrogen-fixing symbiosis with actinobacteria known as Frankia. In comparison to uninoculated plants, plants in symbiosis with Frankia have significantly improved plant growth, total biomass, and nitrogen and chlorophyll content which enhance the development of actinorhizal plants and their resistance to abiotic stresses. However, to optimize the adaptation of actinorhizal species to different environments, selection of both symbiotic partners is necessary. Frankia strains vary in their sensitivity and response to stress including salinity, heavy metals, extreme pH and drought. In this paper, we review the response of different Frankia strains to environmental stresses and their role that they play in the adaptation of actinorhizal plants to stressful conditions