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

Abstract

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.

New Insights on Plant Salt Tolerance Mechanisms and Their Potential Use for Breeding

“…Similarly to what occurs in legume–rhizobia symbioses, it has been reported that inoculation with the microsymbiont Frankia improves the host plant salinity tolerance (Reddell et al, 1986; Ng, 1987; Oliveira et al, 2005; Ngom et al, 2016a). Frankia strains CcI3 and CeD significantly improved Casuarina glauca and Casuarina equisetifolia plant growth, shoot, root, and total dry weight, proline and chlorophyll contents according to the symbiotic association (Ngom et al, 2016a). According to Ng (1987), inoculated C. equisetifolia plants exhibited greater growth (shoot, root, and total dry weight) compared to uninoculated plants under saline conditions.…”

“…Among three Frankia isolates used separately as inoculums of C. glauca , Thr which was more in vitro sensitive to salt stress, was the most effective strain in planta . A recent study showed that inoculation of C. glauca plants with the salt-sensitive CcI3 strain improved plants growth under saline conditions while in C. equisetifolia plants the salt-tolerant strain CeD was more effective (Ngom et al, 2016a). These results suggest there is no correlation between in vitro salt tolerance of Frankia strains and their effectiveness in association with plants under salt stressed conditions (Girgis et al, 1992; Ngom et al, 2016a).…”

“…A recent study showed that inoculation of C. glauca plants with the salt-sensitive CcI3 strain improved plants growth under saline conditions while in C. equisetifolia plants the salt-tolerant strain CeD was more effective (Ngom et al, 2016a). These results suggest there is no correlation between in vitro salt tolerance of Frankia strains and their effectiveness in association with plants under salt stressed conditions (Girgis et al, 1992; Ngom et al, 2016a). Thus, as what was observed in Rhizobium –legumes symbiosis, effectiveness of established actinorhizal symbioses to saline conditions is primary dependent on the salt tolerance of the host plant (Girgis et al, 1992).…”

Comparative Proteomic Analysis of Nodulated and Non-Nodulated Casuarina glauca Sieb. ex Spreng. Grown under Salinity Conditions Using Sequential Window Acquisition of All Theoretical Mass Spectra (SWATH-MS)

“…In fact, preliminary growth tests of free-living Frankia Thr in the presence of increasing NaCl concentrations indicated that this strain was indeed salt tolerant, maintaining a normal growth curve at least up to 600 mM NaCl (unpublished data). Also, studies developed by other research groups revealed that despite the fact that salt stress response is rather diverse among Frankia strains in vitro [26], a similar impact of salt stress was observed in C. glauca plants nodulated by "salt tolerant" and "salt sensitive" strains, respectively [9]. In short, the level of salt tolerance in vitro did not predict the level of salt tolerance in planta.…”

Improvement of tree growth in salt-affected soils under greenhouse conditions using a combination of peanut shells and microbial inoculation

“…The role of biofertilizers such as rhizobia (Diouf et al, 2005;Thrall et al, 2008), Frankia (Diagne et al, 2013;Ngom et al, 2016), arbuscular mycorrhizal fungi (Evelin et al, 2009;Kohler et al, 2010) and plant growth-promoting rhizobacteria (Paul and Nair, 2008;Kohler et al, 2010) in plants tolerating salt stress has been well established in many studies. The bacterialmycorrhizal-legume tripartite symbiosis is currently being suggested as a possible solution to reforestation (Kohler et al, 2010 ;Diagne et al, 2013;Soliman et al, 2014;Ngom et al, 2016;Zhu et al, 2016).…”