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Rhizosphere bacterial diversity and community structure are important factors involving in plant growth. However, the exact process of how plant rhizosphere bacterial community structures is assembled remains unclear. To investigate the role of bacterial chemotaxis to rhizosphere secretions in the establishment of rhizosphere microbial community in Casuarina equisetifolia , we screened bacteria strains derived from the rhizosphere of Casuarina equisetifolia L. using top three chemicals of the plant root exudates (2,4-di-tert-butylphenol, methyl stearate, and arginine) as chemoattractant. Among 72 bacterial strains, five showed strong chemotaxis to 2,4-di-tert-butylphenol, six to methyl stearate, and eleven to arginine, with the highest bacterial chemotaxis occurring at a concentration of 60 μM. This indicates that arginine is a more important chemoattractant than 2,4-di-tert-butylphenol, methyl stearate in the establishment of rhizosphere microbial community in Casuarina equisetifolia . Bacterial community assembly analysis using different chemoattractants and chemoattractants-plus-bacteria combinations were then performed by burying laboratory prepared bags of sterlized soil into C. equisetifolia forest. Bacteria diversity and enrichment analyses using 16S rDNA sequencing at 7 and 14 days after burying showed that arginine-plus- Ochrobactrum sp. and Pantoea sp. treatment exhibited the greatest similarity to the natural forest bacterial community. Our date provides new insights into how chemoattractants and chemotactic bacteria strains shape the rhizosphere microbial community of C. equisetifolia , which constitutes foundational information for future management of these communities.

Casuarina trees are planted along the coast from Hainan province in South China to the Zhoushan Islands of Zhejiang province in Southeastern China. Three key species, Casuarina equisetifolia, Casuarina cunninghamiana and Casuarina glauca, are used as windbreaks, in agroforestry systems, and for the production of timber and fuel wood. Frankia have been studied in China since 1984. Today, Frankia research fields are very wide, and cover morphology, physiology and genetic diversity, and the application of inocula for specific purposes on poor quality sites. In this paper, we review the role of Frankia inoculations in nurseries and casuarina plantations in China and discuss the benefits of inoculation.

* Although it is now well-established that decorated lipo-chitooligosaccharide Nod factors are the key rhizobial signals which initiate infection/nodulation in host legume species, the identity of the equivalent microbial signaling molecules in the Frankia/actinorhizal association remains elusive. * With the objective of identifying Frankia symbiotic factors we present a novel approach based on both molecular and cellular pre-infection reporters expressed in the model actinorhizal species Casuarina glauca. * By introducing the nuclear-localized cameleon Nup-YC2.1 into Casuarina glauca we show that cell-free culture supernatants of the compatible Frankia CcI3 strain are able to elicit sustained high frequency Ca super(2+) spiking in host root hairs. Furthermore, an excellent correlation exists between the triggering of nuclear Ca super(2+) spiking and the transcriptional activation of the ProCgNIN:GFP reporter as a function of the Frankia strain tested. These two pre-infection symbiotic responses have been used in combination to show that the signal molecules present in the Frankia CcI3 supernatant are hydrophilic, of low molecular weight and resistant to chitinase degradation. * In conclusion, the biologically active symbiotic signals secreted by Frankia appear to be chemically distinct from the currently known chitin-based rhizobial/arbuscular mycorrhizal signaling molecules. Convenient bioassays in Casuarina glauca are now available for their full characterization.

Casuarina glauca is an actinorhizal plant that establishes root-nodule symbiosis with N sub(2)-fixing bacteria of the genus Frankia. This plant is highly recalcitrant to extreme environmental conditions such as salinity and drought. The aim of this study was to evaluate the impact of salt stress on the symbiotic relationship between C. glauca and Frankia Thr, focusing on N and C metabolism. Symbiotic and non-symbiotic plants were exposed to 0, 200, 400 and 600 mM NaCl. The following analyses were performed: stable carbon ( delta super(13)C) and nitrogen ( delta super(15)N) isotope signature; nitrogenase activity in nodules (acetylene reduction assay); and gene expression of a set of genes involved in nodule infection and N/C metabolism (qRT-PCR). Data were analysed using two-way ANOVA. Salt stress induced an enrichment in delta super(13)C and delta super(15)N, reflecting a negative impact of salt in the relative water content and N sub(2) fixation, respectively. Furthermore, nitrogenase activity in nodules was insignificant already at 200 mM NaCl, consistent with the expression patterns of nifH as well as of plant genes involved in nodule induction and metabolism. The ability of C. glauca to thrive under highly saline environments is not dependent on the symbiosis with Frankia.

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.

NAC (NAM, ATAF and CUC)-like transcription factors, a class of plant-specific transcription factors, play a pivotal role in plant growth, development, metabolism, and stress response. Notably, a specific subclass of NAC family, known as SNAC (stress-responsive NAC), is particularly involved in the plant’s response to abiotic stress. As a very useful tree, Casuarina equisetifolia L. also has excellent stress resistance properties. To explore gene resources of C. equisetifolia which are associated with stress resistance and the molecular mechanisms that it employed is very helpful to its molecular-assisted breeding. In this study, 10 CeSNAC transcription factors were identified by constructing the phylogenetic tree of 94 CeNACs from the genome of C. equisetifolia L. together with 79 SNAC in different plant species. Phylogenetic tree analysis revealed that these 10 CeSNAC genes are classified into the ATAF (Arabidopsis transcription activation factor), NAP (NAC-like, activated by AP3/P1), and AtNAC3 subfamilies of the NAC family, all featuring the typical NAM (no apical meristem) domain, with the exception of CeSNAC7. In addition, all NAC transcription factors, except CeSNAC9, were localized in the nucleus. Examination of the CeSNAC promoter unveiled the presence of stress response elements such as a STRE (stress responsive element), an MBS (MYB binding site), an ABRE (abscisic acid responsive element) and a LTR (low temperature responsive element). Under various stress treatments, the majority of CeSNAC expressions exhibited induction in response to low temperature, drought, and high salt treatments, as well as ABA (abscisic acid) treatment. However, CeSNAC6, CeSNAC7, and CeSNAC9 were found to be inhibited specifically by drought treatment. Additionally, only CeSNAC3 and CeNAC9 expression was hindered while the rest of the CeSNAC expression were induced by MeJA (methyl jasmonate) treatment. These findings shed light on the relationship between different CeSNAC genes and their responses to abiotic stress conditions, providing valuable insights for further research into CeSNAC functions and aiding the development of stress-resistant varieties in C. equisetifolia.

Although it is now well‐established that decorated lipo‐chitooligosaccharide Nod factors are the key rhizobial signals which initiate infection/nodulation in host legume species, the identity of the equivalent microbial signaling molecules in the Frankia/actinorhizal association remains elusive. With the objective of identifying Frankia symbiotic factors we present a novel approach based on both molecular and cellular pre‐infection reporters expressed in the model actinorhizal species Casuarina glauca. By introducing the nuclear‐localized cameleon Nup‐YC2.1 into Casuarina glauca we show that cell‐free culture supernatants of the compatible Frankia CcI3 strain are able to elicit sustained high frequency Ca²⁺ spiking in host root hairs. Furthermore, an excellent correlation exists between the triggering of nuclear Ca²⁺ spiking and the transcriptional activation of the ProCgNIN:GFP reporter as a function of the Frankia strain tested. These two pre‐infection symbiotic responses have been used in combination to show that the signal molecules present in the Frankia CcI3 supernatant are hydrophilic, of low molecular weight and resistant to chitinase degradation. In conclusion, the biologically active symbiotic signals secreted by Frankia appear to be chemically distinct from the currently known chitin‐based rhizobial/arbuscular mycorrhizal signaling molecules. Convenient bioassays in Casuarina glauca are now available for their full characterization.

Land salinization is a major constraint for the practice of agriculture in the world. Considering the extent of this phenomenon, the rehabilitation of ecosystems degraded by salinization has become a priority to guarantee food security in semi-arid environments. The mechanical and chemical approaches for rehabilitating salt-affected soils being expensive, an alternative approach is to develop and utilize biological systems utilizing salt-tolerant plant species. Casuarina species are naturally halotolerant, but this tolerance has been shown to be improved when they are inoculated with arbuscular mycorrhizal fungi (AMF) and/or nitrogen-fixing bacteria (Frankia). Furthermore, Casuarina plantations have been proposed to promote the development of plant diversity. Thus, the aim of the current study was to evaluate the impact of a plantation comprising the species Casuarina inoculated with AMF and Frankia on the diversity of the sub-canopy and adjacent vegetation. Work was conducted on a plantation comprising Casurina equisetifolia and C. glauca variously inoculated with Frankia and Rhizophagus fasciculatus prior to field planting. The experimental area of 2500 m2 was divided into randomized blocks and vegetation sampling was conducted below and outside of the Casuarina canopy in 32 m2 plots. A total of 48 samples were taken annually over 3 years, with 24 taken from below the Casuarina canopy and 24 from outside the canopy. The results obtained show that co-inoculation with Frankia and Rhizophagus fasciculatus improves the height and survival rate of both species. After 4–5 years, there was greater species diversity and plant biomass in the sub-canopy environment compared with that of the adjacent environments. Our results suggest that inoculation of beneficial microbes can improve growth of Casuarina species and that planting of such species can improve the diversity of herbaceous vegetation in saline environments.

Background and Aims The commonly observed tradeoff between plant water use efficiency (WUE) and nitrogen use efficiency (NUE) has been attributed to physiological constraints in the leaf. We examined if a similar trade-off can occur between WUE and phosphorus use efficiency (PUE) and if changes in NUE and PUE in response to water and nutrient supply can be related to microbial N and P immobilisation. Methods We studied water and nutrient use efficiencies in leaves of four tree species (Eucalyptus sideroxylon, Eucalyptus tereticornis, Casuarina cunninghamiana, and Pinus radiata) that were grown under rainout shelters for 16 months at low and high levels of water and nutrient supply. Results Across all four species, WUE increased when water supply was low and nutrient supply was high, while NUE increased when water supply was high and nutrient supply was low. As a consequence, a trade-off was found between WUE and NUE for all four species. In contrast, no trade-off was found between WUE and PUE, likely because PUE and microbial Ñ immobilisation in the soil unexpectedly increased with high nutrient supply. Conclusions With variable water and nutrient supply, physiological constraints generate a trade-off between WUE and NUE, but not between WUE and PUE; the latter may have been obscured by microbial control over P supply to plants.

Root endosymbioses vitally contribute to plant nutrition and fitness worldwide. Nitrogen-fixing root nodulation, confined to four plant orders, encompasses two distinct types of associations, the interaction of legumes (Fabales) with rhizobia bacteria and actinorhizal symbioses, where the bacterial symbionts are actinomycetes of the genus Frankia. Although several genetic components of the host-symbiont interaction have been identified in legumes, the genetic basis of actinorhiza formation is unknown. Here, we show that the receptor-like kinase gene SymRK, which is required for nodulation in legumes, is also necessary for actinorhiza formation in the tree Casuarina glauca. This indicates that both types of nodulation symbiosis share genetic components. Like several other legume genes involved in the interaction with rhizobia, SymRK is also required for the interaction with arbuscular mycorrhiza (AM) fungi. We show that SymRK is involved in AM formation in C. glauca as well and can restore both nodulation and AM symbioses in a Lotus japonicus symrk mutant. Taken together, our results demonstrate that SymRK functions as a vital component of the genetic basis for both plant-fungal and plant-bacterial endosymbioses and is conserved between legumes and actinorhiza-forming Fagales.