Characterization of four defense-related genes up-regulated in root nodules of Casuarina glauca

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)

Casuarina glauca displays high levels of salt tolerance, but very little is known about how this tree adapts to saline conditions. To understand the molecular basis of C. glauca response to salt stress, we have analyzed the proteome from branchlets of plants nodulated by nitrogen-fixing Frankia Thr bacteria (NOD+) and non-nodulated plants supplied with KNO3 (KNO3+), exposed to 0, 200, 400, and 600 mM NaCl. Proteins were identified by Short Gel, Long Gradient Liquid Chromatography coupled to Tandem Mass Spectrometry and quantified by Sequential Window Acquisition of All Theoretical Mass Spectra -Mass Spectrometry. 600 proteins were identified and 357 quantified. Differentially Expressed Proteins (DEPs) were multifunctional and mainly involved in Carbohydrate Metabolism, Cellular Processes, and Environmental Information Processing. The number of DEPs increased gradually with stress severity: (i) from 7 (200 mM NaCl) to 40 (600 mM NaCl) in KNO3+; and (ii) from 6 (200 mM NaCl) to 23 (600 mM NaCl) in NOD+. Protein-protein interaction analysis identified different interacting proteins involved in general metabolic pathways as well as in the biosynthesis of secondary metabolites with different response networks related to salt stress. Salt tolerance in C. glauca is related to a moderate impact on the photosynthetic machinery (one of the first and most important stress targets) as well as to an enhancement of the antioxidant status that maintains cellular homeostasis.

Is salt stress tolerance in Casuarina glauca Sieb. ex Spreng. associated with its nitrogen-fixing root-nodule symbiosis? An analysis at the photosynthetic level

Casuarina glauca is an actinorhizal tree which establishes root-nodule symbiosis with N2-fixing Frankia bacteria. This plant is commonly found in saline zones and is widely used to remediate marginal soils and prevent desertification. The nature of its ability to survive in extreme environments and the extent of Frankia contribution to stress tolerance remain unknown. Thus, we evaluated the ability of C. glauca to cope with salt stress and the influence of the symbiosis on this trait. To this end, we analysed the impact of salt on plant growth, mineral contents, water relations, photosynthetic-related parameters and non-structural sugars in nodulated vs. non-nodulated plants. Although the effects on photosynthesis and stomatal conductance started to become measurable in the presence of 200 mM NaCl, photochemical (e.g., photosynthetic electron flow) and biochemical (e.g., activity of photosynthetic enzymes) parameters were only strongly impaired when NaCl levels reached 600 mM. These results indicate the maintenance of high tissue hydration under salt stress, probably associated with enhanced osmotic potential. Furthermore, the maintenance of photosynthetic assimilation potential (A(max)), together with the increase in the quantum yield of down-regulated energy dissipation of PSII (Y(NPQ)), suggested a down-regulation of photosynthesis instead of photo-damaging effects. A comparison of the impact of increasing NaCl levels on the activities of photosynthetic (RubisCO and ribulose-5 phosphate kinase) and respiratory (pyruvate kinase and NADH-dependent malate dehydrogenase) enzymes vs. photosynthetic electron flow and fluorescence parameters, revealed that biochemical impairments are more limiting than photochemical damage. Altogether, these results indicate that, under controlled conditions, C. glauca tolerates high NaCl levels and that this capacity is linked to photosynthetic adjustments.

Actinorhizal plant defence-related genes in response to symbiotic Frankia

Actinorhizal plants have become increasingly important as climate changes threaten to remake the global landscape over the next decades. These plants are able to grow in nutrient-poor and disturbed soils, and are important elements in plant communities worldwide. Besides that, most actinorhizal plants are capable of high rates of nitrogen fixation due to their capacity to establish root nodule symbiosis with N2-fixing Frankia strains. Nodulation is a developmental process that requires a sequence of highly coordinated events. One of these mechanisms is the induction of defence-related events, whose precise role in a symbiotic interaction remains to be elucidated. This review summarises what is known about the induction of actinorhizal defence-related genes in response to symbiotic Frankia and their putative function during symbiosis.