Growth Conditions Determine the DNF2 Requirement for Symbiosis

Abstract

Rhizobia and legumes are able to interact in a symbiotic way leading to the development of root nodules. Within nodules, rhizobia fix nitrogen for the benefit of the plant. These interactions are efficient because spectacularly high densities of nitrogen fixing rhizobia are maintained in the plant cells. DNF2, a Medicago truncatula gene has been described as required for nitrogen fixation, bacteroid’s persistence and to prevent defense-like reactions in the nodules. This manuscript shows that a Rhizobium mutant unable to differentiate is not sufficient to trigger defense-like reactions in this organ. Furthermore, we show that the requirement of DNF2 for effective symbiosis can be overcome by permissive growth conditions. The dnf2 knockout mutants grown in vitro on agarose or Phytagel as gelling agents are able to produce nodules fixing nitrogen with the same efficiency as the wild-type. However, when agarose medium is supplemented with the plant defense elicitor ulvan, the dnf2 mutant recovers the fix− phenotype. Together, our data show that plant growth conditions impact the gene requirement for symbiotic nitrogen fixation and suggest that they influence the symbiotic suppression of defense reactions in nodules.

Metabolic profiling of two maize (Zea mays L.) inbred lines inoculated with the nitrogen fixing plant-interacting bacteria Herbaspirillum seropedicae and Azospirillum brasilense

“…The acetylene reduction assay (ARA) was conducted with maize plants cultivated in the gnotobiotic system shown in Figure A (b) in S1 File, using the protocol of Berrabah et al [53]. This system consisted of a glass serum bottle containing 5 plants and sealed with a cotton gauze, allowing aeration.…”

“…For the ARA, 25 mL of acetylene were injected through the rubber stopper with a syringe and 200μl of gas samples were analyzed for ethylene production at 0, 6, 8, and 72 hours after acetylene injection. Ethylene was quantified by gas chromatography as described in Berrabah et al [53], using a 7820A Gas Chromatograph from Agilent Technologies (Massy, France) equipped with a flame ionization detector and a GS-Alumina column (50 m x 0.53 mm) with hydrogen as carrier gas. Column temperature and gas flow rate were 120°C and 7.5 mL min -1 , respectively.…”

A nonRD receptor‐like kinase prevents nodule early senescence and defense‐like reactions during symbiosis

“…The symbiosis is efficient as a result of the chronic and massive bacterial invasion of the nodule cells. Despite the massive colonization of their tissues, plants do not develop defense reactions in response to the endosymbionts (bacteroids) (Berrabah et al, 2014). The relationship between immunity and symbiosis has been investigated before, but the majority of these studies focused on the early step of the symbiosis, that is, the infection process (Zamioudis & Pieterse, 2012).…”

“…We recently described the DNF2 gene that is required for bacteroid differentiation and persistence in M. truncatula (Bourcy et al, 2013b). The nodules of the dnf2 mutant display typical traits, suggesting defense-like reactions such as the induction of defense genes, the accumulation of phenolic compounds and the death of the symbiotic partner (Bourcy et al, 2013a,b;Berrabah et al, 2014). DNF2 encodes a nodule-specific phosphatidylinositol phospholipase C X domain containing protein, but the link between plant defense repression and the biochemical activity of DNF2 has not yet been described.…”

Ohr and OhrR Are Critical for Organic Peroxide Resistance and Symbiosis in Azorhizobium caulinodans ORS571

“…After 20 min, the seedlings were transferred to pots containing sterilized vermiculites and grown in a plant growth chamber at 28 • C [55]. N 2 -fixing activity was measured by the acetylene reduction assay (ARA test), as reported previously [63]. Fifteen nodules per plant were placed into a 20 mL vial sealed by a butyl rubber septum.…”