H2O2 is required for optimal establishment of the Medicago sativa/Sinorhizobium meliloti symbiosis

Abstract

The symbiotic interaction between Medicago sativa and Sinorhizobium meliloti RmkatB(++) overexpressing the housekeeping catalase katB is delayed, and this delay is combined with an enlargement of infection threads. This result provides evidence that H(2)O(2) is required for optimal progression of infection threads through the root hairs and plant cell layers.

FIG. 1.

Catalase activity of Sinorhizobium meliloti strains: Rm1021 (lane 1) and RmkatB⁺⁺ (lane 2) in exponential (A) and stationary (B) phases. The locations of KatA, KatB, and KatC are indicated according to the work of Sigaud et al. (29). Twenty-five micrograms of total protein was loaded on each lane.

FIG. 2.

Number of infection events 5 days after infection with Rm1021 and RmkatB⁺⁺ strains. Experimental data were assessed for statistical significance by means of the Student t test; * indicates significant difference (P < 0.05). Abbreviations: CCRH, colonized curled root hair; IIT, initiated IT; EIT, extended ITs; YN, young nodule; WT, wild type.

FIG. 3.

Structure of ITs in 5-day-old nodules. Nodules were induced by the Rm1021 (A and C) and RmkatB⁺⁺ (B and D) strains. Black arrows show ITs, and open arrowheads show nuclei. Scale bars = 10 μm.

FIG. 4.

Detection of H₂O₂ accumulation as electron-dense precipitate formed in the presence of cerium chloride (arrows) in ITs 5 days after infection with Rm1021 (A and B) and RmkatB⁺⁺ (C and D). (A, C, and D) Transversal cross section of ITs. (B) Longitudinal cross-section showing the release of bacteria inside the plant cell. Magnitudes of pictures are indicated with scale bars.