A possible role of class 1 plant hemoglobin at the early stage of legume-rhizobium symbiosis

Abstract

Leguminous plants form root nodules, in which symbiotic rhizobia fix atmospheric nitrogen and supply the fixation products to their host plants as a nitrogen source. On the process of establishing the symbiosis, rhizobia induce genes involved in the defense system of their host plants. However, the host defense responses will be cancelled by unknown mechanism. We focused on nitric oxide (NO) as a key molecule of plant defense system and class 1 plant hemoglobin (Hb) as a scavenger of NO. The inoculation of a symbiotic rhizobium, Mesorhizobium loti MAFF303099, induced transiently NO production and expression of a class 1 Hb gene LjHb1 in the roots of a model legume Lotus japonicus. In this addendum, we show that the lipopolysaccharide of M. loti induces NO production and expression of LjHb1 in L. japonicus, and we propose the role of NO and Hb at the early stage of symbiosis.

Keywords: Lotus japonicus; Mesorhizobium loti; defense; hemoglobin; lipopolysaccharide; nitric oxide; rhizobium; symbiosis.

Figure 1

(A) A schematic representation of a model for the role of NO and LjHb1 at the recognition between plant and its microsymbiont and plant pathogens. (B) LjHb1 expression and (C) NO production by inoculation with crude LPS of M. loti MAFF303099. Control plants were treated with sterilized water. Total RNAs were isolated from whole plants at indicated time after inoculation. Relative amounts of transcripts normalized by control plants are shown. Data represent the average of three independent experiments with standard error. Images of the roots in (C) were taken at 4 h after inoculation by fluorescence microscopy using DAF-FM DA as a NO detector.