Nicotianamine synthase specifically expressed in root nodules of Lotus japonicus

Abstract

In dicotyledonous plants, nicotianamine synthase (NAS) is thought to play a role in the intercellular transport of iron (Fe). Fe is an essential metal for nitrogen-fixing root nodules of legumes, prompting us to characterize the role of the NAS gene in detail. We previously compared gene-expression profiles in ineffective nodules formed on a Lotus japonicus Fix(-) mutant, sen1, with those in wild-type-effective nodules, and showed that expression of an expressed sequence tag (EST) clone encoding an NAS (EC 2.5.1.43) homologue was repressed in the ineffective nodules. In the present study, two EST clones encoding NAS homologues were found in the EST database. We named them LjNAS1 and LjNAS2. Both were detected as single-copy genes in the L. japonicus genome, and conferred NAS activities in transformed Saccharomyces cerevisiae. LjNAS2 was expressed only in nodules, but LjNAS1 was expressed mainly in leaves, stems, and cotyledons. The level of LjNAS2 transcripts was highest in the nodules 24 days after inoculation with Mesorhizobium loti, and was localized in vascular bundles within the nodules. Expression of LjNAS2 was suppressed in ineffective nodules formed on Fix(-) mutants other than sen1. By contrast, nitrogenase activities of nodules were not influenced in LjNAS2-suppressed plants. We discuss the role of LjNAS2 from the aspect of Fe translocation in nodules.

Iron: an essential micronutrient for the legume-rhizobium symbiosis

“…A protein potentially involved in the redistribution of nodule iron is L. japonicas NA synthase 2 (LjNAS2; Hakoyama et al, 2009). NA synthase catalyses the formation of NA, a phytosiderophore precursor that is present in all plants and forms complexes with a range of metals including Fe(II) and Fe(III) (Curie et al, 2009).…”

“…NA synthase catalyses the formation of NA, a phytosiderophore precursor that is present in all plants and forms complexes with a range of metals including Fe(II) and Fe(III) (Curie et al, 2009). LjNAS2 is expressed in nodule vascular bundles, is nodule specific and its expression reaches a maximum at 24 DAI (Hakoyama et al, 2009). Suppression of LjNAS2 expression specifically by RNAi silencing did not have an affect on nitrogen fixation, suggesting that LjNAS2 is not involved in iron supply to the nodule (Hakoyama et al, 2009).…”

Fixating on metals: new insights into the role of metals in nodulation and symbiotic nitrogen fixation

“…Burton et al (1998) estimated that around 50% of the total nodular iron is recycled in the seed, in a process that is likely to be reminiscent of leaf senescence (Shi et al, 2012). Although no senescence-upregulated metal transporter has been identified, a senescent nodule-specific nicotianamine (NA) synthase has been cloned (Hakoyama et al, 2009). The synthesis of NA, the molecule responsible for intracellular and phloem metal transport (Curie et al, 2009), indicates that the released metals are transported within the phloem using an unknown Yellow Stripe-like (YSL) transporter, since YSLs are responsible for NA-metal transport (Curie et al, 2009).…”

Metal Transport in the Rhizobium-Legume Symbiosis

“…As stated earlier, YSLs are involved in long-distance metal traffi cking in which the metal substrate forms complexes with nicotianamine (DiDonato et al 2004). Although no YSL transporter has as yet been identifi ed in nodules, the presence of the nodule-specifi c senescence-induced nicotianamine synthase LjNAS2 indicates that this mechanism should be in place (Hakoyama et al 2009). In this process of recycling metals, ferritins would also participate.…”