Plant Physiology Preview
Published on April 27, 2007; 10.1104/pp.107.097063
Received February 1, 2007
Accepted April 19, 2007
TILLING Mutants of Lotus japonicus Reveal that Nitrogen Assimilation and Fixation can Occur in the Absence of Nodule-enhanced Sucrose Synthase
Irmtraud Horst , Tracey Welham , Simon Kelly , Takakazu Kaneko , Shusei Sato , Satoshi Tabata , Martin Parniske , and Trevor L. Wang *
Metabolic Biology, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK; Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK; Kazusa DNA Research Institute, 2-6-7 Kazusa-kamatari, Kisarazu, Chiba 292-0818, Japan; Dept. Biologie 1, Ludwig-Maximilian-Universität, Maria-Ward-Str. 1a, D-80638 München, Germany
* Corresponding author; email: trevor.wang{at}bbsrc.ac.uk.
In all plant species studied to date, sucrose synthase occurs as multiple isoforms. The specific functions of the different isoforms are for the most part not clear. Six isoforms of sucrose synthase have been identified in the model legume Lotus japonicus (Regel) Larsen - the same number as in Arabidopsis and rice. The genes encoding these isoforms are differentially expressed in all plant organs examined although one, LjSUS4, is only expressed in flowers. LjSUS1 is the most highly expressed in all plant organs tested except root nodules, where LjSUS3 accounts for more than 60% of the total SUS transcripts. One gene, LjSUS2, produces two transcripts due to alternative splicing, a feature not observed in other species to date. We have isolated plants carrying ethyl methane sulfonate (EMS)-induced mutations in several SUS genes by TILLING (Targeting Induced Local Lesions IN Genomes) reverse genetics and examined the effect of null alleles of two genes, LjSUS1 and LjSUS3, on nodule function. No differences were observed between the mutants and wild-type plants under glasshouse conditions but there was evidence for a nitrogen-starvation phenotype in the sus3-1 mutant and a severe impairment of growth in the sus1-1/sus3-1 double mutant under specific environmental conditions. Nodules of sus3-1 mutant plants retained a capacity for nitrogen fixation under all conditions. Thus, nitrogen fixation can occur in L. japonicus nodules even in the absence of LjSUS3, the major nodule-induced isoform of SUS and LjSUS1 must also contribute to the maintenance of nitrogen assimilation.
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