IN INOCULANTS Nodulaid - 17th International Nitrogen Fixation ...

17 th International Congress on Nitrogen Fixation
Fremantle, Western Australia
27 November – 1 December 2011
Session Details: Monday 28 November 2011
Plenary Session 1
1100 - 1230
Authors: Fang Xie, Giulia Morieri, Jeremy Murray, Anne L. Heckmann, Anne Edwards, Jiyoung Kim,
Giles E. D. Oldroyd and J. Allan Downie
John Innes Centre Norwich Research Park, Colney, Norwich, NR4 7UH, UK
Presentation Title: Characterisation of a legume pectate lyase required for initiation and growth of infection
threads during nodule infection by rhizobia
Presentation Time: 1100 - 1130
It has long been recognised that nodule infection by rhizobia requires localised degradation of plant cell walls to
enable infection threads to initiate and grow. In root hair infection foci, the cell wall is locally degraded enabling
penetration by the bacteria, and this is coupled with the synthesis of a new infection thread wall, but relatively
little is known about the proteins required for these processes. A microarray analysis identified Medicago
truncatula genes more strongly induced by Nod factors from WT than a (nodL) mutant of Sinorhizobium meliloti,
which is reduced for infection. Among the genes identified was a predicted pectate lyase. In parallel work we
identified two allelic Lotus japonicus mutants, which are defective for infection thread growth and nodule
infection. Fine mapping indicated that the mutation was located in a region of about 38 genes. Analysis of the
equivalent region of M. truncatula showed that the pectate lyase gene identified from the transcriptomics was
one of these 38 genes. Sequencing of the orthologous gene from the two L. japonicus mutants identified different
mutations in each, and the cloned WT allele complemented the mutant phenotype. Therefore the mutations in
this pectate lyase gene caused the infection defect. In vitro biochemical assays with the purified WT and mutant
protein demonstrated that the gene encodes a plant pectinase and we conclude that this activity contributes to
the cell-wall degradation that is required to initiate infection thread growth in root hairs. It also appears to be
required for subsequent extension of the infection thread across cell-cell junctions. The pectate lyase gene is
induced following Nod-factor activation of the common symbiosis pathway and requires the transcriptional
regulators NIN and NSP1 for its normal induction. Chromosome immunoprecipitation and promoter binding
experiments indicated that the pectate lyase gene is directly regulated by NIN. These results show that there is
specific Nod-factor induction of genes that play a role in initiating root infection by rhizobia.
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2011