Abstract
Background. During the colonization of root and nodule tissues of legumes by rhizobia, bacterial cells are immersed in a plant extracellular matrix which includes arabinogalactan protein-extensins (AGPE).
 Materials and methods. Immunogold electron microscopy with monoclonal antibodies MAC204 and MAC236 was used to analyse the distribution and abundance of epitopes of AGPE in wild-type and symbiotically defective pea mutants.
 Results. In the nodules of the wild-type line SGE, both AGPE epitopes were detected to the same extent in the matrix of infection threads and infection droplets. In the nodules of the mutant line SGEFix-1 (sym40), the level of labelling by MAC204 was significantly higher than with SGE in both infection threads and infection droplets, but the level of labelling by MAC236 was only increased in the infection droplets. In the mutant line SGEFix-2 (sym33-3), a relatively high level of both epitopes was observed among all analysed genotypes. The double mutant line RBT3 (sym33-3, sym40) showed an intermediate level of labelling for both epitopes in infection threads compared with the parental mutants. In SGEFix-1, an abnormal distribution of both epitopes was observed in the intercellular space matrix. The MAC204 epitope was found in the cell walls of SGEFix-1 and in the infection thread walls of SGEFix-2, whereas in RBT3 this epitope was detected in both types of walls.
 Conclusions. The sym33-3 and sym40 mutations have different effects on the accumulation of AGPE epitopes recognised by MAC204 and MAC236. This indicates that both the Sym33 and the Sym40 genes affect the composition of AGPE in the matrix of infection threads and infection droplets.
Highlights
Colonization of plant cells of legumes with rhizobial cells is accompanied by a progressive remodelling of the plant-microbial interface, leading to symbiotic nitrogen fixation within root nodules
In the nodules of the wild-type SGE line, immunolabelling with MAC204 and MAC236 was observed in infection threads (Fig. 1, a, d) and infection droplets (Fig. 1, b, e)
We have previously studied the distribution of arabinogalactan protein-extensin (AGPE), recognised by monoclonal antibody MAC265, in the nodules of the wild-type line SGE and in the ineffective mutants SGEFix–-1 and SGEFix–-2 and in double mutants carrying the sym33-3 allele [8]
Summary
Colonization of plant cells of legumes with rhizobial cells is accompanied by a progressive remodelling of the plant-microbial interface, leading to symbiotic nitrogen fixation within root nodules. Because of the very high content of tyrosine residues, it has been proposed that AGPE macromolecules may become cross-linked and insolubilized under oxidative and peroxide-rich conditions This cross-linking might serve to regulate the growth of the infection thread itself as a result of a progressive fluid-to-solid transition in the embedding matrix of AGPE [1, 7, 9, 10]. In the nodules of the wild-type line SGE, both AGPE epitopes were detected to the same extent in the matrix of infection threads and infection droplets. The sym and sym mutations have different effects on the accumulation of AGPE epitopes recognised by MAC204 and MAC236 This indicates that both the Sym and the Sym genes affect the composition of AGPE in the matrix of infection threads and infection droplets
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