MsPG3 , a Medicago sativa polygalacturonase gene expressed during the alfalfa– Rhizobium meliloti interaction

Abstract

Polygalacturonase (PG) is one of the most important enzymes associated with plant cell wall degradation. It has been proposed to participate in the early steps of the Rhizobium-legume interaction. We have identified two classes of cDNA fragments corresponding to two classes of PG genes in the Medicago genome. One of this class, represented by E 2 in M. truncatula and Pl 1 in M. sativa, seems to be related to previously characterized plant PG genes expressed in pollen. We have isolated the genomic clone containing the entire gene corresponding to the second class (E 3 ). We showed that MsPG3 is a single gene in the Medicago genome coding for PG. By reverse transcription-PCR, MsPG3 expression was detected in roots 1 day after Rhizobium inoculation. The early induction of the MsPG3, as also seen by in situ hybridization experiments, supports its involvement in the early stages of the Rhizobium-legume infection process. In addition, by analyzing the expression of a MsPG3 promoter-gus construct in Vicia hirsuta-transgenic root nodules, we showed that MsPG3 was expressed in all cells of nodule primordia and in the cells of the invasion zone. By Northern blot, MsPG3 transcripts are not detected in various Medicago tissues, indicating that the function of this gene is related closely to symbiosis. Thus, our results strongly suggest the involvement of MsPG3 gene during meristem formation and͞or in the infection process, probably by facilitating cell wall rearrangement, penetration of the bacteria through the root hair wall, or infection thread formation and release of bacteria in plant cells. MsPG3 represents a class of PG genes, distinct from the pollenspecific genes, and it is the first pectic encoded enzyme demonstrated to be involved in Rhizobium-legume symbiosis.

Rhizobium etli HrpW is a pectin-degrading enzyme and differs from phytopathogenic homologues in enzymically crucial tryptophan and glycine residues

“…Cell wall degradation is essential to processes such as root hair initiation, fruit ripening, pollen tube elongation and leaf abscission. In addition, symbiosis-specific expression of polygalacturonases and pectin methylesterases was observed during the early steps of the S. meliloti-Medicago interaction (Munoz et al, 1998;Rodriguez-Llorente et al, 2004). So, instead of degrading plant cell walls themselves, rhizobia might induce the expression of degradative host enzymes to the same effect, and a deficiency such as that caused by mutation of hrpW could be masked.…”

Physiological roles of plant glycoside hydrolases

“…It has been proposed that some cell wall-degrading enzymes could be involved in the early steps of Rhizobiumlegume interactions (Muñoz et al 1998;Rodriguez-Llorente et al 2004). It is probable that these enzymes facilitate penetration by hydrolysing the cell wall of bacteria in plant cells.…”

Infection and Invasion of Roots by Symbiotic, Nitrogen-Fixing Rhizobia during Nodulation of Temperate Legumes

“…It was recently shown that an alfalfa polygalacturonase gene (MsPG3) was induced specifically in roots in response to S. meliloti (111). The gene was induced as early as 1 day after spot inoculation, and high levels of expression of this gene were seen in the nodule meristem, infection zone, and interzone II-III of nodules.…”