plant surface microbiology.pdf

6 Signalling in the Rhizobia–Legumes Symbioses 113
channel. The long search for the molecular identification of the gene responsible
for the regulation of nodule number on the host plants (supernodulation)
was successful. Characterized were a receptor like kinase with the HAR
1 gene (Krusell et al. 2002; Nishimura et al. 2002) and the GmNARK gene from
soybeans, a CLAVATA 1 like receptor kinase (Searle et al. 2002). HAR 1 and
NARK are the same genes from different species, as summarized by Downie
and Parniske (2002).
Besides Nod factors, rhizobia also excrete other components relevant for
the symbiosis development, e.g., type III secretion systems (TTSSs). Rhizobial
TTSS clusters contain an open reading frame, homologous to ysc and hrc
genes (Bogdanove et al. 1996; Hueck 1998; Marie et al. 2001). Two proteins
have been identified in Rhizobium NGR234, secreted in a TTSS dependent
way: nolX and y4xL.In Bradyrhizobium japonicum a new two-component system,
ElmS and ElmR, has recently been identified, coding for a putative regulator
protein and a putative sensor histidine kinase with unknown functions
(Mühlencoert and Müller 2002). In agreement with the results from cytological
observations in the chapter of F. Dazzo (see Chap 27, this Vol.), the population
density-dependent expression of Bradyrhizobium japonicum nodulation
genes has been reported (Loh et al. 2002). Induction of nod genes is high
at low culture densities and is repressed at high population densities. The
expression of NolA and NodD2 was mediated by an extracellular factor
excreted into the medium. Two rhizobia species with a very broad host range
are Rhizobium strain NGR234 and Rhizobium fredii USDA257 (Pueppke and
Broughton 1999). They nodulate a wide range of mimosoid legumes, especially
Acacia species and also the nonlegume Parasponia andersonii. In a few
cases, only Rhizobium fredii USDA257 could nodulate some host plants such
as Glycine max and Glycine soja. The most important result was that there is
no relationship between the origin of the host plants and the ability of the
strains to nodulate specific host plant species. The strain NGR234 shows significant
dynamics of the genome architecture (Mavingui et al. 2002) with a
large-scale DNA rearrangement, cointegration and excision exist between the
three replicons, the symbiotic plasmids, the megaplasmid and the chromosome.
Going from the laboratory to the field under natural conditions, especially
in agricultural soils, we must realize that there are not only a large number of
soil, microbial and plant factors involved, but nowadays also a large number
of agrochemicals present in small quantities on seeds and also on emerging
roots (Johnsen et al. 2001). Pesticide effects on specific populations of soil
bacteria have been demonstrated for Rhizobium species (Ramos and Ribeiro
1993) and with nitrifying bacteria (Martinez-Toledo et al. 1992). The degradation
of these pesticides by the microbial communities in soils is another relevant
aspect, contributing to the complexity of effects of molecules on the plant
surface microbial interaction (Soulas and Lagacherie 2001). A large number
of resistance genes in Rhizobium species is a strategy to deal with many