Molecular mechanisms of attachment of Rhizobium bacteria to plant roots

Abstract

Attachment of bacteria to plant cells is one of the earliest steps in many plant-bacterium interactions. This review covers the current knowledge on one of the best-studied examples of bacterium-plant attachment, namely the molecular mechanism by which Rhizobium bacteria adhere to plant roots. Despite differences in several studies with regard to growth conditions of bacteria and plants and to methods used for measuring attachment, an overall consensus can be drawn from the available data. Rhizobial attachment to plant root hairs appears to be a two-step process. A bacterial Ca(2+)-binding protein, designated as rhicadhesin, is involved in direct attachment of bacteria to the surface of the root hair cell. Besides this step, there is another step which results mainly in accumulation and anchoring of the bacteria to the surface of the root hair. This leads to so-called firm attachment. Depending on the growth conditions of the bacteria, the latter step is mediated by plant lectins and/or by bacterial appendages such as cellulose fibrils and fimbriae. The possible role of these adhesions in root nodule formation is discussed.

Systemic Regulation of Soybean Nodulation by Acidic Growth Conditions

“…Low-pH conditions can also negatively impact the growth of plants and rhizobia (Brockwell et al, 1991;Marschner, 1991;Smit et al, 1992;Hungria and Stacey, 1997;Sadowsky et al, 1998;Morón et al, 2005) and can decrease the synthesis of legume flavonoids and rhizobia Nod factor (McKay and Djordjevic, 1993;Hungria and Stacey, 1997). These factors may have also contributed to the down-regulation of early nodulation gene expression observed here (Figs.…”

“…This is especially important considering that nitrate acts through GmNARK in a root-localized fashion to suppress nodule development, and hence, a local mechanism is in place that could have been usurped by acid to exert nodulation control. Indeed, the debilitating effects of low pH on plant and rhizobia health (Brockwell et al, 1991;Marschner, 1991;Sadowsky et al, 1998) and on their symbiotic interactions (Smit et al, 1992;Hungria and Stacey, 1997;Morón et al, 2005) have long been considered local rhizosphere effects induced directly by H + ions. This now appears to be only one component of pH-regulated nodulation control.…”

“…The attachment of rhizobia to legume root hairs requires important Ca 2+ -dependent adhesions (Smit et al, 1992). However, low-pH conditions limit the availability of Ca 2+ in soils, which is suggested to impair rhizobia attachment, infection, and infection thread formation (Caetano-Anollés et al, 1989).…”

Insights into bacterial cellulose biosynthesis by functional metagenomics on Antarctic soil samples

“…Although this production remains negligible compared with plant production, its environmental impact is highly importance. Indeed, the cellulose production by bacteria was associated to plant root and Rhizobium interaction (Smit et al, 1992), to biofilm formation (Branda et al, 2005;Da Re and Ghigo, 2006) and to plant infection by phytopathogens (Matthysse and McMahan, 1998;Rodríguez-Navarro et al, 2007). Our finding gives new insights into a better understanding of these processes that affect both micro and macro ecosystems.…”

A Small GTPase of the Rab Family Is Required for Root Hair Formation and Preinfection Stages of the Common Bean–RhizobiumSymbiotic Association

“…These responses can occur independently, as was shown by the phenotype of different mutants, in which bacterial infection, and related epidermal responses, is observed in the absence of nodule formation and vice versa (Gleason et al, 2006;Tirichine et al, 2006aTirichine et al, , 2006bTirichine et al, , 2007Murray et al, 2007). After the initial chemical communication, the physical association begins with the attachment of the bacteria to the surface of the root through plant lectins and polysaccharides present on the surface of the bacteria (Smit et al, 1992;Diaz et al, 1995). A colony is formed on the surface of the root hair, and the normal polar growth of this cell is arrested in a process that is dependent on Nod factor perception (Esseling et al, 2003).…”