The rhizobial adhesion protein RapA1 is involved in adsorption of rhizobia to plant roots but not in nodulation

Abstract

The effect of the rhizobium adhesion protein RapA1 on Rhizobium leguminosarum bv. trifolii adsorption to Trifolium pratense (red clover) roots was investigated. We altered RapA1 production by cloning its encoding gene under the plac promoter into the stable vector pHC60. After introducing this plasmid in R. leguminosarum bv. trifolii, three to four times more RapA1 was produced, and two to five times higher adsorption to red clover roots was obtained, as compared with results for the empty vector. Enhanced adsorption was also observed on soybean and alfalfa roots, not related to R. leguminosarum cross inoculation groups. Although the presence of 1 mM Ca2+ during rhizobial growth enhanced adsorption, it was unrelated to RapA1 level. Similar effects were obtained when the same plasmid was introduced in Rhizobium etli for its adsorption to bean roots. Although root colonization by the RapA1-overproducing strain was also higher, nodulation was not enhanced. In addition, in vitro biofilm formation was similar to the wild-type both on polar and on hydrophobic surfaces. These results suggest that RapA1 receptors are present in root but not on inert surfaces, and that the function of this protein is related to rhizosphere colonization.

RapA2 Is a Calcium-binding Lectin Composed of Two Highly Conserved Cadherin-like Domains That Specifically Recognize Rhizobium leguminosarum Acidic Exopolysaccharides

“…gation and adhesion of R. leguminosarum cells (6,8). Due to the expansion of the sequence databases, it is now clear that the Ra domains are conserved far beyond rhizobia (9).…”

“…Therefore, it was argued that the surface receptor of RapA1 may be some structure of the acidic EPS. RapA1 overproduction increased the attachment of R. leguminosarum R200 cells to clover roots (8), suggesting its involvement in colonization or biofilm formation.…”

Rhizobial communities in symbiosis with legumes: genetic diversity, competition and interactions with host plants

“…Concerning the differences in the availability of particular substrates in the soil versus an endosymbiotic environment [75][76][77][78][79], it is possible that such `environmental diversification` is one of the reasons for maintaining a considerable metabolic differentiation within local rhizobial populations [52]. Moreover, rhizobial competitiveness may be affected by some bacterial traits involved with bacterial motility [80], which is essential for rhizobial migration towards legume roots, bacterial susceptibility to plant molecular signals [81][82][83], essential in chemotaxis and the recognition between a compatible plant host and its microsymbionts and the production of adhesins responsible for adsorption of rhizobia to plant roots [84,85]. The importance of abiotic properties of the soil environment, such as salinity or pH, should also be pointed out.…”

“…Total global BNF is estimated at 100-300 Tg N per annum, and the amount of N 2 derived from rhizobial-legume symbioses Tg N per annum) is comparable with the total amount of N fixed industrially in fertilizer production (80)(81)(82)(83)(84)(85)(86)(87)(88)(89)(90) Tg N per annum) [1,2]. Even though crop, pasture and wild legumes assimilate and accumulate BNF-derived nitrogen to differing levels [3], in most cases, symbiotically reduced N 2 covers more than half of total nitrogen requirements of a plant [2,4].…”

Mutation of praR in Rhizobium leguminosarum enhances root biofilms, improving nodulation competitiveness by increased expression of attachment proteins

“…These Rap proteins are calcium-binding lectins containing cadherin-like domains that bind to the acidic exopolysaccharide (Abdian et al ., 2013), which is essential for attachment both in vitro and to root hairs (Russo et al ., 2006; Williams et al ., 2008). Increased expression of one of these proteins, RapA1, enhanced attachment to roots and increased infection competitiveness (Mongiardini et al ., 2008; 2009). Cellulose fibrils play a role in biofilm growth (called cap formation) on root hairs after initial attachment, although a mutant unable to produce cellulose nodulated normally and apparently was not reduced for nodulation competitiveness (Smit et al ., 1987; Laus et al ., 2005; Williams et al ., 2008).…”