NopP, a phosphorylated effector of Rhizobium sp. strain NGR234, is a major determinant of nodulation of the tropical legumes Flemingia congesta and Tephrosia vogelii

Abstract

SummaryRhizobium sp. NGR234 nodulates many plants, some of which react to proteins secreted via a type three secretion system (T3SS) in a positive-( Flemingia congesta , Tephrosia vogelii ) or negative-( Crotalaria juncea , Pachyrhizus tuberosus ) manner. T3SSs are devices that Gram-negative bacteria use to inject effector proteins into the cytoplasm of eukaryotic cells. The only two rhizobial T3SS effector proteins characterized to date are NopL and NopP of NGR234. NopL can be phosphorylated by plant kinases and we show this to be true for NopP as well. Mutation of nopP leads to a dramatic reduction in nodule numbers on F. congesta and T. vogelii . Concomitant mutation of nopL and nopP further diminishes nodulation capacity to levels that, on T. vogelii , are lower than those produced by the T3SS null mutant NGR W W W W rhcN . We also show that the T3SS of NGR234 secretes at least one additional effector, which remains to be identified. In other words, NGR234 secretes a cocktail of effectors, some of which have positive effects on nodulation of certain plants while others are perceived negatively and block nodulation. NopL and NopP are two components of this mix that extend the ability of NGR234 to nodulate certain legumes.

Hijacking of leguminous nodulation signaling by the rhizobial type III secretion system

“…However, this and other studies show that B. japonicum is unable to nodulate on either En1282 or Clark-rj 1 , suggesting that B. elkanii possesses a unique T3E(s) responsible for activation of nodulation signaling. Although we have not yet detected any B. elkanii unique T3E candidate genes, B. elkanii encodes several genes for T3E candidates, including nopL, nopM, and nopP, which have been reported to play either a positive or a negative role in symbiosis in other rhizobia (9,25). Intriguingly, CCaMK and LHK1, mentioned above, are protein kinases that play pivotal roles in nodulation and should participate in phosphorylation systems (26).…”

“…Intriguingly, CCaMK and LHK1, mentioned above, are protein kinases that play pivotal roles in nodulation and should participate in phosphorylation systems (26). As a number of T3Es in plant pathogens are known to target host kinases (27), and the rhizobial T3Es NopL and NopP are phosphorylated in host cytoplasm (9), it is conceivable that some T3Es perturb the host phosphorylation state for nodulation.…”

“…Disruption of ttsI in B. elkanii was performed using a previously described method (9). Briefly, ttsI-containing fragments were amplified by PCR using primers ttsI_F and ttsI_R (Table S6) from the USDA61 genome.…”

“…frondosus, whereas the number of nodules on the roots of Lotus halophilus significantly increases when inoculated with a T3SS-null mutant (8). Meanwhile, the T3Es NopL and NopP of NGR234 have positive effects on symbiosis with their host legumes (9). Both NopL and NopP are phosphorylated by plant protein kinases, and NopL can prevent the complete induction of pathogenesis-related defense proteins when expressed in Lotus japonicus roots (10).…”

Functional Analysis of the Type 3 Effector Nodulation Outer Protein L (NopL) from Rhizobium sp. NGR234

“…strain NGR234 (20) possesses a functional type 3 secretion system (21), which can affect nodulation of host plants either positively or negatively. Up to now, four nodulation outer proteins (Nops) of NGR234 have been identified as type 3 effectors, namely NopL (21-24), NopP (25,26), NopT (27,28), and NopM (28). Their functions remain unclear, however.…”