, 2008). Other secreted proteins, find more including NopD, NopJ, NopL, NopM, NopP, and NopT, have been described as effector proteins (Bartsev et al., 2004; Skorpil et al., 2005; Rodrigues et al., 2007; Dai et al., 2008; Kambara et al., 2009). Nodule formation and nitrogen fixation in legume roots are the result of a symbiotic process characterized by

a complex exchange of signals between the plant and the bacterium. Plant flavonoids induce the production of rhizobial Nod factors responsible for the first morphological and physiological events that trigger nodule development. As for Nod factors, expression of rhizobial T3SS components and effectors is induced by flavonoids and NodD as the gene encoding the TtsI transcriptional factor contains a nod box consensus sequence in its promoter region (Krause et al., 2002; Marie et al., 2004). TtsI binds to tts boxes in the promoter regions of genes encoding T3SS components, inducing their transcription (Wassem et al., 2008). Mesorhizobium loti forms a symbiotic association with Lotus spp. The sequencing of the M. loti MAFF303099 genome has revealed the presence of all the genes required to encode a T3SS (Kaneko et al., 2000a, b). Regulation of the M. loti MAFF303099 T3SS is the same as in other rhizobia because its ttsI homolog is preceded by a nod box. Using a bioinformatic approach, we have previously searched

for other tts box-controlled genes. We identified three new T3SS putative check details effectors in M. loti MAFF303099 (proteins encoded by mlr6331, mlr6358, and mlr6361) (Sánchez et al., 2009) and determined the NodD/flavonoid-transcriptional regulation for another putative Tideglusib effector previously described for M. loti MAFF303099 (a protein encoded by mlr6316) (Hubber et al., 2004). We also determined that the N-terminal region of Mlr6361 and Mlr6358 directs the secretion of the protein through T3SS (Sánchez et al., 2009). We have previously found that a M. loti rhcN mutant strain

is negatively affected in nodulation competitiveness on Lotus glaber (now called Lotus tenuis) (Sánchez et al., 2009). The rhcN gene encodes a protein similar to RhcN of Rhizobium sp. strain NGR234, a T3SS protein that shares characteristics of ATPase and whose mutation abolishes T3SS secretion (Viprey et al., 1998). Okazaki et al. (2010) analyzed the nodulation efficiency of an M. loti mutant which lacked a region of the chromosome-containing genes encoding for both structural components of T3SS and putative secreted proteins and demonstrated that the presence of T3SS affected nodulation positively on Lotus corniculatus and Lotus filicaulis but negatively on Lotus halophilus, Lotus peregrinus, and Lotus subbiflorus. Okazaki et al. also observed no significant differences, between this mutant and the wild-type strain, in nodulation ability on Lotus japonicus Gifu B-129. Transcriptional analysis applied to Lo. japonicus Gifu B-129 inoculated with the M.