Splice variants of the SIP1 transcripts play a role in nodule organogenesis in Lotus japonicus

SymRK-interacting protein 1 (SIP1) has previously been shown to interact with the symbiosis receptor kinase, SymRK, in Lotus japonicus. A longer variant of the SIP1 transcript, SIP1L, was isolated and characterized. SIP1L contains an additional 17 amino acids that make its C-terminus a complete heat shock protein 20 (Hsp20)-like domain. In contrast to SIP1S, the longer splicing variant SIP1L could not interact with SymRK. Both SIP1L and SIP1S transcripts could be detected in developing nodules and other plant tissues, although the former was always more abundant than the latter. SIP1L and SIP1S formed heteromeric protein complexes, which were co-localized in the plasma membrane, cytoplasm and nuclei. Expression of SIP1-RNAi in transgenic hairy roots resulted in impairment in the nodule and arbuscular mycorrhizal development, suggesting an important role of SIP1 in the common symbiosis pathway. Overexpression of either SIP1L or SIP1S increased the number of nodules formed on transgenic hairy roots, indicating a positive role of SIP1 in nodulation. The SIP1S-like transcript was not detected in other higher plants tested, and the SIP1L-like proteins of these plants were capable of interacting with the SymRK orthologs. It is proposed that the loss of the ability of SIP1L to interact with SymRK in Lotus is compensated by the expression of a shorter splicing variant, SIP1S, which binds SymRK and may play a role in relaying the symbiosis signals to downstream cellular events.