Analysis of the Lateral Suppressor Pathway in Tomato

In seed plants, axillary meristems (AM) play a crucial role in the elaboration of shoot architecture. AMs are initiated in the boundary region between the shoot apical meristem and leaf primordia. Lateral suppressor (Ls), a putative transcription factor, is one of the most important and most specific regulators of this process. To unravel the Ls pathway we have identified downstream targets of Ls by comparing transcript profiles of leaf axils between wild-type and ls-1 (RNA-Seq analysis). Comparing the RPKM (reads per kilobase of exon model per million mapped reads) values of wild-type and ls-1, we found that 13 genes are downregulated and 47 are upregulated in ls-1. Selected candidate genes were further characterized by qRT-PCR and RNA in-situ hybridizations.

In tomato, Ls mRNA accumulates not only in axils of leaf primordia, but also at the distal boundary of leaflet primordia, as shown by RNA in-situ hybridization. However, phenotypic defects in ls-1 have only been described for axillary bud formation and flower development. We observed that wild-type tomato plants occasionally form ectopic shoots on leaves, a feature, which is lost in the ls-1 mutant. Similar defects in AM and ectopic meristem formation were found in gob-3, a loss-of-function allele of Goblet (Gob). Furthermore, transcripts of Gob co-localize with Ls in leaf axils and in distal boundaries of leaflets. To explain these phenotypic similarities, we are currently looking for possible interactions between these two boundary genes.