The tomato NAC transcription factor SlNAM2 is involved in flower-boundary morphogenesis

Being composed of several whorls of distinct floral organs, the flower is one of the most complex organs in the plant. As such, the formation and maintenance of boundaries that separate the meristem from the floral organ primordium and adjacent organs are critical for its normal development. In Arabidopsis, the miR164-regulated NAM genes play key roles in floral-boundary specification. By contrast, much less is known about floral-boundary establishment in the model crop tomato. It was found that the miR164-regulated NAM gene GOBLET is expressed in the floral meristem-organ boundaries and its loss-of-function mutant produces flowers with fused organs, indicating its requirement for tomato floral-boundary formation. It was found here that sly-miR164 targets the transcripts of three additional uncharacterized NAM genes in developing flowers. It is shown that, after floral-boundary initiation, the NAM gene Solyc03g115850 (SlNAM2) is expressed as stripes that mark the boundaries between sepals and between different floral whorls. Furthermore, ectopic accumulation of SlNAM2-encoding transcripts caused various growth-suppression and extraorgan phenotypes typically observed in plants over-expressing known boundary genes. Flower-specific silencing of sly-miR164-targeted NAM genes (AP1>>MIR164) caused defects in the separation of sepals and floral whorls indicating abnormal boundary specification. However, supplementing these NAM-deficient flowers with miR164-resistant SlNAM2 suppressed their fusion phenotypes and completely restored floral boundaries. Together, our results strongly suggest that SlNAM2 participates in the establishment of tomato flower whorl and sepal boundaries.