SHOOT MERISTEMLESS is Required for the Proper Internode Patterning and the Sepal Separation in Arabidopsis

SHOOT MERISTMLESS (STM) encoding a homeodomain protein plays a vital role in the specification of shoot meristem, the source of undifferentiated cells for the above-ground organ development in Arabidopsis thaliana. The roles of STM still remain to be elucidated as stm is deficient in the late organ development. Here, we have isolated a recessive filamentous gynoecium (fig)-1 displaying slightly reduced floral meristem and weak sepal fusion defect by introducing the 35S enhancer tag into the poltergeist-6. A truncated tag was localized at the 4628 base pairs (bps) upstream from the STM start codon in fig-1 led to the decrease in the STM transcripts. Two additional knocked-down mutant alleles, stm-p1 and stm-p3 harboring T-DNA insertions at 468 and 2225 bps upstream from the start codon, respectively displayed the pennywise (pny)-like clustered internode together with the sepal fusion defects. In stm-p1, the expression of KNOTTED-1 LIKE FROM A. THALIANA6 was enhanced and stm-p1 brevipedicelus-1 developed extremely compact internodes. The sepal fusion phenotypes of stm-p1 and stm-p3 were comparable to that of cup-shaped cotyledon1 (cuc1) and CUC1 expression was reduced in stm-p1. Together, these results suggest that STM functions genetically in association with PNY and CUC1 for the proper inflorescence development and the sepal separation, respectively.