A PLETHORA3/7 transcription factor shapes cucumber shoot architecture [RNA-Seq]

PLETHORA transcription factors (PLTs) are master regulators of plant development. In Arabidopsis, loss of shoot meristematic PLTs leads to destabilised phyllotaxis, but whether these factors have similar functions in more distantly related species, is not known. Here, we investigated the role of PLTs in shaping cucumber shoot architecture. Through transcriptional analysis, we determined that two of the four homologs - CsPLT3/7 and CsPLT5 - are expressed in complementary domains in the shoot apex. In situ hybridisation showed that whereas CsPLT3/7 was expressed in the shoot apical meristem summit and leaf axils, CsPLT5 transcripts localised beneath the CsPLT3/7 domain and to the adaxial side of leaf primordia. Both transcription factors recognised an ANT-like consensus motif and bound to SAM organising genes in vitro. Using a panel of TILLING mutants of each of the four CsPLT homologs, we only identified strong phenotypic defects for Csplt3/7 mutants. The combination of Csplt1/2 and Csplt3/7 proved embryonic lethal. In young seedlings, loss of CsPLT3/7 caused absence of a shoot apical meristem, which regenerated de novo shoot from a flat shoot apex during subsequent seedling development. Mature Csplt3/7 shoots exhibited defects in internode length stabilisation, revealing both extremely short and long internodes, stem architecture, flower morphology, and axillary organ initiation. Moreover, phyllotactic patterns were slightly shifted from a spiral towards a distichous orientation. We thus provide the first evidence of a role for PLTs in shoot development outside of Arabidopsis. Overall design: RNA profiling of 6 dpg shoot apices of cucumber '9930' seedlings grown on soil plates and in germination pouches