Premise of the study: On a compound leaf, leaflet primordia are repetitively formed along the apical-basal axis, with the direction varying among taxa. Why and how the directions vary among species is yet to be solved, although a change in a single factor was proposed to cause the variation. In this study, we compared two species in the Papaveraceae with different directions of leaflet initiation, Chelidonium majus subsp. asiaticum (basipetal) and Eschscholzia californica (acropetal). Because E. californica has been studied in some detail, we focused on C. majus and asked how basipetal pattern is achieved. •
Methods: Since only immature leaf primordial tissue has leaflet-generating competency, we performed histological and gene expression analyses on markers of the tissue maturation state. In addition, we performed a time-course analysis of leaf primordial growth. •
Key results: Quantitative reverse transcription-PCR analysis demonstrated that a putative regulator of tissue maturation in C. majus, the CINCINNATA homolog, had higher expression in apical parts than in basal parts during the organogenetic phase. In contrast, expression of the CIN homolog was not elevated in either the apical or basal parts in E. californica during the organogenetic phase. •
Conclusions: In C. majus, apical parts of leaf primordia have already lost leaflet-generating competency during the organogenetic phase. We propose that precocious progression of the maturation process instructs basipetal progression of leaflet initiation in C. majus. This is not the mirror image of data on E. californica, which shows the opposite direction in leaflet formation, indicating that variation in direction is not attributable to a change in a single factor.
Keywords: CINCINNATA; Chelidonium majus subsp. asiaticum; Eschscholzia californica; Papaveraceae; acropetal; basipetal; compound leaf development; organogenetic phase; tissue maturation; trichome.