Squamosa promoter-binding protein-like 9 (SPL9) regulates age-dependent stem elongation and flowering in Lilium

miR156-targeted lily SPL9 primarily regulates stem elongation, which begins in the adult vegetative phase, and subsequently promotes flowering. Herbaceous perennial plants typically have a long juvenile phase and the timing of stem elongation varies depending on species. Lilium species undergo a long juvenile phase. When they reach their adult vegetative phase, their stems begin to elongate, followed by flower induction. microRNA156 (miR156)-targeted SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL) 9 is among the major factors regulating the juvenile-to-adult and vegetative-to-reproductive transitions, and lily SPL9 was predicted to play a different role than that of annual plants. This study investigated the function of lily SPL9 in stem elongation and flowering. miR156-resistant SPL9 (rSPL9) was overexpressed in L. formosanum. Gene expression was assessed under chilling and non-chilling conditions in juvenile and adult plants of wild-type L. longiflorum or an Asiatic hybrid lily. rSPL9-overexpressing lily plants exhibited elongated stems and flowering under non-inductive (non-chilling) conditions. Because the transformed plants developed numerous cauline leaves and time lags existed between the initiation of stem elongation and flower induction, rSPL9 appeared to first stimulate stem elongation and subsequently induce flowering. SPL9 transcripts predominantly accumulated in the shoot apices of adult lily plants, and their accumulation levels increased with stem growth and decreased after the transition to the reproductive phase. Chilling did not affect SPL9 expression. miR156 suppressed SPL9 expression at the post-transcriptional level and SPL9 mRNA accumulation levels were low in a juvenile phase. Therefore, SPL9 is significantly involved in the lily life cycle, mainly regulating stem elongation in the adult vegetative phase, although the role of SPLs in stem elongation is largely unknown in other species.