Integrating meristem initiation and floral fate control: toward de novo flower organogenesis in vitro

Plant tissue culture is a cornerstone of green biotechnology, facilitating key applications such as genetic transformation and clonal propagation. In tissue culture, meristems serve as the source of regenerative growth, enabling complete plants to develop from small explants or single cells. While tissue culture accelerates propagation, inducing flowering remains a critical bottleneck for breeding programs, as flowering typically requires an extended vegetative phase. The ability to induce direct flowering <i>in vitro</i>, effectively bypassing the vegetative phase, would decouple reproduction from endogenous developmental timing and environmental cues. We frame this approach as <i>de novo</i> flower organogenesis (DNFO), analogous to <i>de novo</i> shoot (DNSO) and root organogenesis (DNRO), to conceptualize strategies for directly inducing floral meristems. DNFO could accelerate breeding cycles, streamline the development of transgenic lines, and enhance experimental throughput. Meristems are typically induced via hormone supplementation or ectopic expression of developmental regulators (DRs). However, regenerated plants still undergo sequential fate transitions before floral induction. This transition is mainly controlled by floral pathway integrators (FPIs), such as <i>FLOWERING LOCUS T</i> (<i>FT</i>) and <i>SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1</i> (<i>SOC1</i>), and floral meristem identity (FMI) genes, including <i>LEAFY</i> (<i>LFY</i>) and <i>APETALA1</i> (<i>AP1</i>). In this review, we examine the potential to simultaneously induce meristems and floral fate to enable DNFO in tissue culture. Although constitutive or early expression of FPIs and FMIs accelerates flowering <i>in planta</i> and <i>in vitro</i>, it has so far not been sufficient to induce flowers directly from undifferentiated tissue. We propose that this limitation may reflect the absence of UNUSUAL FLORAL ORGANS (UFO), a cofactor of LFY that is required for the full activation of floral identity programs. We hypothesize that spatiotemporally coordinated co-expression of <i>UFO</i> and <i>LFY</i> could bypass intermediate developmental stages and trigger direct formation of floral meristems, hence flowers, in <i>in vitro</i> culture.