Single-nucleus RNA Sequencing and MRNA Hybridization Indicate Key Bud Events and LcFT1 and LcTFL1-2 MRNA Transportability During Floral Transition in Litchi

In flowering plants, floral induction signals intersect at the shoot apex to modulate meristem determinacy and growth form. Here, we report a single-nucleus RNA sequence analysis of litchi apical buds at different developmental stages. A total of 41 641 nuclei expressing 21 402 genes were analyzed, revealing 35 cell clusters corresponding to 12 broad populations. We identify genes associated with floral transition and propose a model that profiles the key events associated with litchi floral meristem identity by analyzing 567 identified floral meristem cells at single cell resolution. Interestingly, single-nucleus RNA-sequencing data indicated that all putative FT and TFL1 genes were not expressed in bud nuclei, but significant expression was detected in bud samples by RT-PCR. Based on the expression patterns and gene silencing results, we highlight the critical role of LcTFL1-2 in inhibiting flowering and propose that the LcFT1/LcTFL1-2 expression ratio may determine the success of floral transition. In addition, the transport of LcFT1 and LcTFL1-2 mRNA from the leaf to the shoot apical meristem is proposed based on in situ and dot-blot hybridization results. These findings allow a more comprehensive understanding of the molecular events during the litchi floral transition, as well as the identification of new regulators.