<i>PavSPL</i> Expression Dynamics in Fruits and Seeds and in Relation to Endocarp Lignification Status During the Transition from Development to Ripening in Sweet Cherry

The transition to ripening in non-climacteric species is governed by several signals, including hormones that enhance or counteract the abscisic acid (ABA)-promoting effect. The SQUAMOSA Promoter-binding protein-Like (SPL) transcription factors are involved in ripening through the modulation of anthocyanin biosynthesis. In sweet cherry fruits, several miR156-targeted <i>PavSPLs</i> are expressed before and during ripening. Recently, some <i>PavSPLs</i> were found in the transition from development to ripening in cultivars contrasting in maturity time. Additionally, several forms of miR156 were expressed in sweet cherry seeds of an early-season cultivar. In this work, we addressed the relevance of endocarp lignification and <i>PavSPLs</i> expression for the transition to ripening. First, we characterized early- and late-season sweet cherry cultivars, ‘Celeste’ and ‘Regina’, focusing on fruit and seed development, endocarp lignification, and <i>PavSPL</i> expression profile. Fruit growth dynamics revealed an earlier onset of color development and lignification in ‘Celeste’, while ‘Regina’ exhibited a prolonged lag phase and delayed embryo development. Transcript profiling at the light green stage showed a higher expression of <i>PavSPL</i> genes in fruits and identified cultivar-specific expressions, especially between ‘Regina’ and ‘Celeste’ seeds. Co-expression networks linked <i>PavSPLs</i> to genes involved in lignin and anthocyanin biosynthesis. We focused on <i>PavSPL2</i> and <i>PavSPL9</i>, which were targeted by mtr-miR156a and gma-miR156f. Both <i>PavSPLs</i> and miRNAs were expressed in fruits and seeds at the yellow stage, an advanced point in the transition to ripening in sweet cherry. Exogenous application of auxin-related compounds in the mid-season cultivar ‘Lapins’ modulated endocarp lignification and pigmentation. Notably, <i>p</i>-IBA treatment, which enzymatically targets the lignin pathway, transiently increased anthocyanin accumulation and reduced lignin deposition, effects that correlated with the downregulation of <i>PavSPL</i> gene expression. These findings highlight the interplay between lignification, color evolution, and pigment biosynthesis during the transition from development to ripening in sweet cherry fruits, and suggest a role for <i>PavSPL</i> genes in this transition.