Abstract text: In tomato fruit ripening, the transition from symplasmic to apoplastic phloem unloading is accompanied by extensive cell wall remodelling. Callose deposition at plasmodesmata (PD) is a key regulator of symplasmic permeability. However, whether callose is developmentally regulated at PD, and its turnover influences cell wall structure and mechanics during fruit ripening have not been defined.

Immunolabelling revealed that callose accumulation peaked at 4 weeks post-anthesis, demonstrating stage-specific regulation. This stage coincided with a minimum in Young’s modulus measured at PD wall domains by AFM nanoindentation, such that stiffness was lowest when callose levels were maximal. To test whether callose turnover governs cell wall mechanics, we reduced callose by transiently overexpressing two tomato β-1,3-glucanase genes. Callose depletion led to an increase in stiffness within PD wall regions, establishing that manipulation of callose levels is sufficient to alter the elastic modulus of plant cell wall. This mechanical shift was accompanied by reduced cellulose microfibril diameter and disrupted organisation.

Together, these findings identify callose turnover as a determinant of wall stiffness and architecture. Callose therefore functions not only as a regulator of symplasmic communication, but as a mechanically consequential polymer linking plasmodesmatal dynamics to cell wall remodelling during fruit growth and ripening.