Woody plants must acclimate to environmental stresses, including soil salinity, for proper growth and development. Microtubule reorganization supports plant survival in saline-rich soils, but the underlying molecular mechanism in tree species remains unclear. In this study, we identified a salinity stress response mechanism in hybrid poplar seedlings. This mechanism involves regulation of microtubule dynamics by the microtubule-associated protein PLASMA MEMBRANE–ASSOCIATED CATION BINDING PROTEIN 1a (PagPCaP1a). Salinity stress induced PagPCaP1a expression and phase separation of PagPCaP1a protein to form PagPCaP1a condensates in a calcium-dependent manner. The formation of PagPCaP1a condensates was partially driven by the VEEEKK motif within the carboxyl terminus of the protein, which rapidly depolymerizes microtubules under salinity stress. Our study reveals that the liquid-liquid phase separation of PagPCaP1a represents an additional regulatory layer for microtubule depolymerization, and we propose an effective strategy to manipulate the phase separation of PagPCaP1a to improve plant stress tolerance.

Microtubules undergo dynamic remodeling in response to diverse abiotic stress in plants. The plant-specific IQ67 DOMAIN (IQD) family proteins serve as microtubule-associated proteins, playing multifaceted roles in plant development and response to abiotic stress. However, the biological function of IQD genes in apple remains unclear. In this study, we conducted a comprehensive analysis of the Malus domestica genome, identifying 42 IQD genes distributed across 17 chromosomes and categorized them into four subgroups. Promoter analysis revealed the presence of stress-responsive elements. Subsequent expression analysis highlighted the significant upregulation of MdIQD17 and MdIQD28 in response to cold treatments, prompting their selection for further functional investigation. Subcellular localization studies confirmed the association of MdIQD17 and MdIQD28 with microtubules. Crucially, confocal microscopy and quantification revealed diminished microtubule depolymerization in cells transiently overexpressing MdIQD17 and MdIQD28 compared to wild-type cells during cold conditions. In conclusion, this study provides a comprehensive analysis of IQD genes in apple, elucidating their molecular mechanism in response to cold stress.