A Grass-Specific Structural Myosin VIII Regulates Protoxylem Development and Hydraulic Conductance in Sorghum

Sorghum (Sorghum bicolor), a C4 grass adapted to hot semi-arid environments, depends on reinforced xylem vessels to maintain hydraulic conductance under high evaporative demand. During xylem differentiation, coordinated microtubule and actin dynamics guide secondary cell wall (SCW) deposition; however, whether actin-based motors regulate vascular architecture and hydraulic performance has remained unknown. Here, we identify HEAT-SENSITIVE 1 (HS1), a previously uncharacterized myosin VIII, as a central regulator of protoxylem integrity and water transport in sorghum. The hs1 mutant exhibited severe leaf scorching under field conditions, accompanied by pronounced protoxylem defects under controlled environments, including vessel collapse, reduced lumen area, and attenuated lignified SCWs. These structural abnormalities compromised longitudinal hydraulic conductance, diminished whole-plant water use, and rendered developing leaves unable to meet transpirational demand, resulting in transient water deficit and secondary tissue injury. HS1 encoded a grass-specific myosin VIII with a grass-specific N-terminal extension exhibiting high intrinsic disorder in N-terminal and lineage-specific substitutions in the motor domain. Single-cell transcriptome analyses positioned HS1 within differentiating protoxylem cells of developing leaves, revealing pronounced temporal and cell type specificity. Furthermore, bulk transcriptome profiling and quantitative lignin measurements indicated that HS1 is required for proper lignin deposition during protoxylem differentiation, linking an actin-based motor to wall reinforcement. Reduced nucleotide diversity at the HS1 locus further supports strong evolutionary constraint, suggesting an adaptive role in sorghum hydraulic resilience. Together, these findings establish HS1 as an actin-based motor controlling xylem architecture and hydraulic function in plants, a previously unrecognized component of grass hydraulic regulation not evident in Arabidopsis.