The parallel alignment of stiff cellulose microfibrils in plant-cell walls mediates anisotropic growth [1, 2]. This is largely controlled by cortical microtubules, which drive the insertion [3, 4] and trajectory of the cellulose synthase (CESA) complex at the plasma membrane [5-7]. The CESA interactive protein 1 (CSI1) acts as a physical linker between CESA and cortical microtubules [8-10]. Here we show that the inflorescence stems of csi1 mutants exhibit subtle right-handed torsion. Because cellulose deposition is largely uncoupled from cortical microtubules in csi1, we hypothesize that strictly transverse deposition of microfibrils in the wild-type is replaced by a helical orientation of uniform handedness in the mutant and that the helical microfibril alignment generates torsion. Interestingly, both elastic and viscous models for an expanding cell predict that a net helical orientation of microfibrils gives rise to a torque [11, 12]. We indeed observed tilted microfibrils in csi1 cells, and the torsion was almost absent in a csi1 prc1 background with impaired cellulose synthesis. In addition, the stem torsion led to a novel bimodal and robust phyllotactic pattern in the csi1 mutant, illustrating how growth perturbations can replace one robust mathematical pattern with a different, equally robust pattern.