Prior secondary cell wall formation is required for gelatinous layer deposition and posture control in gravi-stimulated aspen

Plants can modify their shape and cell structures in response to gravity. Cell walls especially secondary cell walls (SCW) are crucial for maintaining the shape of cells and reinforcing wood structure. Generally, in hardwood trees, tension wood is formed along the upper side of the bending stem and contains wood fiber cells having a gelatinous layer (G-layer) which is successively formed inside of the SCW. In a previous study, we generated nst/snd quadruple-knockout aspens, in which SCW formation was impaired in 99% of wood fiber cells. In the present study, we produced nst/snd triple-knockout aspens, in which a large number of wood fibers had thinner SCW than WT and some had no SCW. As the SCW layers are always formed prior to the G-layer deposition, the nst/snd mutants raise an interesting question of whether the mutants can form G-layers without SCW and control their postures in response to alteration of gravitational direction. All genotypes produced tension wood characterized by growth eccentricity and reduction of vessel frequency when grown on an incline; however, triple mutants only slightly recovered upright growth and quadruple mutants were hard to maintain their postures. Generated mutants clearly exhibited that the G-layers were formed in the wood fibers having the SCW but not in those lacking the SCW. Our results indicate that the SCW deposition is essential for the G-layer formation and posture control. Each wood fiber cell may recognize its developmental stage of the cell wall to initiate formation of the G-layer responding to gravistimulation.