Evidence for Contractile Roots in Several Species of Yucca.

Yucca seedlings can longitudinally contract and radially expand their roots, pulling the seedlings down into the soil for protection from evapotranspiration and heat. This contraction occurs through a rearrangement of microtubules in the perivascular cortical tissue. Plant microtubule arrays are correlated with cellulose microfibrils in the cell wall, and therefore plant cell growth and polarity. Within the cortical array of the basal region of the root, microtubules can reorient and in turn cause the cell to grow laterally. Anatomical changes in root tissue were studied in many species of yucca. Physical measurements of seedling roots in soil were used to quantify the amount of contraction over a period of time. Root contraction was observed in all of the species studied. All seedling roots measured shortened significantly over a period of three weeks. Ridges also formed in the contracted basal region if the root epidermis, providing external evidence of contraction. Hand, paraffin, and plastic sections were examined for a broadening of cortical cells. Within the perivascular cortical tissue, lateral elongation of cells was verified using microscopy. Microtubule reorientation was observed using immunofluorescence microscopy within contracting root cells within the cortical tissue