Changes in root cap pH are required for the gravity response of the Arabidopsis root

Although the columella cells of the root cap have been identified as the si te of gravity perception, the cellular events that mediate gravity signalin g remain poorly understood. To determine if cytoplasmic and/or wall pH medi ates the initial stages of root gravitropism, we combined a novel cell wall pH sensor (a cellulose binding domain peptide-Oregon green conjugate) and a cytoplasmic pH sensor (plants expressing pH-sensitive green fluorescent p rotein) to monitor pH dynamics throughout the graviresponding Arabidopsis r oot. The root cap apoplast acidified from pH 5.5 to 4.5 within 2 min of gra vistimulation. Concomitantly, cytoplasmic pH increased in columella cells f rom 7.2 to 7.6 but was unchanged elsewhere in the root. These changes in ca p pH preceded detectable tropic growth or growth-related pH changes in the elongation zone cell wall by 10 min. Altering the gravity-related columella cytoplasmic pH shift with caged protons delayed the gravitropic response. Together, these results suggest that alterations in root cap pH likely are involved in the initial events that mediate root gravity perception or sign al transduction.