Tension wood (TW) is an important strategy for plant adaptation to a changing environment. However, the initial biological signals triggered by gravitational stimulation remain intriguing. Here, we observed lower cytosolic Ca2+ density in phloem parenchymal cells (PPCs) adjacent to cambium on the top of the horizontal stem during early gravity induction, and LaCl3-treated and PagGLR3.3 knockout plants exhibited a low Ca2+ density in PPCs and showed the typical feature of tension wood (TW) such as the formation of gelatinous layers in the stems. In addition, low Ca2+ also caused accumulation of superoxide anion in cambium, which led to the elevated indole-3-acetic acid (IAA) level. The rapid division but advanced programmed cell death (PCD) in cambium cells were detected in both gravistimulated and superoxide anion activator (methyl viologen) treated plants, suggesting a early differentiation of the xylem cells during TW formation. Our study provides new insights into the important role of the Ca2+, superoxide anionin and IAA cascade in the early induction of TW in response to gravitropism.