Secretion as a key component of gravitropic growth: implications for annexin involvement in differential growth.

Abstract

Materials for cell wall growth are delivered to the wall by secretory vesicles, and recent papers have highlighted the role of secretion in plant growth generally and plant gravitropic growth specifically. Annexins, calcium-dependent membrane-binding proteins, are well documented to be key participants in the process of secretion, and here we present autoradiographic and immunolocalization data specifically linking annexins with the enhanced secretion that accompanies growth, including the asymmetric growth of gravitropism, in Arabidopsis. For immunolocalization studies we used monospecific anti-annexin antibodies raised against divergent 31-mer peptides from two annexins expressed in the growth zone of roots and hypocotyls. Autoradiography following H3-galactose incorporation was used to show a clear correlation of the active secretion of tritiated wall materials with the immunolocalization patterns of a specific individual annexin in the same cells at the same developmental stage. We also observed an asymmetric distribution pattern of annexins and tritiated sugar that was almost identical for them on the faster growing (upper) side of downward curving primary roots. In animals the process of secretion is accompanied by the delivery of cellular ATP to the outside of cells, where it serves multiple signaling roles. We have documented that high levels of ATP in the ECM of plants can inhibit gravitropic growth of elongating roots, which raises the possibility that ATP must be removed from the ECM of growth zones to maintain that growth. Consistent with this possibility we have found that the abundance of mRNA encoding ATP-hydrolyzing apyrases in Arabidopsis is highest in high growth regions of hypocotyls and roots and drops in zones where growth is inhibited, e.g., in hypocotyls by phytochrome activation. Our data add valuable new information on the molecular components that mediate the impact of secretion on plant growth.