Forms a concentration gradient along the pollen tube growth path, with a lower level in the stigma papilla cell wall and a higher level in the transmitting tract extracellular matix of the style. Expressed in the inflorescence and in the transmitting tract of the pistil. Detected in roots, stems, cauline leaves, cotyledons, hypocotyls, guard cells, pistils, sepals, stamen filaments and vascular bundles of roots but not of leaves. Not expressed in petals, anthers or pollen. Expressed in style and papilla cells when pollination occurs. Detected in the mature embryo sacs of ovules before and after fertilization. Down-regulated by microRNA 408 (miR408) via AGO1 and AGO2 in response to low copper availability. No visible phenotype.