Various tissues respond to auxin in a wide variety of ways. This physiological response specificity is partially due to tissue-specific differences in gene expression. In the growing root tip, application of auxin rapidly inhibits cell expansion. A rapid transient increase in cytosolic Ca2+ concentration ([Ca2+]cyt) as a response to global auxin exposure was monitored via laser scanning confocal microscopy of a genetically-encoded FRET based biosensor. This response is present in cells located in the growing root tip, particularly in epidermal cells of the elongation zone. The response is specific to this region and not detectable in other tissues, such as the mature root, hypocotyl, lateral root cap, and columella. In the growing root tip, transcriptional regulation of a subset of auxin-induced genes displays Ca2+ dependence based on treatment with the pharmacological Ca2+ channel blocker La3+. To learn more about the extent of this regulation using an unbiased approach, RNA sequencing was performed on these samples revealing approximately 1400 auxin-induced genes, half of which were Ca2+-dependent. Quantitative PCR revealed that the La3+ effect on auxin-induced gene expression was absent in tissues that lack a Ca2+ response. This potential Ca2+-dependent gene expression, in combination with the tissue- specific cytosolic Ca2+ response, may play a role in the specificity of the auxin-induced growth- inhibition of the growing root tip.
