Arrowroot powder or flour is the ground rhizomes of the West Indian M. arundinacea arrowroot plant, relative of ginger. It is also made from similar tubers. Some of the tubers are poisonous until they are cooked. You can read more about the details here :University of Florida
Per 2 teaspoons arrowroot powder needed substitute:
The best Keto substitute for arrowroot thickener in baking is to use Xanthan Gum. The reason for this as described by Bob's Red Mill, is because "first, it works as an emulsifier, encouraging liquids that normally don't like one another to mix together. Second, it works as thickener, increasing the viscosity of liquids and batters. Third, it can create a creamy texture."
CraftyCookery.com suggests using 1/2 teaspoon xanthan gum per cup of gluten free flour for recipes for breads and doughs.
For cookies, cakes and muffins, use 1/4 teaspoon xanthan gum per cup of gluten free flour.
Some of the arrowroot you purchase may actually be made from tapioca root. You can purchase arrowroot powder in many well-stocked grocery stores or purchase it online at Amazon.com
The true arrowroot plant is the Bermuda Arrowroot and is native to Central America. It is classified as Maranta arundinacea.