MAT - A flat piece of coarse material used for wiping one's feet, or as a decorative or protective floor covering.
MOO - The characteristic sound made by a cow or bull.
TOO - Likewise.
OAT - Widely cultivated cereal grass, typically Avena sativa.
TOT - A small child.
ATOM - The smallest, indivisible constituent part or unit of something.
MOAT - A deep, wide defensive ditch, normally filled with water, surrounding a fortified habitation.
MOOT - Subject to discussion (originally at a moot); arguable, debatable, unsolved or impossible to solve.
TOOT - The noise of a horn or whistle.
MOTTO - A sentence, phrase, or word, forming part of an heraldic achievement.
TOMATO - A widely cultivated plant, Solanum lycopersicum, having edible fruit.