Involved in auxin transport. Positive regulator of the auxin signaling pathway involved in gravitropism, plant growth and grain development. Mostly expressed in the vascular tissues of leaves, culms and young panicles, especially in hulls. Rapidly induced by auxin (IAA) and analogs, indole-3-butyric acid (IBA) and naphthylacetic acid (NAA). Expression level is gradually decreased as the panicle matures. Decreased sensitivity to auxin and N-1-naphthylphthalamic acid, an auxin transport inhibitor. Reduced plant height and decreased panicle length, and grain size and weight. Altered auxin transport. Good candidate to improve plant biomass and grain productivity in various species. The over-expressing mutants Bg1-D and BG1-OE exhibit increased grain size with both bigger length and width, and associated with longer epidermis cells of both palea and lemma in spikelet hull as well as increases of both number and area of the parenchyma cells in the hull. Some genes associated with cell cycle and cell expansion are up-regulated in the panicles. Enhanced growth of other tissues at both the vegetative and reproductive stages are also observed. Increased sensitivity to auxin (IAA) in terms of root elongation/inhibition, as well as altered IAA distribution and transport. Belongs to the BIG GRAIN 1 (BG1) plant protein family.