Provided are plants that express, or overexpress, type III Gγ protein AGG3. Such plants exhibit faster vegetative and reproductive growth, accompanied by an increase in photosynthetic efficiency. Constitutive or seed-specific expression of AGG3 in Camelina increases seed size, seed mass, and seed number per plant by 15-40%, effectively resulting in significantly higher oil yield per plant. AGG3-expressing Camelina plants also exhibit improved stress tolerance. Use of AGG3 is therefore an effective biotechnological tool to dramatically increase stress tolerance and plant yield, including oil, in agricultural and horticultural crops.