Plants synthesize proteins from amino acids, which are the building blocks of proteins. Amino acids are organic compounds that contain an amino group and a carboxyl group. They are obtained through various sources such as soil, air, and water, and are essential for the growth and development of plants.

Option 1, fatty acids, are compounds used by plants for energy storage and membrane synthesis, but they are not directly involved in protein synthesis. Similarly, option 2, sugar, is a carbohydrate that serves as an energy source for plants, but it is not the primary source for protein synthesis. Option 4, starch, is a complex carbohydrate that is used by plants to store energy, but it is not directly involved in protein synthesis.

Thus, the correct answer is option 3, amino acids, as they are the fundamental components used by plants to synthesize proteins.