Some years ago, in a 1000~ level biology course we learned that the DNA essentially encodes formulas for creating proteins from amino acids. While the human body can synthesize many many amino acids, some we are unable to synthesize and we dub these as essential amino acids because we must obtain them from our diet. However, surely something must be capable of synthesizing the amino acids, otherwise they would not make it up the food chain to us, which opens the door to the possibility that animals may have varying capabilities in regards to amino acid synthesis. Presumably, this is why cats have to obtain a high protein diet, as they have high need for protein but a poor ability to synthesize them?
Moreover, in doing basic research into nutrition to plan a regular workout schedule there is some need to distinguish protein sources as not all protein sources are considered to be complete proteins because they do not provide all amino acids, or otherwise do not provide them in the right balance. Particularly, many fruits and vegetables tend to be poor sources of protein.
For plants which are incomplete protein sources, they would seem to lack the ability to synthesize some amino acids. They also would not be able to obtain these through diet, as they're plants. Consequently, does this mean that some plants may not encode for a specific amino acid throughout their entire DNA?