The essential amino acids needed in human diets like Lys, Met, Thr and Trp are present in low level in plant foods.

http://www.ncbi.nlm.nih.gov/pubmed/16164407

Plants represent the major source of food for humans, either directly or indirectly through their use as livestock feeds.

Plant foods are not nutritionally balanced because they contain low proportions of a number of essential metabolites, such as vitamins and amino acids, which humans and a significant proportion of their livestock cannot produce on their own.

Among the essential amino acids needed in human diets, Lys, Met, Thr and Trp are considered as the most important because they are present in only low levels in plant foods.