Triticum aestivum, otherwise known as bread wheat, is the most widely grown crop in the world.
Italics are used in the sentence above because the italicized words are:

Triticum aestivum is italicized because it is the scientific name of the organism. Therefore, this option is correct. Words that the reader may not know is incorrect. Triticum aestivum is italicized because it is the scientific name of the organism. Additionally, a description of the organism is included afterward, in order to assist the reader. Intended to be emphasized by the author is incorrect. Triticum aestivum is italicized because it is the scientific name of the organism. The author does not intend to emphasize this name over anything else in the sentence.
A proper title of something is incorrect. Triticum aestivum is italicized because it is the scientific name of the organism. It is not a proper title, but rather, an organism.