Thesis writing style: when to italicize? In a thesis or journal you write all sentences yourself, but on some occasions you prefer to take a piece of text from another source. If you do, should you use quotation marks only, or italicize it, or use both, to identify that you have not written it?

Example for quoting directly from a source:


  
  This can, according to Someone et al. [1], be seen as "an excellent way of representing a piece of text quoted from another
  source", and therefore it's emphasized.
  This can, according to Someone et al. [1], be seen as "an excellent way of representing a piece of text quoted from another
  source", and therefore it's emphasized.
  This can, according to Someone et al. [1], be seen as an excellent way of representing a piece of text quoted from another
  source, and therefore it's emphasized.
  


Example for a definition with a source:


  
  Wikipedia defines italic as "a semi-cursive, slightly sloped style of handwriting and calligraphy that was developed during the
  Renaissance in Italy" [2].
  Wikipedia defines italic as "a semi-cursive, slightly sloped style of handwriting and calligraphy that was developed during the
  Renaissance in Italy" [2].
  Wikipedia defines italic as a semi-cursive, slightly sloped style of handwriting and calligraphy that was developed during the
  Renaissance in Italy [2].
  


Are there are any style guides or recommendations that refer to italicized text, for either quotes or definitions? And related to the two examples, what is the recommended way to distinguish between a quote and a definition?

Edit: note that if a style guide is provided, the guide should be followed. This question relates to situations where no style guide is required.
There can be specific exceptions for journals, which may have conventions preserved unchanged from the past, but the general principle is to be typographically simple while conveying the necessary information. Quotation marks are required to unambiguously indicate that a certain sequence of text is a literal quotation; the quote itself should then not modify the format of the text, thus should be roman when the original is roman, italic when it is italic, and so on. Thus, your first option would be the correct choice.