There is no universal answer to this, as there are many ways of indicating emphasis in plain text. Your best bet is to read the submission guidelines of the publication that you're submitting to, as each may have different requirements.
In the case of Daily Science Fiction, they have a page of story formatting guidelines. As far as I can tell, they don't want any formatting at all.
Stories must be submitted as plain text* via our web form. Please don't put your address, word count, title, name or other information in the form. Please don't retype your entire story--copy and paste it into the form (once you've saved your document as plain text). If a story is accepted for publication, we'll contact you to incorporate any formatting and layout needed.
*Plain text means no smart quotes, no curvy apostrophes, no magic ellipses, no clever Microsoft formatting that sometimes makes your story harder to read.
I would personally find this somewhat strange, but defying submission guidelines is a certain means to have your story rejected out of hand. If I were submitting to this magazine, I'd look for a way to get clarification on this.
However, in general: Read the submission guidelines. Every publication may have slightly different ones.