A Scottish court has begun hearing a £750,000 defamation case against Green MSP Andy Wightman.
Paul O'Donoghue claimed a blog written by the MSP about his business, Wildcat Haven, falsely suggested it was a "tax haven" and damaged its reputation.
Lawyers for Mr Wightman contend that the blog contained "legitimate comment and criticism" stemming from his "legitimate concerns" about the firm.
The hearing at the Court of Session in Edinburgh is set to run for eight days.
Wildcat Haven is seeking a payment of £750,000 from the politician, as well as a court order to prevent him from republishing the blog online.
Mr O'Donoghue said the blog - titled "Wildcat Haven: Bumblebee haven or tax haven" - contained false statements about the business, which sells "souvenir plots" of land to members of the public.
Mr Wightman who had questioned this business practice, contending that the buyers of the plots being sold by the company would not "actually in fact or in law acquire part of the land" and could not register them with the Land Registry of Scotland.
Mr O'Donoghue, a former lecturer at Chester University, said the firm "explicitly say that they cannot register the land", and said the blog was a "biased attack" on his company.
He said the title of the blog was "clearly and only about offshore tax havens", saying that "falsely accusing a small company dedicated to saving the wildcat as a tax haven is not in the public interest."
He added: "It wasn't responsible journalism, Give us a call. Ring us up. Ask what is going on - don't smear."
Mr Wightman, an MSP for the Lothian region, denies Mr O'Donoghue's claims and wants Lord Clark to throw out the case.
The hearing continues on Wednesday.