There is a very interesting ‘opinion editorial’ by Edward N. Trifonov on The Scientist website about what constitutes life.
Trifonov first unveils the philosophical concerns about “life” and re-opens the long-standing debate over whether viruses can be considered to be life. On viruses he says:
“Viruses are simple, having often only protein coat and one or a few molecules of nucleic acids. But this is only a minute part of their description. For reproduction they require the whole complexity of the higher organisms in which they reside. But the “individuality” of the virus is encoded in its genome. Viruses do not invade any arbitrary organism, but rather target specific host species, dictated by the virus’s own genes.”
Then with the even simpler viroids:
“The most primitive plant disease agents, which consist of only RNA. They invade a plant and force it to produce the viroid RNA. This RNA does not encode any proteins, but serves merely to direct the expression of relevant host proteins and cellular processes. That is, the viroid, if considered as a living organism with non-traditional life cycle, is just an RNA, with the sequence instructing its own propagation (via the host), all ingredients and copying devices provided by the host’s cells.”
He then moves on to consider the idea of “simplissimus”.
“Whether we stop the reductionist strive here, at replicating RNA, or continue even further down, to the variety of abiotic syntheses, depends on what we define as life. The border between life and nonlife may, actually, be placed anywhere within the realm of the abiotic processes.”
To read more: The Scientist