Fig. 3. PTGS-deficient mutants are hypersusceptible to infection by CMV. Non-infected PTGS-deficient sgs mutants of Arabidopsis (C) grow as non-infected wild-type plants (A), whereas disease symptoms are much more pronounced in CMV-infected mutants (D) compared with CMV-infected wild-type plants (B). Infected mutants have very small stems, are completely sterile and eventually die, whereas infected wild-type plants develop stems with elongated internodes and are still able to form seeds. The difference in symptom severity caused by CMV infection is due to an average fivefold overaccumulation of viral RNA in the mutants.