Software systems need software architectures to improve their scalability and maintenance. However, many agile practitioners claim that the upfront design of software architectures is an investment that does not pay off, since customers can rarely appreciate the value delivered by architectures. Furthermore, conventional architectural practices may be considered unacceptable from the Agile values and principles perspective. In this paper, the development of working architectures in agile iterations is presented as an attempt to solve the problem of designing software architectures in Agile. This contribution is based on the new concept of Plastic Partial Component (PPC). PPCs are highly malleable components that can be partially described, what increases the flexibility of architecture design. PPCs based architectures let reinforce some of the agile values and principles. Our experience of putting this contribution into practice is illustrated through the agile development of a Testing Framework for Biogas Plants.