Senescence: developmental program or timetable?
New Phytologist: 2008 179:575-579
The concept of the â€˜programâ€™ is widely used by developmental biologists and generally everyone knows what it means. However, with the advent of Systems Biology there is an influx into the biological sciences of researchers from other disciplines, such as computing, mathematics and engineering, in which â€˜programâ€™ is also a technical term. If Systems Biology is to keep its promises, it is important to ensure that everyone engaged in the analysis of programmed processes in living cells is talking the same language. Arising from discussions in two recent conferences (Wingler, 2007; Thomas, 2008), this Letter takes a critical look at the notion of a program as conceived and studied by plant developmental biologists, focusing particularly on our area of interest, leaf senescence.
A program is a number of events that occur in a predetermined way, and developmental programs are believed to behave, by and large, like computer .exe files: signal molecules, kinases and transcription factors are often activated in sequence, leading to the development of, for example, an organ or a metabolic state. The plasticity of plant development, however, shows that developmental programs are not fixed but are instead continuously modulated by external and internal factors, to yield a plant body well adapted to its environment.
Developmental programs have often been studied by analysing pathway mutants, but in recent years profiling methodologies, such as DNA microarrays, have become the techniques of choice for dissecting the sequence of events during a developmental process (Schmid et al., 2005). Every approach has its inherent problems, and we will, in this contribution, argue that, at least when leaf senescence is considered, the concept of a developmental program raises fundamental questions.