Consequences of the inherent developmental plasticity of organ and tissue relations

Varied cell lineages, random elements of stomata and vein patterns, and unpredictable details of branch relations are all examples of evidence for an inherent plasticity of development that need not be dependent on environmental cues. Such plastic form could be generated by a combination of programs and ‘Developmental Selection’, a principle that resembles Darwinian processes, albeit without genetic differences. Both internal and environmental cues could act by modifying the outcome of this selection. Adaptive responses to environmental heterogeneity cannot be strictly separated from the underlying plasticity of unperturbed development. The proximal mechanisms and the genetic specification of the outcome of developmental selection require an excess developmental potential, one that includes many unused alternatives. The choice between these alternatives depends on preset hierarchies, but this choice can be perturbed according to the environment as well as the internal conditions, including the ones due to random developmental mistakes. Form is specified as a balance between signals of the various components of the organism, without a strict determination of their precise numbers and locations. This requires that developing tissues and organs ‘inform’ the plant about their states and respond according to the signals and substrates they receive. Further, the varied responses must be integrated so as to form and maintain an organized, functional whole. Unexpected and unrecognized traits of known hormones, and especially auxin, suggest concrete knowledge about such mechanisms. Plasticity that is based on developmental selection allows plants in a community to adjust their individual forms to those of their close neighbors. It could have important evolutionary consequences: mutations that have favorable effects on one process could be accommodated by plastic adjustments of other parts of the functional plant. On the other hand, genetic information required only for unusual conditions could be expected to deteriorate because it is not in constant use, and in fact plasticity that requires such is at best rare. At the conceptual level, plasticity calls for integrating reductionist and organismal thinking. A greater challenge is for the concurrent consideration of both proximal and Darwinian mechanisms.