Intrinsic regulation of cambial growth

Secondary growth in trees is an attractive system for explaining, through concerted research into mRNA, proteomics, and cell biology, how eukaryotic cellular differentiation is regulated. Differentiation pursuits by genetically uniform cambial derivatives are diverse, less than perfectly repetitive in time and space, and readily modified experimentally. Within each zone of both xylo- and phloiogenesis, competence for at least pluripotent, and not uncommonly totipotent, development evidently is retained. Thus, hypothetical concepts of cellular differentiation 'programs' and 'determined histogenesis' lack support beyond formation and continuing perpetuation of cambium as template for production of similarly shaped and sized daughter cells. The several distinct developmental zones of wood formation manifest metabolic plateaus, and their transitional regions indicate where equilibrium becomes overbalanced and cascades to the next step, changing cells biochemically, hence anatomically, into differentiated states. It remains unclear if differential gene expression during wood formation is strictly of a quantitative nature or if it also varies qualitatively. In addition to selective transcription, another plausible regulatory mechanism is quantitatively varying but still totipotent expression of so-called 'housekeeping' genes modulated through spatial information and changing environments, for example, at levels of mRNA supply and stability. The environment of fusiform and ray cells of the cambial region comprises, in addition to dynamic maintenance metabolism, fluxes in phytohormones, carbohydrates, water, O2, root nutriment, and physical factors capable of influencing both gene expression and enzyme kinetics. In addition to phloem and xylem transport, intercellular communication is normal to cambium and its differentiating derivatives; thus, the procambium-cambium continuum appears to be a living 'fibre' communication network plausibly serving to integrate growth and development throughout the whole plant.