Two significant gaps in our understanding of plant growth control remain:
• How environmental, nutritional, and growth factor cues are perceived and processed by sensory networks
• Mechanistic detail on how such networks control and coordinate the activity of cell growth, division, and expansion
Although increasing numbers of genes involved in these mechanisms are uncovered, very little is still known about how these genes interact to form a regulatory network that couples exogenous and endogenous signals to orchestrate growth responses.
Rapid progress in our understanding of environmental (specifically nutrient) control of adaptive growth responses in plants would be very much facilitated if a minimal set of parameters necessary for analyzing how spe cific cues effect changes in growth processes were determined in future experiments. These include the establishment of size at cell birth, kinematic analysis of the spatio-temporal scale and pattern of growth, ploidy analysis, and final cell size. The analysis of several of these parameters is still very challenging, but novel technical approaches, for example FRET-based sensors (Looger et al. 2005), and approaches that could help determine ploidy levels with cellular resolution (Matzke et al. 2005) are being developed. Although comprehensive data sets reflecting genome-wide responses at the level of gene expression, the proteome, and various post-translational modifications are becoming available, I posit that as long as these are obtained from, for example, whole tissues, which correspond to mixed populations of cells undertaking different, often opposite responses, they will be confusing and potentially misleading. Fortunately, novel tools and techniques are becoming available that should soon allow the analysis of such genome-wide responses at the cellular level (Birnbaum et al. 2005; Casson et al. 2005; Lee et al. 2006; Mace et al. 2006; Schad et al. 2005).
Finally, a conceptual debate about the most efficient and comprehensive experimental approaches for characterization of growth signaling is also necessary. Recent analysis of large collections of systematically generated knock-out mutants in budding yeast have led to revised views of signaling pathways. Instead of essentially linear pathways with only few lateral inputs, it has been proposed that much larger numbers of genes and their products participate in signaling networks with many products, associated in complexes, contributing quantitatively to signaling in minor ways (Friedman and Per-rimon 2007). These conclusions have been drawn on the basis of end-point results, for example the quantitative effect of loss-of-function mutations on a specific trait under investigation. Such approaches are useful for assembly of a collection of cellular components even peripherally involved in signaling. However, the defining feature of signaling networks are that they respond dynamically to constant changes of specific cues to orchestrate desired outcomes at the cellular, organ, or whole-plant level by processing cues and propagating resultant signals. Thus, signaling networks contain two types of components:
(i) those that change their activity as they process and transduce signals, and
(ii) those that are minor accomplices to assist signal flux. To understand how the environment controls adaptive growth responses, we must focus on those network components that change properties when signaling is active and on their targets by examining the behavior of such networks under conditions of dynamic change.
Acknowledgements The support of the Biotechnology and Biological Sciences Research Council (BBSRC), the Royal Society, The Darwin Trust, and The Samuel Roberts Noble Foundation for work in the Doerner Lab is gratefully acknowledged. I apologize to colleagues whose work was not mentioned due to space restrictions.
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