Whereas there is no doubt of the powerful informational directives a plant cell receives from the five major hormones, there is also no doubt that the absolute intensity of the response can be modulated by the presence of other signal molecules. Just as pH, temperature, ionic concentrations, osmolarity and many other physical factors can all determine the quantitative nature of a response, so may chemically induced molecular changes in ion channels, plasmodesmatal connections, membrane domains and transcriptional/translational/post-translational events shift cell performance from optimal to sub- or supra-optimal. The brassi-nosteroids, jasmonates, oligosaccharins and peptides described here, while not the major players in plant growth control, can all exert at different times and in different ways a differentiation- and performance-directed influence in both the short and long terms. They are substances that can modify the effectiveness of hormones and the capabilities and potential of the cell. At any one time, each cell is exposed to a multi-array of external and internal signals. In all instances, it is the cell itself that must discriminate and sort the inputs; the response that ensues then becomes the outcome of the precise limitations placed upon that cell by its own target state.
We should perhaps add that all the signalling molecules we have considered are essentially plant produced and plant orientated, but our consideration is not exhaustive. For example, polyamines are common to both eukaryotes and prokaryotes and as low molecular weight compounds they can regulate and influence metabolic events (Kumar et al., 1997). Sugars (sucrose or glucose) levels in a cell operate a complex signalling network with ethylene and abscisic acid to inhibit or promote specific growth responses (Leon and Sheen, 2003). Such substances are universal signal molecules. Perhaps as every cell is a target cell, so, given the appropriate circumstances, every molecule can become, at least temporarily, a signal molecule.
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