Effects of extracellular ATP and ADP on growth and development of Arabidopsis seedlings
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In mammalian cells, it is well documented that extracellular ATP is a signaling molecule. Recent studies suggest that extracellular ATP may regulate cellular activities in plants as well. In this dissertation, two different physiological responses (root gravitropism and the elongation of etiolated hypocotyls) of Arabidopsis seedlings to exogenously applied ATP or ATPγS are documented and discussed. First, this dissertation describes that certain concentrations of exogenous ATP specifically block root gravitropic responses and increase the response sensitivity of roots to exogenously added auxin in seven-day-old light-grown Arabidopsis seedlings. These concentrations of extracellular ATP also decrease basipetal auxin transport in both maize and Arabidopsis roots, as well as increases the retention of 3H-IAA in root tips of maize. These findings suggest that inhibition of root gravitropic responses maybe due to a disturbance of auxin distribution in the roots caused by the presence of exogenously applied ATP. The inhibitory effects of extracellular ATP on auxin distribution may occur at the level of auxin export. In addition, this dissertation demonstrates that ATP and ADP are able to stimulate etiolated hypocotyl elongation at concentrations lower than 1 mM, and to inhibit hypocotyl elongation at concentrations of 2 mM or greater. A similar dosedependent response curve can also be observed for seedlings grown in MS media supplied with either ATPγS or ADPβS, however the concentration required to observe a similar promotion and inhibition pattern is only one-tenth of that recorded for the growth effects of either ATP or ADP on seedlings. The inhibition by ATP of etiolated hypocotyl elongation is partially mediated by ethylene, since ATPγS up-regulates the mRNA abundance of genes in the ethylene synthesis and signaling pathways, and the ethylene-insensitive mutant ein 2 is partially insensitive to the inhibitory effects of ATPγS on hypocotyl elongation. ATPγS up-regulates the expression of several stress-related MAPkinase pathway genes that are also turned on during stress responses. Furthermore, various stresses such as cold, touch, hypotonic and hypertonic conditions are found to stimulate the release of ATP from the cell. Taken together with other data from the Roux laboratory, we propose that ATP is one of the key signaling molecules in mediating plant stress responses.