MCB-9417761 Tallman The long-term objective of the project is to elucidate the molecular mechanisms that regulate senscence of cells and chloroplasts of plant leaves. Guard cells which flank stomata fail to undergo a senescence (aging) process like that which takes place in cells of the underlying tissue (mesophyll). Thus a system to culture individual guard cells has been developed to study the induction and regulation of senescence. Guard cells protoplasts (GCP, guard cells isolated with cell walls removed) have been successfully cultured for a week at 36-40o C, but lower temperatures were not effective at maintaining the guard cells as guard cells, but allowed redifferentiation and callus formation. The following model is suggested to account for the temperature sensitivity: 1) At lower temperatures (24-34oC) the plant hormone auxin induces synthesis of ethylene. 2) Ethylene synthesis results in cellular senescence or a growth response that results in callus formation. 3) Ethylene synthesis is inhibited at higher temperatures. 4) At higher temperatures, abscisic acid (ABA) counteracts the auxin/cytokinin response, thereby preventing redifferentiation of GCP. To test this model the following experiments are done: 1) The GCP are cultured at the higher temperature in ABA to test if ABA itself could induce dedifferentiation. 2) Examine the levels of ethylene produced by GCP at lower temperatures. 3) Test whether other conditions which which suppress ethylene action also allow the GCP to maintain a non-senescent state. This is a project supported through the Research at Undergraduate Institutions (RUI) program. Over the prior award period of three years there were 26 undergraduates involved in research. A similar level of involvement is expected in this award. %%% The long-term objective the project is to elucidate the molecular mechanisms that regulate the decline during aging (senescence) of cells and chloroplasts of plant leaves. Guard cells wh ich flank stomata fail to undergo an aging process like that which takes place in cells of the underlying tissue (mesophyll). Thus a system to culture individual guard cells has been developed to study the induction and regulation of senescence. In this set-up, guard cells protoplasts (GCP, guard cells isolated with cell walls removed) are successfully cultured for fairly long periods (days) at elevated temperatures (36-40o C). Lower temperatures are not effective at maintaining the guard cells as guard cells, but allow redifferentiation and general cell growth or callus formation. The hypothesis is that the plant growth regulator, ethylene, causes the cells to senesce at lower temperatures, but is not produced by the guard cells at higher tempertures. Also at lower temperatures, other growth regulators like auxin and cytokinin are active, resulting in higher growth rates and cellular dedifferentiation. These hypotheses are tested by measuring the levels of ethylene produced by the cells at different temperatures and by examining if exogeneously added growth regulators such as abscisic acid interfere with the aging process. The study leads to a better understanding of why guard cells fail to senesce and the mechnisms that induce and regulate senescence in plant cells. ***
