IDENTIFICATION, CHARACTERIZATION, AND PURIFICATION OF A TOBACCO ENDONUCLEASE ACTIVITY-INDUCED UPON HYPERSENSITIVE RESPONSE CELL-DEATH

Programmed cell death (pcd) is activated during the hypersensitive res ponse (HR) of plants to avirulent pathogens, We have recently shown th at, similar to pcd in animal cells, nuclei of cells undergoing HR cell death contain fragmented nuclear DNA (nDNA). Here, we report that cel l death occurring during the HR is accompanied by an increase in the a ctivity of several deoxyribonucleases. Induction of nuclease activitie s was coordinated with cell death and may account for the degradation of nDNA during the HR. HR-associated nuclease activities were not indu ced during senescence, following necrotic cell death resulting from ab iotic stress, or in response to induction of plant defense mechanisms by salicylic acid, HR-associated nuclease activities were stimulated b y Ca2+ and inhibited by EGTA, EDTA, and Zn2+. At least one of the HR-a ssociated nuclease activities was detected in nuclei purified from lea ves undergoing pcd, A nuclease with an electrophoretic mobility simila r to that of the nuclease activity found in nuclei isolated from leave s undergoing HR cell death was purified, Our findings are in accordanc e with some of the biochemical events that occur during pcd in animal cells. However, further analysis of the pattern of nDNA fragmentation and the corresponding structural changes that occur in the nuclei of t obacco cells undergoing HR cell death revealed that these features may have differences from those that take place during apoptosis in anima l cells.