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Nucleases (SSNs) which preferentially degrade single-stranded nucleic acids have been implicated in programmed cell death pathways, including senescence, in higher plants, though the biological roles of such enzymes are obscure. An assessment has been made of changes in SSN activity that occur during dark-induced senescence of the primary leaf of barley. Specifically, changes have been studied of SSN activity in differently aged sections of primary leaf of seedlings grown either under a 16 h light/8 h dark cycle or following transfer to continuous darkness. Prolonged darkness caused an increase in SSN activity, due to four different SSN isoforms, a major activity of ca. 37 kD and minor activities of ca. 36.5, 40.1 and 43 kD. The increase occurred most rapidly and to a greater extent, in older cells towards the leaf tip, with a progressively smaller increase towards the leaf base. Increased SSN activity was apparent within 8–12 h of darkness, but did not persist or accumulate in plants grown under a light/ dark cycle. The results show that, within the primary leaf, in addition to light/dark control, factors related to leaf tissue age also participate in the control of enzyme activity. Possible roles of SSNs in leaf senescence and cell death are discussed. |
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