FACTORS AFFECTING SINGLE STRAND-PREFERRING NUCLEASE ACTIVITY DURING LEAF AGING AND DARK-INDUCED SENESCENCE IN BARLEY (HORDEUM-VULGARE L.)

Nucleases (SSNs) which preferentially degrade single-stranded nucleic acids have been implicated in programmed cell death pathways, includin g senescence, in higher plants, though the biological roles of such en zymes are obscure. An assessment has been made of changes in SSN activ ity that occur during dark-induced senescence of the primary leaf of b arley. Specifically, changes have been studied of SSN activity in diff erently aged sections of primary leaf of seedlings grown either under a 16 h light/8 h dark cycle or following transfer to continuous darkne ss. Prolonged darkness caused an increase in SSN activity, due to four different SSN isoforms, a major activity of cn. 37 kD and minor activ ities 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 p rogressively smaller increase towards the leaf base. Increased SSN act ivity was apparent within 8-12 h of darkness, but did not persist or a ccumulate in plants grown under a light/dark cycle. The results show t hat, within the primary leaf, in addition to light/dark control, facto rs related to leaf tissue age also participate in the control of enzym e activity. Possible roles of SSNs in leaf senescence and cell death a re discussed. (C) 1998 Elsevier Science Ireland Ltd. All rights reserv ed.