Leaf Senescence Induced by Mild Water Deficit Follows the Same Sequence of Macroscopic, Biochemical, and Molecular Events as Monocarpic Senescence in Pea

Abstract

We have compared the time course of leaf senescence in pea (Pisum sativum L. cv Messire) plants subjected to a mild water deficit to that of monocarpic senescence in leaves of three different ages in well-watered plants and to that of plants in which leaf senescence was delayed by flower excision. The mild water deficit (with photosynthesis rate maintained at appreciable levels) sped up senescence by 15 d (200°Cd), whereas flower excision delayed it by 17 d (270°Cd) compared with leaves of the same age in well-watered plants. The range of life spans in leaves of different ages in control plants was 25 d (340°Cd). In all cases, the first detected event was an increase in the mRNA encoding a cysteine-proteinase homologous to Arabidopsis SAG2. This happened while the photosynthesis rate and the chlorophyll and protein contents were still high. The 2-fold variability in life span of the studied leaves was closely linked to the duration from leaf unfolding to the beginning of accumulation of this mRNA. In contrast, the duration of the subsequent phases was essentially conserved in all studied cases, except in plants with excised flowers, where the degradation processes were slower. These results suggest that senescence in water-deficient plants was triggered by an early signal occurring while leaf photosynthesis was still active, followed by a program similar to that of monocarpic senescence. They also suggest that reproductive development plays a crucial role in the triggering of senescence.Senescence is the final phase of leaf development, during which a large part of leaf nitrogen, carbon, and minerals is recycled to other organs of the plant (Noodén, 1988a). It consists of an ordered sequence of physiological, biochemical, and ultrastructural changes, involving a decline in functions associated to carbon assimilation and a massive degradation of macromolecules (RNAs, proteins, and lipids) and chlorophylls (Buchanan-Wollaston, 1997; Gan and Amasino, 1997; Noodén et al., 1997). This process relies on the execution of a specific genetic program, which is under the control of a high and complex regulation by various endogenous and environmental factors (Smart, 1994; Gan and Amasino, 1997). It is sped up by water or nitrogen deficits (Merrien et al., 1981; Wolfe et al., 1988a Wolfe et al., , 1988b and delayed when reproductive sinks are removed (Lindoo and Noodén, 1976; Noodén, 1988b; Wolfe et al., 1988a).Acceleration of leaf senescence is thought to be adaptative in plants subjected to water shortage (a) because it reduces the water demand cumulated over the whole plant cycle, thereby avoiding water deficit during seed filling, and (b) because it allows recycling of scarce resources to the reproductive sinks. However, in crops species, early leaf senescence usually correlates with lower yield because cumulative photosynthesis is reduced (Fischer and Kohn, 1966; Merrien et al., 1981; Gifford and Jenkins, 1982; Wolfe et al., 1988a). Selection based on delayed leaf senescence ("stay-green" plants) under drought c...

WRKY54 and WRKY70 co-operate as negative regulators of leaf senescence in Arabidopsis thaliana

“…Leaf senescence occurs in an age-dependent manner (Hensel et al , 1993; Nooden and Penney, 2001) influenced by various endogenous factors including developmental cues and reproductive growth (Gan and Amasino, 1995; Pic et al , 2002; Riefler et al , 2006). In this context, cytokinin, a phytohormone implicated in cell proliferation control during leaf development, acts as a negative regulator of senescence.…”

Salt stress and senescence: identification of cross-talk regulatory components

“…By triggering a wide range of cellular events, salinity, like other abiotic stresses, superimposes its downstream effects on the existing developmental signalling processes. This leads to the activation of whole-plant responses, such as growth reduction, changes in biomass allocation, leaf senescence, and death of plants (Volkmar et al , 1998; Munns, 2002; Pic et al , 2002; Munné-Bosch and Alegre, 2004). It has been proposed that many salt stress-triggered processes, such as a decline in photosynthetic activity or an increase in membrane damage, reflect a hastening of the naturally occurring senescence process (Dwidedi et al , 1979; Dhindsa et al , 1981).…”

An Arabidopsis Mitogen-Activated Protein Kinase Cascade, MKK9-MPK6, Plays a Role in Leaf Senescence

“…Senescence can also be initiated prematurely by a number of exogenous environmental stresses, including light and temperature stress, dehydration, nutrient stress, and pathogen infection (Beers and McDowell, 2001;Pic et al, 2002;Xiong et al, 2005;Hopkins et al, 2007). When the endogenous signals or environmental stresses are perceived by the plant, the signals are subsequently transmitted, resulting in changes in gene expression and/or physiological activities.…”