Senescence-induced loss in photosynthesis enhances cell wall β-glucosidase activity

Abstract

A link between senescence-induced decline in photosynthesis and activity of beta-glucosidase is examined in the leaves of Arabidopsis. The enzyme is purified and characterized. The molecular weight of the enzyme is 58 kDa. It shows maximum activity at pH 5.5 and at temperature of 50 degrees C. Photosynthetic measurements and activity of the enzyme are conducted at different developmental stages including senescence of leaves. Senescence causes a significant loss in total chlorophyll, stomatal conductance, rate of evaporation and in the ability of the leaves for carbon dioxide fixation. The process also brings about a decline in oxygen evolution, quantum yield of photosystem II (PS II) and quantum efficiency of PS II photochemistry of thylakoid membrane. The loss in photosynthesis is accompanied by a significant increase in the activity of the cell wall-bound beta-glucosidase that breaks down polysaccharides to soluble sugars. The loss in photosynthesis as a signal for the enhancement in the activity of the enzyme is confirmed from the observation that incubation of excised mature leaves in continuous dark or in light with a photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) that leads to sugar starvation enhances the activity of the enzyme. The work suggests that in the background of photosynthetic decline, the polysaccharides bound to cell wall that remains intact even during late phase of senescence may be the last target of senescing leaves for a possible source of sugar for remobilization and completion of the energy-dependent senescence program.

In vivo extraction of Arabidopsis cell turgor pressure using nanoindentation in conjunction with finite element modeling

“…However, although the cell no longer expands in mature cells, cell‐wall components continue to be introduced into the wall, leading to an increase in cell‐wall thickness (Wei and Lintilhac, ). In samples from senescing tissue, the cell walls serve as a source of nutrients for the cell and start to thin (Mohapatra et al ., ). The results obtained using our approach (Figure a) match these observations.…”

“…These biological facts are in agreement with our results, which show that the relaxation time of the cell wall reduces with age (Table ), implying that the cell wall in a young cell is more viscous than a mature wall. In a senescing cell, because photosynthesis has ceased, the cell is utilizing sugars in the wall (Mohapatra et al ., ), and the cell wall begins to lose its integrity and structural support.…”

Plant Responses to Sugar Starvation

“…The influence of sugar on leaf senescence was assessed on excised leaves and on leaves of intact plants. The effect of sugar starvation was achieved by keeping the plant material in darkness or by its spraying with photosynthetic inhibitors (Mohapatra et al 2010). To examine the effects of sugar on senescence of various flower parts, cut flowers were treated with various sucrose concentrations in the vase solution (Azad et al 2008, Arrom andMunné-Bosch 2012).…”

Structural Changes in Senescing Oilseed Rape Leaves at Tissue and Subcellular Levels Monitored by Nuclear Magnetic Resonance Relaxometry through Water Status

“…In terms of leaf senescence at the cell level, where chloroplasts are degraded sequentially, relative organelle volume does not seem to be greatly modified, the vacuole remains intact, and in darkness-induced senescence the number of chloroplasts per cell decreases only slightly (Keech et al, 2007). However, major changes in metabolic fluxes and cell water relationships are expected during the senescence program that may be associated with macromolecule catabolism, organic solute synthesis, transport and remobilization, and cell structure reconfiguration such as chloroplast evolution to gerontoplast (Hoertensteiner, 2006;Zhang et al, 2010) through the autophagy process (Wada et al, 2009), accumulation of senescence-associated vacuoles (Otegui et al, 2005), and cell wall degradation (Mohapatra et al, 2010).The senescent leaves of oilseed rape (Brassica napus), a major oleiferous crop, generally fall while still maintaining a high N content (about 2.5%-3% [w/w] of the dry matter; Malagoli et al, 2005). In addition to the environmental impact of this leaking of N out of the plant, the low capacity to remobilize foliar N is associated with a high requirement for N fertilization to meet * Address correspondence to laurent.leport@univ-rennes1.fr.…”