Extracellular Invertase Is an Essential Component of Cytokinin-Mediated Delay of Senescence[W]

Abstract

Leaf senescence is the final stage of leaf development in which the nutrients invested in the leaf are remobilized to other parts of the plant. Whereas senescence is accompanied by a decline in leaf cytokinin content, exogenous application of cytokinins or an increase of the endogenous concentration delays senescence and causes nutrient mobilization. The finding that extracellular invertase and hexose transporters, as the functionally linked enzymes of an apolasmic phloem unloading pathway, are coinduced by cytokinins suggested that delay of senescence is mediated via an effect on source-sink relations. This hypothesis was further substantiated in this study by the finding that delay of senescence in transgenic tobacco (Nicotiana tabacum) plants with autoregulated cytokinin production correlates with an elevated extracellular invertase activity. The finding that the expression of an extracellular invertase under control of the senescence-induced SAG12 promoter results in a delay of senescence demonstrates that effect of cytokinins may be substituted by these metabolic enzymes. The observation that an increase in extracellular invertase is sufficient to delay leaf senescence was further verified by a complementing functional approach. Localized induction of an extracellular invertase under control of a chemically inducible promoter resulted in ectopic delay of senescence, resembling the naturally occurring green islands in autumn leaves. To establish a causal relationship between cytokinins and extracellular invertase for the delay of senescence, transgenic plants were generated that allowed inhibition of extracellular invertase in the presence of cytokinins. For this purpose, an invertase inhibitor was expressed under control of a cytokinin-inducible promoter. It has been shown that senescence is not any more delayed by cytokinin when the expression of the invertase inhibitor is elevated. This finding demonstrates that extracellular invertase is required for the delay of senescence by cytokinins and that it is a key element of the underlying molecular mechanism.

Cis- and trans-zeatin differentially modulate plant immunity

“…Since cZ shows neither significant impact on SA accumulation nor on Pst proliferation but still reduced symptom development, a more general CK impact on plant fitness and the physiological status has to be considered. In addition to the induction of distinct defense reactions the classical and well established physiological CK effects such as the induction of apoplasmic phloem unloading and sink metabolism 15,16 as well as the delay of senescence through invertase activity, 17 contribute to maintain plant tissue integrity and thereby plant fitness. The maintenance of tissue integrity integrated within the whole plant-pathogen interaction can extend the time frame during which distinct defense reactions act on the pathogen itself.…”

“…4 Detached leaves of 7-10-wk-old N. tabacum SR1 plants, grown under greenhouse conditions, 4 were fed via petioles with 10 μM cZ (Olchemim Ltd.,), tZ (Duchefa), diluted from 100 mM stock solutions in 0.5 M NaOH (Roth) or accordingly diluted NaOH solvent control for 24 h prior to Pst infection. 4,17 After this pre-treatment, leaves were infiltrated with 10 6 cfu/ml Pst in 10 mM MgCl 2 and leaves were put back on water. The bacteria were derived from a exponentially growing liquid culture in LB medium supplemented with 20 mg/l tetracycline (Sigma-Aldrich).…”

Metabolic Regulation of Leaf Senescence in Sunflower (Helianthus annuus L.) Plants

“…The increase in soluble sugars may also be ascribed to senescence causing a loss of functional and structural integrity in cell membranes, thereby boosting membrane lipid catabolism and hence sugar production by gluconeogenesis (Buchanan-Wollaston et al, 2003b;Lim et al, 2007). Leaf senescence is a plastic process triggered by a variety of external and internal factors (Weaver & Amasino, 2001;Buchanan-Wollaston et al, 2003a;Balibrea-Lara et al, 2004;Wingler et al, 2006). Senescence reduces photosynthetic carbon fixation, but is important for the recycling of nitrogen and other nutrients (Díaz et al, 2005;Wingler et al, 2005).…”

In Arabidopsis thaliana, the invertase inhibitors AtC/VIF1 and 2 exhibit distinct target enzyme specificities and expression profiles

“…The molecular mechanisms of this downregulation of CWI and VI activities are not yet fully understood but one specific mechanism operates via complex formation of CWI and VI with proteinaceous invertase inhibitors [17][18][19]. A recent study proposed a crucial role to an apoplastic invertase inhibitor in the control of leaf senescence [3].…”

“…Acid invertases are of exceptional importance as they are the only enzymes able to cleave sucrose in extracellular compartments such as the vacuole (vacuolar invertase; VI) or the apoplastic space (cell wall invertase; CWI). Acid invertases are responsible for sucrose unloading from the conducting tissues and for the adjustment of the hexose/sucrose ratio, which affects plant development including programmed cell death [3]. Thus, the hexose/sucrose ratio acts as an important metabolic signal, which dramatically affects gene expression profiles [4][5][6][7][8].…”