Effects of senescence-induced alteration in cytokinin metabolism on source-sink relationships and ontogenic and stress-induced transitions in tobacco

Abstract

Senescence and reserve mobilization are integral components of plant development, are basic strategies in stress mitigation, and regulated at least in part by cytokinin. In the present study the effect of altered cytokinin metabolism caused by senescence-specific autoregulated expression of the Agrobacterium tumefaciens IPT gene under control of the P(SAG12) promoter (P(SAG12)-IPT) on seed germination and the response to a water-deficit stress was studied in tobacco (Nicotiana tabacum L.). Cytokinin levels, sugar content and composition of the leaf strata within the canopy of wild-type and P(SAG12)-IPT plants confirmed the reported altered source-sink relations. No measurable difference in sugar and pigment content of discs harvested from apical and basal leaves was evident 72 h after incubation with (+)-ABA or in darkness, indicating that expression of the transgene was not restricted to senescing leaves. No difference in quantum efficiency, photosynthetic activity, accumulation of ABA, and stomatal conductance was apparent in apical, middle and basal leaves of either wild-type or P(SAG12)-IPT plants after imposition of a mild water stress. However, compared to wild-type plants, P(SAG12)-IPT plants were slower to adjust biomass allocation. A stress-induced increase in root:shoot ratio and specific leaf area (SLA) occurred more rapidly in wild-type than in P(SAG12)-IPT plants reflecting delayed remobilization of leaf reserves to sink organs in the transformant. P(SAG12)-IPT seeds germinated more slowly even though abscisic acid (ABA) content was 50% that of the wild-type seeds confirming cytokinin-induced alterations in reserve remobilization. Thus, senescence is integral to plant growth and development and an increased endogenous cytokinin content impacts source-sink relations to delay ontogenic transitions wherein senescence in a necessary process.

Plant growth promotion by 18:0-lyso-phosphatidylethanolamine involves senescence delay

“…15 Source-sink relations and reserve mobilization are key processes in both developmental and stress-induced ontogenic transitions and in the senescence process. 16 Generally speaking, allocation of resources is governed by changing metabolic gradients established by supply and demand that arise as a consequence of the dynamic between anabolic and catabolic respiratory processes. Of the carbohydrate-metabolizing enzymes, Ac INV contributes significantly to increased sugar availability for the establishment of metabolic sinks [17][18][19] it has been identified as an integral component of the molecular mechanism of cytokinin-mediated senescence delay 20 and activity is induced on exposure of tissue to LPE.…”

Conditional up-regulation of cytokinin status increases growth and survival of sugarcane in water-limited conditions

“…This is an important result as it confirms that RAB17 promoter is active in sugarcane, is induced by stress (via stressinduced ABA), and facilitated the production of biologically active CKs in quantities required to elicit physiological and growth responses. This is also consistent with the strategy of conditional expression of IPT driven by other promoters like P SAG12 in tobacco (Cowan et al, 2005), wheat (Sykorova et al, 2008), creeping bentgrass (Xu et al, 2009) and cassava (Zhang et al, 2010) under drought stress. ABA levels observed in the plants here strongly correlated with the degree of stress.…”

“…This result is in line with the reports of IPT up-regulation in P SAG12 ::IPT transgenic tobacco (Cowan et al, 2005), wheat (Sykorova et al, 2008) creeping bentgrass (Xu et al, 2009) and cassava (Zhang et al, 2010) in response to drought stress treatment. In addition, we observed that IPT expression was also up-regulated under well-watered treatment in most of the P Ubi ::IPT plants which is expected as Ubi is a constitutive promoter (Christensen and Quail, 1996), however there was no obvious difference in the phenotype of these lines.…”

“…The explanation for reduce CK content under water stress is either a reduction in CK biosynthesis or enhanced CK degradation (Hwang et al, 2012). Under drought stress, biologically important CKs isopentenyladenine (iP), isopentenyladenosine (iPR), and zeatin-types CK decreased in leaves of tobacco (Cowan et al, 2005), wheat (Sykorova et al, 2008), creeping bentgrass (Xu et al, 2009) and cassava (Zhang et al, 2010).…”

Delayed leaf senescence induces extreme drought tolerance in a flowering plant

“…The P SAG12 -IPT expression was activated at the onset of senescence in the basal leaves of tobacco plants and induced CK biosynthesis, which inhibited leaf senescence and, in turn, diminished P SAG12 -IPT activity. Nevertheless, P SAG12 -IPT transgenic plants displayed altered source-sink relations and nutrient deficiency in young leaves (7), delayed flowering (8), and reduced seedling establishment in response to water-deficit stress (9). We speculated that if the IPT gene could be expressed during plant maturation, but before the onset of senescence, CK production would not be limited to the old leaves, and thus, CK production would not alter nitrogen mobilization within the plant, and source/sink relationships would not be affected.…”