The concept that photosynthetic flux is influenced by the accumulation of photo-assimilate persisted for 100 years before receiving any strong experimental support. Precise analysis of the mechanisms of photosynthetic responses to sink activity required the development of a battery of appropriate molecular techniques and has benefited from contemporary interest in the effects of elevated CO2 on photosynthesis. Photosynthesis is one of the most highly integrated and regulated metabolic processes to maximize the use of available light, to minimize the damaging effects of excess light and to optimize the use of limiting carbon and nitrogen resources. Hypotheses of feedback regulation must take account of this integration. In the short term, departure from homeostasis can lead to redox signals, which cause rapid changes in the transcription of genes encoding photosystems I and II. End-product synthesis can exert short-term metabolic feedback control through Pi recycling. Beyond this, carbohydrate accumulation in leaves when there is an imbalance between source and sink at the whole plant level can lead to decreased expression of photosynthetic genes and accelerated leaf senescence. In a high CO2 world this may become a more prevalent feature of photosynthetic regulation. However, sink regulation of photosynthesis is highly dependent on the physiology of the rest of the plant. This physiological state regulates photosynthesis through signal transduction pathways that co-ordinate the plant carbon : nitrogen balance, which match photosynthetic capacity to growth and storage capacity and underpin and can override the direct short-term controls of photosynthesis by light and CO2. Photosynthate supply and phytohormones, particularly cytokinins, interact with nitrogen supply to control the expression of photosynthesis genes, the development of leaves and the whole plant nitrogen distribution, which provides the dominant basis for sink regulation of photosynthesis.

Sink regulation of photosynthesis

Drought, the most prominent threat to agricultural production worldwide, accelerates leaf senescence, leading to a decrease in canopy size, loss in photosynthesis and reduced yields. On the basis of the assumption that senescence is a type of cell death program that could be inappropriately activated during drought, we hypothesized that it may be possible to enhance drought tolerance by delaying drought-induced leaf senescence. We generated transgenic plants expressing an isopentenyltransferase gene driven by a stress- and maturation-induced promoter. Remarkably, the suppression of drought-induced leaf senescence resulted in outstanding drought tolerance as shown by, among other responses, vigorous growth after a long drought period that killed the control plants. The transgenic plants maintained high water contents and retained photosynthetic activity (albeit at a reduced level) during the drought. Moreover, the transgenic plants displayed minimal yield loss when watered with only 30% of the amount of water used under control conditions. The production of drought-tolerant crops able to grow under restricted water regimes without diminution of yield would minimize drought-related losses and ensure food production in water-limited lands.

/pdf/delayed-leaf-senescence-induces-extreme-drought-tolerance-in-25ylzrvfg4.pdf

Delayed leaf senescence induces extreme drought tolerance in a flowering plant

Senescence in green plants is a complex and highly regulated process that occurs as part of plant development or can be prematurely induced by stress. In the last decade, the main focus of research has been on the identification of senescence mutants, as well as on genes that show enhanced expression during senescence. Analysis of these is beginning to expand our understanding of the processes by which senescence functions. Recent rapid advances in genomics resources, especially for the model plant species Arabidopsis, are providing scientists with a dazzling array of tools for the identification and functional analysis of the genes and pathways involved in senescence. In this review, we present the current understanding of the mechanisms by which plants control senescence and the processes that are involved.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1467-7652.2003.00004.x

The molecular analysis of leaf senescence--a genomics approach.

The complex juvenile/maturity transition during a plant's life cycle includes growth, reproduction, and senescence of its fundamental organs: leaves, flowers, and fruits. Growth and senescence of leaves, flowers, and fruits involve several genetic networks where the phytohormone ethylene plays a key role, together with other hormones, integrating different signals and allowing the onset of conditions favorable for stage progression, reproductive success and organ longevity. Changes in ethylene level, its perception, and the hormonal crosstalk directly or indirectly regulate the lifespan of plants. The present review focused on ethylene's role in the development and senescence processes in leaves, flowers and fruits, paying special attention to the complex networks of ethylene crosstalk with other hormones. Moreover, aspects with limited information have been highlighted for future research, extending our understanding on the importance of ethylene during growth and senescence and boosting future research with the aim to improve the qualitative and quantitative traits of crops.

/pdf/ethylene-role-in-plant-growth-development-and-senescence-5jpulgnpiq.pdf

Ethylene Role in Plant Growth, Development and Senescence: Interaction with Other Phytohormones

Extracellular invertase is the key enzyme of an apoplasmic phloem unloading pathway and catalyses the hydrolytic cleavage of the transport sugar sucrose released into the apoplast. This mechanism contributes to long-distance assimilate transport, provides the substrate to sustain heterotrophic growth and generates metabolic signals known to effect various processes of primary metabolism and defence responses. The essential function of extracellular invertase for supplying carbohydrates to sink organs was demonstrated by the finding that antisense repression of an anther-specific isoenzyme provides an efficient method for metabolic engineering of male sterility. The regulation of extracellular invertase by all classes of phytohormones indicates an essential link between the molecular mechanism of phytohormone action and primary metabolism. The up-regulation of extracellular invertase appears to be a common response to various biotic and abiotic stress-related stimuli such as pathogen infection and salt stress, in addition to specific stress-related reactions. Based on the observed co-ordinated regulation of source/sink relations and defence responses by sugars and stress-related stimuli, the identified activation of distinct subsets of MAP kinases provides a mechanism for signal integration and distribution within such complex networks. Sucrose derivatives not synthesized by higher plants, such as turanose, were shown to elicit responses distinctly different from metabolizable sugars and are rather perceived as stress-related stimuli.

Extracellular invertase: key metabolic enzyme and PR protein

We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin synthesis, i.e. targeted to senescing leaves and negatively auto-regulated (PSAG12‐IPT), thus preventing developmental abnormalities. Plants were grown with growthlimiting nitrogen supply. Compared to the wild-type, endogenous levels of free zeatin (Z)- and Z riboside (ZR)type cytokinins were increased up to 15-fold (total ZR up to 100-fold) in senescing leaves, and twofold in younger leaves of PSAG12‐IPT. In these plants, the senescenceassociated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more ( 15 N-labelled) N than younger leaves, associated with reduced shoot N accumulation (‐60%) and a partially inverted canopy N profile in PSAG12‐IPT plants. While root N accumulation was not affected, N translocation to non-senescing leaves was progressively reduced. We discuss potential consequences of these modified sink‐source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2000.00544.x

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

The contribution of pre-defoliation reserves and current assimilates to leaf and root growth was examined in Lolium perenne L. during regrowth after defoliation. Differential steady-state labelling with 13 C (CO 2 with δ 13 C = -0.0281 and -0.0088) and 15 N (NO 3 - with 1.0 and 0.368 atom percentage, i.e. δ 15 N = 1.742 and 0.0052, respectively) was applied for 2 weeks after defoliation. Rapidly growing tissues were isolated, i.e. the basal elongation and maturation zones of the most rapidly expanding leaves and young root tips, with a biomass turnover rate > 1 d -1 . C and N weights of the elongation zone showed a transient decline. The dry matter and C concentration in fresh biomass of leaf growth zones transiently decreased by up to 25% 2 d after defoliation, while the N concentration remained constant. This 'dilution' of growth zone C indicates a decreased net influx of carbohydrates relative to growth-related influx of water and N in expanding cells, immediately after defoliation. Recovery of the total C and N weights of the leaf elongation zone coincided with net incorporation of currently absorbed C and N, as shown by the kinetics of δ 13 C and atom percentage 15 N in the growth zones after defoliation. C isotope discrimination (Δ 13 C) in leaf growth zones was about 23‰, 1-2‰ higher than the Δ in root tips. Δ 15 N in the leaf and root growth zones was 10 ± 3‰. The leaf elongation zones (at 0-0.03 m from the tiller base) and the distant root tips (about 0.2 m from the base) exhibited similar kinetics of current C and N incorporation. The amount of pre-defoliation C and N in the growth zones, expressed as a fraction of total C and N, decreased from 1.0 to 0.5 at 3 (C) and 5 (N) d after defoliation, and to 0.1 at 5 (C) and 14 (N) d after defoliation. Thus, the dependence of growth zones on current assimilate supply was significant, and stronger for C than for N. The important roles of current assimilates (as compared to pre-defoliation reserves) and 'dilution' of dry matter in regrowth after defoliation are discussed in relation to the method of labelling and the functional and morphological heterogeneity of shoot tissues.

Kinetics and relative significance of remobilized and current C and N incorporation in leaf and root growth zones of Lolium perenne after defoliation: assessment by 13C and 15N steady-state labelling

R. De Visser

Papers

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

Kinetics and relative significance of remobilized and current C and N incorporation in leaf and root growth zones of Lolium perenne after defoliation: assessment by 13C and 15N steady-state labelling