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Leaf senescence can be induced by stress or aging, sometimes in a synergistic manner. It is generally acknowledged that the ability to withstand senescence-inducing conditions can provide plants with stress resilience. Although the signaling and transcriptional networks responsible for a delayed senescence phenotype, often referred to as a functional stay-green trait, have been actively investigated, very little is known about the subsequent metabolic adjustments conferring this aptitude to survival. First, using the individually darkened leaf (IDL) experimental setup, we compared IDLs of wild-type (WT) Arabidopsis (Arabidopsis thaliana) to several stay-green contexts, that is IDLs of two functional stay-green mutant lines, oresara1-2 (ore1-2) and an allele of phytochrome-interacting factor 5 (pif5), as well as to leaves from a WT plant entirely darkened (DP). We provide compelling evidence that arginine and ornithine, which accumulate in all stay-green contexts--likely due to the lack of induction of amino acids (AAs) transport--can delay the progression of senescence by fueling the Krebs cycle or the production of polyamines (PAs). Secondly, we show that the conversion of putrescine to spermidine (SPD) is controlled in an age-dependent manner. Thirdly, we demonstrate that SPD represses senescence via interference with ethylene signaling by stabilizing the ETHYLENE BINDING FACTOR1 and 2 (EBF1/2) complex. Taken together, our results identify arginine and ornithine as central metabolites influencing the stress- and age-dependent progression of leaf senescence. We propose that the regulatory loop between the pace of the AA export and the progression of leaf senescence provides the plant with a mechanism to fine-tune the induction of cell death in leaves, which, if triggered unnecessarily, can impede nutrient remobilization and thus plant growth and survival. [ABSTRACT FROM AUTHOR]

In annual plants with determinate growth, sugar accumulation signals high carbon availability once growth has ceased, resulting in senescence-dependent nutrient recycling to the seeds. However, this senescence-inducing effect of sugars is abolished at cold temperature, where sugar accumulation is important for protection. Here, natural variation was exploited to analyse the effect of chilling on interactions between leaf senescence, sugars, and phytohormones in Arabis alpina, a perennial plant with indeterminate growth. Eight accessions of A. alpina originating from between 2090 and 3090 m above sea level in the French Alps were used to identify heritable adaptations in senescence, stress response, sugars, and phytohormones to altitude. Accessions from high altitudes showed an enhanced capacity for sucrose accumulation and a diminished loss of chlorophyll in response to chilling. At warm temperature, sucrose content was negatively correlated with chlorophyll content, and sucrose treatment induced leaf senescence. Chilling resulted in lower indole-3-acetic acid, but higher zeatin and jasmonic acid contents. Interactions between sugar and phytohormones included a positive correlation between sucrose and jasmonic acid contents that may be involved in promoting the stress-dependent decline in chlorophyll. These findings reveal regulatory interactions that underlie adaptation in the senescence and stress response to chilling. [ABSTRACT FROM AUTHOR]

Abstract: Despite plant age is a key factor to understand the performance and survival of any given species in the field, little is known about its influence on the endogenous levels of cytokinins in juvenile individuals of perennial plants. In this study, we hypothesized that plant age in juvenile mastic trees may affect cytokinin levels and lead to significant changes in cytokinin-related processes, such as leaf growth and pigment accumulation. Two groups of juvenile plants aged 2 and 5 years old were grown under Mediterranean field conditions to evaluate the age-related differences in the endogenous levels of cytokinins and auxins, leaf biomass and area, pigments levels and the efficiency of PSII photochemistry (F v/F m ratio). Furthermore, regression analyses between cytokinin levels and several indicators of stress performance were carried out in a population of juvenile mastic trees. Results showed that endogenous levels of cytokinins, particularly 2-isopentenyladenine (2-iP), increased during periods of intense leaf growth. However, plant age did not affect 2-iP levels, but those of trans-zeatin (Z); 2-year-old plants showing 2-fold higher levels of Z compared to 5-year-old plants, the latter having 11-fold higher total plant biomass. Plant age-related reductions in cytokinins were not associated with reductions in leaf growth in the largest individuals, but with an increased chlorophyll (Chl) a/b ratio. Further regression analyses revealed that although enhanced Chl a/b ratios predispose leaves to suffer photooxidative stress, as indicated by increased anthocyanins levels and lowered F v/F m ratios in juvenile mastic trees, individuals of different ages showed a similar degree of stress tolerance. It is concluded that plant age may alter the endogenous composition of cytokinins and the pattern of pigment accumulation in leaves, but this does not reduce leaf growth or stress tolerance in this species. [Copyright &y& Elsevier]

Leaf senescence is a complex phenomenon occurring in all plant species, but it is still poorly understood in plants grown in Mediterranean field conditions and well-adapted to harsh climatic conditions. To better understand the physiological processes underlying leaf senescence in mastic trees ( Pistacia lentiscus L.), we evaluated leaf growth, water and N content, photosystem II (PSII) photochemistry, lipid peroxidation and levels of photosynthetic pigments, antioxidants, abscisic acid, and salicylic acid and jasmonic acid during the complete leaf lifespan, from early expansion to late senescence in relation to natural climatic conditions in the field. While mature leaves suffered from water and N deficit during late spring and summer, both young (emerging) and old (senescing) leaves were most sensitive to photo-oxidative stress, as indicated by reductions in the Fv/ Fm ratio and enhanced lipid peroxidation during late autumn and winter. Reductions in the Fv/ Fm ratio were associated with low α-tocopherol (vitamin E) levels, while very old, senescing leaves additionally showed severe anthocyanin losses. We have concluded that both young (emerging) and old (senescing) leaves suffer oxidative stress in mastic trees, which may be linked in part to suboptimal temperatures during late autumn and winter as well as to low vitamin E levels. [ABSTRACT FROM AUTHOR]

Increasing plants' photosynthetic efficiency is a major challenge that must be addressed in order to cover the food demands of the growing population in the changing climate. Photosynthesis is greatly limited at the initial carboxylation reaction, where CO 2 is converted to the organic acid 3-PGA, catalyzed by the RuBisCO enzyme. RuBisCO has poor affinity for CO 2 , but also the CO 2 concentration at the RuBisCO site is limited by the diffusion of atmospheric CO 2 through the various leaf compartments to the reaction site. Beyond genetic engineering, nanotechnology can offer a materials-based approach for enhancing photosynthesis, and yet, it has mostly been explored for the light-dependent reactions. In this work, we developed polyethyleneimine-based nanoparticles for enhancing the carboxylation reaction. We demonstrate that the nanoparticles can capture CO 2 in the form of bicarbonate and increase the CO 2 that reacts with the RuBisCO enzyme, enhancing the 3-PGA production in in vitro assays by 20%. The nanoparticles can be introduced to the plant via leaf infiltration and, because of the functionalization with chitosan oligomers, they do not induce any toxic effect to the plant. In the leaves, the nanoparticles localize in the apoplastic space but also spontaneously reach the chloroplasts where photosynthetic activity takes place. Their CO 2 loading-dependent fluorescence verifies that, in vivo , they maintain their ability to capture CO 2 and can be therefore reloaded with atmospheric CO 2 while in planta . Our results contribute to the development of a nanomaterials-based CO 2 -concentrating mechanism in plants that can potentially increase photosynthetic efficiency and overall plants' CO 2 storage.