The Role of Cytokinins in the Development and Metabolism of Barley Leaves III. The Effect on the RNA Metabolism in Various Cell Compartments During Senescence

From Accumulation to Degradation: Reprogramming Polyamine Metabolism Facilitates Dark-Induced Senescence in Barley Leaf Cells

The aim of this study was to analyze whether polyamine (PA) metabolism is involved in dark-induced Hordeum vulgare L. 'Nagrad' leaf senescence. In the cell, the titer of PAs is relatively constant and is carefully controlled. Senescence-dependent increases in the titer of the free PAs putrescine, spermidine, and spermine occurred when the process was induced, accompanied by the formation of putrescine conjugates. The addition of the anti-senescing agent cytokinin, which delays senescence, to dark-incubated leaves slowed the senescence-dependent PA accumulation. A feature of the senescence process was initial accumulation of PAs at the beginning of the process and their subsequent decrease during the later stages. Indeed, the process was accompanied by both enhanced expression of PA biosynthesis and catabolism genes and an increase in the activity of enzymes involved in the two metabolic pathways. To confirm whether the capacity of the plant to control senescence might be linked to PA, chlorophyll fluorescence parameters, and leaf nitrogen status in senescing barley leaves were measured after PA catabolism inhibition and exogenously applied γ-aminobutyric acid (GABA). The results obtained by blocking putrescine oxidation showed that the senescence process was accelerated. However, when the inhibitor was applied together with GABA, senescence continued without disruption. On the other hand, inhibition of spermidine and spermine oxidation delayed the process. It could be concluded that in dark-induced leaf senescence, the initial accumulation of PAs leads to facilitating their catabolism. Putrescine supports senescence through GABA production and spermidine/spermine supports senescence-dependent degradation processes, is verified by H2O2 generation.

Selection of reference genes for qPCR- and ddPCR-based analyses of gene expression in Senescing Barley leaves

Leaf senescence is a tightly regulated developmental or stress-induced process. It is accompanied by dramatic changes in cell metabolism and structure, eventually leading to the disintegration of chloroplasts, the breakdown of leaf proteins, internucleosomal fragmentation of nuclear DNA and ultimately cell death. In light of the global and intense reorganization of the senescing leaf transcriptome, measuring time-course gene expression patterns in this model is challenging due to the evident problems associated with selecting stable reference genes. We have used oligonucleotide microarray data to identify 181 genes with stable expression in the course of dark-induced senescence of barley leaf. From those genes, we selected 5 candidates and confirmed their invariant expression by both reverse transcription quantitative PCR and droplet digital PCR (ddPCR). We used the selected reference genes to normalize the level of the expression of the following senescence-responsive genes in ddPCR assays: SAG12, ICL, AGXT, CS and RbcS. We were thereby able to achieve a substantial reduction in the data variability. Although the use of reference genes is not considered mandatory in ddPCR assays, our results show that it is advisable in special cases, specifically those that involve the following conditions: i) a low number of repeats, ii) the detection of low-fold changes in gene expression or iii) series data comparisons (such as time-course experiments) in which large sample variation greatly affects the overall gene expression profile and biological interpretation of the data.

Dark-induced senescence of barley leaves involves activation of plastid transglutaminases

Transglutaminases (E.C. 2.3.2.13) catalyze the post-translational modification of proteins by establishing ε-(γ-glutamyl) lysine isopeptide bonds and by the covalent conjugation of polyamines to endo-glutamyl residues of proteins. In light of the confirmed role of transglutaminases in animal cell apoptosis and only limited information on the role of these enzymes in plant senescence, we decided to investigate the activity of chloroplast transglutaminases (ChlTGases) and the fate of chloroplast-associated polyamines in Hordeum vulgare L. 'Nagrad' leaves, where the senescence process was induced by darkness (day 0) and continued until chloroplast degradation (day 12). Using an anti-TGase antibody, we detected on a subcellular level, the ChlTGases that were associated with destacked/degraded thylakoid membranes, and beginning on day 5, were also found in the stroma. Colorimetric and radiometric assays revealed during senescence an increase in ChlTGases enzymatic activity. The MS/MS identification of plastid proteins conjugated with exogenous polyamines had shown that the ChlTGases are engaged in the post-translational modification of proteins involved in photosystem organization, stress response, and oxidation processes. We also computationally identified the cDNA of Hv-Png1-like, a barley homologue of the Arabidopsis AtPng1 gene. Its mRNA level was raised from days 3 to 10, indicating that transcriptional regulation controls the activity of barley ChlTGases. Together, the presented results deepen our knowledge of the mechanisms of the events happened in dark-induced senescence of barley leaves that might be activation of plastid transglutaminases.

Are green islands red herrings? Significance of green islands in plant interactions with pathogens and pests

The term green island was first used to describe an area of living, green tissue surrounding a site of infection by an obligately biotrophic fungal pathogen, differentiated from neighbouring yellowing, senescent tissue. However, it has now been used to describe symptoms formed in response to necrotrophic fungal pathogens, virus infection and infestation by certain insects. In leaves infected by obligate biotrophs such as rust and powdery mildew pathogens, green islands are areas where senescence is retarded, photosynthetic activity is maintained and polyamines accumulate. We propose such areas, in which both host and pathogen cells are alive, be termed green bionissia. By contrast, we propose that green areas associated with leaf damage caused by toxins produced by necrotrophic fungal pathogens be termed green necronissia. A range of biotrophic/hemibiotrophic fungi and leaf-mining insects produce cytokinins and it has been suggested that this cytokinin secretion may be responsible for the green island formation. Indeed, localised cytokinin accumulation may be a common mechanism responsible for green island formation in interactions of plants with biotrophic fungi, viruses and insects. Models have been developed to study if green island formation is pathogen-mediated or host-mediated. They suggest that green bionissia on leaves infected by biotrophic fungal pathogens represent zones of host tissue, altered physiologically to allow the pathogen maximum access to nutrients early in the interaction, thus supporting early sporulation and increasing pathogen fitness. They lead to the suggestion that green islands are 'red herrings', representing no more than the consequence of the infection process and discrete changes in leaf senescence.

Plants and biotrophs: a pivotal role for cytokinins?

Plants infected with biotrophic fungal pathogens exhibit reduced photosynthetic rates, nutrient mobilization towards infection sites and, in interactions where discrete pustules are formed, green islands are induced. The ability of cytokinins to mobilize nutrients towards sites of application and to delay senescence led researchers to speculate that cytokinins might be involved in nutrient mobilization and green island formation in plants infected with biotrophic fungi. There is evidence that the reduction in photosynthesis in infected leaves results from early increases in invertase activity, leading to carbohydrate accumulation and the downregulation of photosynthetic metabolism. In this Opinion article, we propose that these seemingly disparate changes in host physiology in infected plants are the result of cytokinin-induced increases in invertase activity occurring early on in the host-pathogen interaction.

Changes in Cell Ultrastructure and Zeatin Riboside Concentrations in Hedera helix, Pelargonium zonale, Prunus avium, and Rubus ulmifolius Leaves Infected by Fungi

Ultrastructural alterations in epidermal and mesophyll cells and variations in endogenous zeatin riboside (ZR) concentrations were studied in leaves of Hedera helix, Pelargonium zonale, Pru-nus avium, and Rubus ulmifolius infected by Colletotrichum trichellum, Puccinia pelargonii-zonalis, Cercospora circumscissa, and Phragmidium violaceum, respectively. Infected tissues showed a marked increase in vesicles, myelin-like structures, and electron-dense bodies associated with plasma membranes. The main changes to the chloroplast included thylakoid swelling and disruption of the chloroplast envelope. The ZR content of the green islands was always higher than that of the yellow, senescent parts of the same leaves; the highest levels of ZR were observed in the green areas of infected Prunus avium (462.2 pmol g^-1 fresh weight [FW]) and Rubus ulmifolius (441.6 pmol g^-1 FW), followed by Pelargonium zonale (263.8 pmol g^-1 FW) and Hedera helix (219.8 pmol g^-1 FW); the yellow zones of the same leaves had lower ZR contents (78.3, 73.9, 73.6, and 18.1 pmol g^-1 FW, respectively). The green islands had almost the same ZR content as the controls (green healthy leaves). These results suggest a relationship between ultrastructural alterations and ZR content of these plant species (blackberry, cherry, English ivy, geranium) in reacting to this type of biotic stress and could confirm the role of cytokinins as senescence-delaying hormones.