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Leaf senescence is a terminal step in plant growth and development. Considerable information on processes and signals involved in this process has been obtained, although comparatively little is known about leaf senescence in monocotyledonous plants. In particular, little is known about players involved in leaf senescence imposed by a prolonged dark treatment. New information has now been unveiled on dark-induced leaf senescence in a monocot, barley. A close association has been found between ubiquitous polyamines, reactive oxygen species (ROS), and senescence of barley leaves during prolonged darkness. Although polyamines (putrescine, spermidine, and spermine) are absolutely essential for critical cellular functions, including regulation of nucleic acids and protein synthesis, macromolecular structural integrity, and signalling, a strong link between polyamines and dark-induced leaf senescence has been found using barley plant as a model of monocots. Interestingly, Arabidopsis polyamine back-conversion oxidase mutants deficient in the conversion of spermine to spermidine and/or spermidine to putrescine do not occur and have delayed entry into dark-induced leaf senescence. This review summarizes the recent molecular, physiological, and biochemical evidence implicating concurrently polyamines and ethylene in dark-induced leaf senescence and broadening our knowledge on the mechanistic events involved in this important plant death process.

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. Electronic supplementary material The online version of this article (doi:10.1007/s00726-014-1912-y) contains supplementary material, which is available to authorized users.