Polyamines - A New Metabolic Switch: Crosstalk With Networks Involving Senescence, Crop Improvement, and Mammalian Cancer Therapy

Abstract

Polyamines (PAs) are low molecular weight organic cations comprising biogenic amines that play multiple roles in plant growth and senescence. PA metabolism was found to play a central role in metabolic and genetic reprogramming during dark-induced barley leaf senescence (DILS). Robust PA catabolism can impact the rate of senescence progression in plants. We opine that deciphering senescence-dependent polyamine-mediated multidirectional metabolic crosstalks is important to understand regulation and involvement of PAs in plant death and re-mobilization of nutrients during senescence. This will involve optimizing the use of PA biosynthesis inhibitors, robust transgenic approaches to modulate PA biosynthetic and catabolic genes, and developing novel germplasm enriched in pro- and anti-senescence traits to ensure sustained crop productivity. PA-mediated delay of senescence can extend the photosynthesis capacity, thereby increasing grain starch content in malting grains such as barley. On the other hand, accelerating the onset of senescence can lead to increases in mineral and nitrogen content in grains for animal feed. Unraveling the "polyamine metabolic switch" and delineating the roles of PAs in senescence should further our knowledge about autophagy mechanisms involved in plant senescence as well as mammalian systems. It is noteworthy that inhibitors of PA biosynthesis block cell viability in animal model systems (cell tumor lines) to control some cancers, in this instance, proliferative cancer cells were led toward cell death. Likewise, PA conjugates work as signal carriers for slow release of regulatory molecule nitric oxide in the targeted cells. Taken together, these and other outcomes provide examples for developing novel therapeutics for human health wellness as well as developing plant resistance/tolerance to stress stimuli.

Keywords: CRISPR/Cas9; autophagy; cancer therapy; cell death; crop improvement; polyamines-nitric oxide conjugates; senescence; transcriptome profiling.

Figure 1

Plant polyamine metabolism pathways and its inhibition. Putrescine (Put), spermidine (Spd), and spermine (Spm) constitute major PAs that are primary amines with two or more amine groups. The biosynthesis of PAs is well established in plants. Put in many plants is synthesized from arginine (Arg) via agmatine catalyzed by Arg decarboxylase (ADC) and from ornithine (Orn) by Orn decarboxylase (ODC) except Arabidopsis in whose genome Orn decarboxylase seems not present. Put is thereafter sequentially converted to Spd and Spm/thermo-Spm (T-Spm) through successive addition of aminopropyl groups from S-adenosylmethionine catalyzed by S-adenosylmethionine decarboxylase (SAMDC). The transfer of the aminopropyl groups is catalyzed by Spd and Spm/T-Spm synthases (SPDS/SPMS/TSPMS), respectively. On the other hand, diamine (DAO) and polyamine (PAO) oxidases work in tandem to deaminate each PA, producing, in the process, the signaling molecule hydrogen peroxide (H₂O₂). Back-conversions from Spm to Put via Spd, and Spm to Spd, are catalyzed by PAOs(bc). Blue boxes indicate inhibitors of polyamine metabolism and blue font indicate polyamines and enzymes taking part in PA metabolism. ADC, arginine decarboxylase; ODC, ornithine decarboxylase; SAMDC, S-adenosylmethionine decarboxylase; SPDS/SPMS, Spd and Spm synthases; DAO, diamine oxidases; PAO, polyamine oxidases; AIH, agmatine iminohydrolase; CPA, N-carbamoyl putrescine amidohydrolase. Inhibitors: 1,4-DB – 1,4-diamino-butanone, AG – diaminoguanidine, CHA – cyclohexylammonium sulphate, D-arginine, DFMA – γ-difluoromethylarginine, DFMO – α-difluoromethylornithine, G – guazatine, MGBG -methylglyoxalbis-(guanylhydrazone).

Figure 2

Regulation of autophagy pathways in cell development of yeast (A) and plants (B) under normal and stress conditions. (A) This model for the mechanism of autophagy and its regulation has been established for yeast, where the process is triggered by amino acid starvation. Multiple signaling autophagy pathways converge at the expression and activity of autophagy genes ATG1 and ATG13. Autophagy is promoted by AMP-activated protein kinase (AMPK). Conversely, autophagy is inhibited by the target of rapamycin kinase (TOR), a central cell growth regulator that integrates growth factor and nutrient signals. Under nutrient sufficiency, high TOR activity prevents ATG13 activation by phosphorylating ATG13 Ser 757 and disrupting the interaction between ATG1 and ATG13. TOR, protein kinase A, AMK (AMP activated kinase), and GCN2 (General Control Nonderepressible2) are kinases operating in autophagy signaling pathway. Elongation initiation factor 2α (E2Fa) and the transcription factor GCN4 regulate expression of ATG1 and 13. (B) Potential TOR signaling pathways in Arabidopsis. The TOR complex, including TOR, RAPTOR, and LST8 (RAPTOR recruits substrates and presents them to TOR for phosphorylation, and LST8 stabilizes the TOR complex), senses and integrates multiple upstream signals such as nutrient starvation. TOR may serve as a negative regulator of autophagy. Some TOR substrates in plants have been identified, including: AML1 (Arabidopsis Mei2-like1, Mei2 is a meiosis signaling molecule that has been suggested to be a potential TOR substrate, also in yeast), EBP1 (ErbB – 3 epidermal growth factor receptor binding protein), S6K (ribosomal p70 S6 kinase), and Tap46 (a regulatory subunits of PP2A (protein 2 phosphatase type 2A), which is phosphorylated by TOR, suggesting that Tap46 is a direct substrate of TOR). These substrates may function to control translation, cell growth, and autophagy (modified from Liu and Bassham, 2012).

Figure 3

Autophagy and dark-induced leaf senescence (DILS) of barley seedlings. (A) Model. (B) Ultrastructure of autophagy symptoms of dark-induced senescing parenchyma cells. Early (days 3 and 7) and late (day 10) events of leaf senescence and time limit for reversal (arrows on the top indicate the point of no return) of the senescence process. During the initial senescence period (day 3 of darkness), tonoplast invagination, presence of small cytoplasmic fragments near or connected with tonoplast and vacuoles, and shrunken protoplasts are apparent. On day 7 of senescence, a few cells show discontinuity of the cell membrane, while by day 10, tonoplast apparently ruptures. Consequently, all the organelles undergo gradual disintegration and localized to the central part of the cell. The cell membrane increasingly loosens and, consequently, the intracellular compartmentation is lost. Cell death during senescence is distinguished by rapidly occurring changes in the chloroplasts, whereas the nucleus and mitochondria are relatively more stable, and their degradation occurs only after the final lytic stage following vacuole tonoplast rupture. In the later stages of cell death, distinguishing specific organelles is not possible. However, the shrinking of the protoplast and deformation of the cell wall are clearly observed (see the micrographs). Autophagy is apparent during ultrastructural observations of senescing parenchyma cells seen as small autophagic bodies inside vacuoles, autophagosomes presence in protoplasts, and tonoplast rupture. The changes, at each stage of DILS, are accompanied by elements of micro-, macro- and mega-autophagy. Autophagy role in the metabolic turnover of cell components as one of the mechanisms of quality control of the leaf senescence is discerned. In this process, nutrients such as carbon, nitrogen and phosphorus are released in the course of degradation of proteins, lipids, sugars and nucleic acids, and then transported to younger leaves, ripening fruits, and for seed formation. Metabolism and selective remobilization of macromolecules, which are crucial in the effective performance of the process, are accompanied first by micro- and then macro-autophagy. These studies have emphasized that the efficient regulation of autophagous process is a symptom of the vitality of senescing cells, which must at every stage maintain the ability to keep homeostasis. Bars, 200 nm days 0 and 3; 500 nm days 7 and 10 (top row of B); bars, 200 nm days 0, 3, 7, and 10 (bottom row of B) (modified from Sobieszczuk-Nowicka et al., 2018).

Figure 4

Polyamine metabolism crosstalks (multidirectional links) with the metabolic network during the induced-senescence process. Polyamines as a metabolic switch for barley leaf senescence is a good model for developing a molecular basis of the process in order to apply such information for developing resilient crops for the future. PA metabolism inhibitors as well as transgenic approaches can be used to over-express and/or silence some of the rate-limiting PA biosynthetic and catabolic genes to test specific barley PA transgenics for their adaptability to leaf senescence phenomenon. PAs may, in future, play a role in reprogramming plant senescence that can be altered for a pro-growth phenotype by exogenously directed application of natural and synthetic PAs. This can help plants to develop tolerance to the broad spectrum of stress factors and thereby lead to higher plant productivity. The delineation of the roles of PAs in senescence should lead also to a better understanding of senescence-related cell death mechanisms and provide new knowledge about PCD in mammalian systems since PAs are universal bioregulators of these processes across kingdoms.