Rice transglutaminase gene: Identification, protein expression, functionality, light dependence and specific cell location

Plant Transglutaminases: New Insights in Biochemistry, Genetics, and Physiology

Transglutaminases (TGases) are calcium-dependent enzymes that catalyse an acyl-transfer reaction between primary amino groups and protein-bound Gln residues. They are widely distributed in nature, being found in vertebrates, invertebrates, microorganisms, and plants. TGases and their functionality have been less studied in plants than humans and animals. TGases are distributed in all plant organs, such as leaves, tubers, roots, flowers, buds, pollen, and various cell compartments, including chloroplasts, the cytoplasm, and the cell wall. Recent molecular, physiological, and biochemical evidence pointing to the role of TGases in plant biology and the mechanisms in which they are involved allows us to consider their role in processes such as photosynthesis, plant fertilisation, responses to biotic and abiotic stresses, and leaf senescence. In the present paper, an in-depth description of the biochemical characteristics and a bioinformatics comparison of plant TGases is provided. We also present the phylogenetic relationship, gene structure, and sequence alignment of TGase proteins in various plant species, not described elsewhere. Currently, our knowledge of these proteins in plants is still insufficient. Further research with the aim of identifying and describing the regulatory components of these enzymes and the processes regulated by them is needed.

AtPng1 knockout mutant of Arabidopsis thaliana shows a juvenile phenotype, morpho-functional changes, altered stress response and cell wall modifications

In order to ascertain the role of plant transglutaminases (TGase) in growth and abiotic stress response, the AtPng1 knock out (KO) line of A. thaliana has been analyzed during plant development and under heat and wound stress. Comparing wild type (WT) and KO lines a 58-kDa band was immunodetected by anti-AtPng1p antibody in the cell wall and chloroplasts only in the WT line. A residual TGase activity, not showing correlation with development nor stress response, was still present in the KO line. The KO line was less developed, with a juvenile phenotype characterized by fewer, smaller and less differentiated cells. Chloroplast TGase activity was insensitive to mutation. Data on stressed plants showed that (i) KO plants under heat stress were more juvenile compared to WT, (ii) different responses between WT and KO lines after wounding took place. TGase activity was not completely absent in the KO line, presenting high activity in the plastidial fraction. In general, the mutation affected A. thaliana growth and development, causing less differentiated cytological and anatomical features.

Changes in antioxidants and leaf pigments of safflower (Carthamus tinctorius L.) affected by exogenous spermine under water deficit

Water deficit is one of the most limiting factors for plant growth and production. Polyamines are osmo-active compounds and have important roles in plant resistance to water limitation. A pot experiment was undertaken in a greenhouse as factorial based on complete randomized block design with three replications to assess the physiological and biochemical responses of safflower to different levels of water supply (100% and 40% field capacity) and spermine (0, 40 and 60 µM). Ascorbate peroxidase and peroxidase activities (POX), malondialdehyde (MDA), hydrogen peroxide (H₂O₂), anthocyanins, soluble protein, soluble sugars and proline contents in shoots increased, while total phenols, flavonoids, and photosynthetic pigments significantly decreased due to water deficit. Foliar spray of spermine mitigated the adverse effects of water deficit by increasing the catalase, superoxide dismutase, POX activities, soluble proteins and photosynthetic pigments, and by decreasing MDA and H₂O₂ contents. Spermine could, therefore, play an important role in protecting photosynthetic system and cellular membranes during drought stress in safflower.

Ectopic expression of CsTGase enhances salt tolerance by regulating polyamine biosynthesis, antioxidant activities and Na+/K+ homeostasis in transgenic tobacco

Transglutaminases (TGases), mediators of the transamidation of specific proteins by polyamines (PA), play critical roles in PA metabolism in animals, but their functions and regulatory mechanisms are largely unknown in plants. In this study, we demonstrated that TGase from cucumber played a protective role in the regulation of PA metabolism under salt stress. The expression of TGase was induced by salt stress in cucumber. Ectopic overexpression of cucumber TGase in tobacco conferred enhanced tolerance to salt stress based on both external symptoms and membrane integrity. Overexpression lines maintained high levels of PAs under salt stress, suggesting that PAs played a vital role in TGase-induced salt tolerance. In contrast, the levels of Na+ content in the wild-type (WT) plants increased, while they decreased in the overexpression plants. The expression levels of several genes related to ion exchange enhanced, and the Na+/K+ ratio decreased by increased TGase activity under salt stress. The activities of the proton-pump ATPase (H+-ATPase), vacuolar H+-ATPase (V-ATPase) and vacuolar H+-pyrophosphatase (PPase) were higher in the overexpression lines than in WT plants under salt stress. Moreover, the malondialdehyde (MDA) and H2O2 contents were significantly lower in the overexpression lines than in WT plants, accompanied by increased antioxidant enzyme activity. Taken together, these findings demonstrate that TGase plays protective roles in response to salt stress, which may promote plant survival by regulating PA metabolism and the Na+/K+ balance under salt stress.

Overexpression of Transglutaminase from Cucumber in Tobacco Increases Salt Tolerance through Regulation of Photosynthesis

Transglutaminase (TGase) is a regulator of posttranslational modification of protein that provides physiological protection against diverse environmental stresses in plants. Nonetheless, the mechanisms of TGase-mediated salt tolerance remain largely unknown. Here, we found that the transcription of cucumber TGase (CsTGase) was induced in response to light and during leaf development, and the CsTGase protein was expressed in the chloroplast and the cell wall. The overexpression of the CsTGase gene effectively ameliorated salt-induced photoinhibition in tobacco plants, increased the levels of chloroplast polyamines (PAs) and enhanced the abundance of D1 and D2 proteins. TGase also induced the expression of photosynthesis related genes and remodeling of thylakoids under normal conditions. However, salt stress treatment reduced the photosynthesis rate, PSII and PSI related genes expression, D1 and D2 proteins in wild-type (WT) plants, while these effects were alleviated in CsTGase overexpression plants. Taken together, our results indicate that TGase-dependent PA signaling protects the proteins of thylakoids, which plays a critical role in plant response to salt stress. Thus, overexpression of TGase may be an effective strategy for enhancing resistance to salt stress of salt-sensitive crops in agricultural production.

The Over-expression of the Plastidial Transglutaminase from Maize in Arabidopsis Increases the Activation Threshold of Photoprotection

Plastidial transglutaminase is one of the most promising enzymes in chloroplast bioenergetics due to its link with polyamine pathways and the cross talk with signals such as Ca(2+) and GTP. Here, we show the effect of the increase of transglutaminase activity in Arabidopsis by using genetic transformation techniques. These lines fulfill their biological cycle normally (normal growth in soil, production of viable seeds) and show a relatively mild increase in transglutaminase activity (127%). These overexpressors of transglutaminase (OE TGase) have an extended stroma thylakoid network (71% higher number of PSIIβ centers), similar chlorophyll content (-4%), higher linear electron flow (+13%), and higher threshold of photoprotection activation (∼100%). On the other hand OE TGase showed a reduced maximum photochemistry of PSII (-6.5%), a smaller antenna per photosystem II (-25%), a lower photoprotective "energization" quenching or qE (-77% at 490 μmol photons m(-2) s(-1)) due to a higher threshold of qE activation and slightly lower light induced proton motive force (-17%). The role of the polyamines and of the transglutaminase in the regulation of chemiosmosis and photoprotection in chloroplasts is discussed.

Proteomic and Physiological Analyses Reveal Putrescine Responses in Roots of Cucumber Stressed by NaCl

Soil salinity is a major environmental constraint that threatens agricultural productivity. Different strategies have been developed to improve crop salt tolerance, among which the effects of polyamines have been well-reported. To gain a better understanding of the cucumber (Cucumis sativus L.) responses to NaCl and unravel the underlying mechanism of exogenous putrescine (Put) alleviating salt-induced damage, comparative proteomic analysis was conducted on cucumber roots treated with NaCl, and/or Put for 7 days. The results showed that exogenous Put restored the root growth inhibited by NaCl. Sixty-two differentially expressed proteins implicated in various biological processes were successfully identified by MALDI-TOF/TOF MS. The four largest categories included proteins involved in defense response (24.2%), protein metabolism (24.2%), carbohydrate metabolism (19.4%), and amino acid metabolism (14.5%). Exogenous Put up-regulated most identified proteins involved in carbohydrate metabolism, implying an enhancement in energy generation. Proteins involved in defense response and protein metabolism were differently regulated by Put, which indicated the roles of Put in stress resistance and proteome rearrangement. Put also increased the abundance of proteins involved in amino acid metabolism. Meanwhile, physiological analysis showed that Put could further up-regulated the levels of free amino acids in salt stressed-roots. In addition, Put also improved endogenous polyamines contents by regulating the transcription levels of key enzymes in polyamine metabolism. Taken together, these results suggest that Put may alleviate NaCl-induced growth inhibition through degradation of misfolded/damaged proteins, activation of stress defense, and the promotion of carbohydrate metabolism to generate more energy.

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.

Proteomic and transcriptomic analysis of rice tranglutaminase and chloroplast-related proteins

The recently cloned rice transglutaminase gene (tgo) is the second plant transglutaminase identified to date (Campos et al. Plant Sci. 205-206 (2013) 97-110). Similarly to its counterpart in maize (tgz), this rice TGase was localized in the chloroplast, although in this case not exclusively. To further characterise plastidial tgo functionality, proteomic and transcriptomic studies were carried out to identify possible TGO-related proteins. Some LHCII antenna proteins were identified as TGO related using an in vitro proteomic approach, as well as ATPase and some PSII core proteins by mass spectrometry. To study the relationship between TGO and other plastidial proteins, a transcriptomic in vivo Dynamic Array (Fluidigm™) was used to analyse the mRNA expression of 30 plastidial genes with respect to that of tgo, in rice plants subjected to different periods of continuous illumination. The results indicated a gene-dependent tendency in the expression pattern that was related to tgo expression and to the illumination cycle. For certain genes, including tgo, significant differences between treatments, principally at the initiation and/or at the end of the illumination period, connected with the day/night cycling of gene expression, were observed. The tgo expression was especially related to plastidial proteins involved in photoprotection and the thylakoid electrochemical gradient.

Polyamines are common players in different facets of plant programmed cell death

Programmed cell death (PCD) is a process that occurs throughout the life span of every plant life, from initial germination of the seed to the senescence of the plant. It is a normal physiological milestone during the plant's developmental process, but it can also be induced by external factors, including a variety of environmental stresses and as a response to pathogen infections. Changes in the morphology of the nucleus is one of the most noticeable during PCD but all the components of the plant cell (cytoplasm, cytoskeleton and organelles) are involved in this fascinating process. To date, relatively little is known about PCD in plants, but several factors, among which polyamines (PAs) and plant growth regulators, have been shown to play an important role in the initiation and regulation of the process. The role of PAs in plant PCD appears to be multifaceted acting in some instances as pro-survival molecules, whereas in others seem to be implicated in accelerating PCD. The molecular mechanism is still under study. Here we present some PCD plant models, focusing on the role of the enzyme responsible for PA conjugation to proteins: transglutaminase (TGase), an enzyme linked with the process of PCD also in some animal models. The role of PAs and plant TGase in the senescence and PCD in flowers, leaf and the self-incompatibility of pollen will be discussed and examined in depth.

Transglutaminases: recent achievements and new sources

Transglutaminases are a family of enzymes (EC 2.3.2.13), widely distributed in various organs, tissues, and body fluids, that catalyze the formation of a covalent bond between a free amine group and the γ-carboxamide group of protein or peptide-bound glutamine. Besides forming these bonds, that exhibit high resistance to proteolytic degradation, transglutaminases also form extensively cross-linked, generally insoluble, protein biopolymers that are indispensable for the organism to create barriers and stable structures. The extremely high cost of transglutaminase of animal origin has hampered its wider application and has initiated efforts to find an enzyme of microbial origin. Since the early 1990s, many microbial transglutaminase-producing strains have been found, and production processes have been optimized. This has resulted in a rapidly increasing number of applications of transglutaminase in the food sector. However, applications of microbial transglutaminase in other sectors have also been explored, but in a much lesser extent. Our group has identified a transglutaminase in the oomycete Phytophthora cinnamomi, which is able to induct defense responses and disease-like symptoms. In this mini-review, we report the achievements in this area in order to illustrate the importance and the versatility of transglutaminases.

Senescence and programmed cell death in plants: polyamine action mediated by transglutaminase

Research on polyamines (PAs) in plants laps a long way of about 50 years and many roles have been discovered for these aliphatic cations. PAs regulate cell division, differentiation, organogenesis, reproduction, dormancy-break and senescence, homeostatic adjustments in response to external stimuli and stresses. Nevertheless, the molecular mechanisms of their multiple activities are still matter of research. PAs are present in free and bound forms and interact with several important cell molecules; some of these interactions may occur by covalent linkages catalyzed by transglutaminase (TGase), giving rise to "cationization" or cross-links among specific proteins. Senescence and programmed cell death (PCD) can be delayed by PAs; in order to re-interpret some of these effects and to obtain new insights into their molecular mechanisms, their conjugation has been revised here. The TGase-mediated interactions between proteins and PAs are the main target of this review. After an introduction on the characteristics of this enzyme, on its catalysis and role in PCD in animals, the plant senescence and PCD models in which TGase has been studied, are presented: the corolla of naturally senescing or excised flowers, the leaves senescing, either excised or not, the pollen during self-incompatible pollination, the hypersensitive response and the tuber storage parenchyma during dormancy release. In all the models examined, TGase appears to be involved by a similar molecular mechanism as described during apoptosis in animal cells, even though several substrates are different. Its effect is probably related to the type of PCD, but mostly to the substrate to be modified in order to achieve the specific PCD program. As a cross-linker of PAs and proteins, TGase is an important factor involved in multiple, sometimes controversial, roles of PAs during senescence and PCD.

Plastid-associated polyamines: their role in differentiation, structure, functioning, stress response and senescence

Polyamines are low-molecular weight biogenic amines. They are a specific group of cell growth and development regulators. In the past decade biochemical, molecular and genetic studies have contributed much to a better understanding of the biological role of polyamines in the plant cell. Substantial evidence has also been added to our understanding of the role of polyamines in plastid development. In developing chloroplasts, polyamines serve as a nitrogen source for protein and chlorophyll synthesis. In chloroplast structure, thylakoid proteins linked to polyamines belong mainly to antenna proteins of light-harvesting chlorophyll a/b-protein complexes. The fact that LHCII oligomeric forms are much more intensely labelled by polyamines, in comparison to monomeric forms, suggests that polyamines participate in oligomer stabilisation. In plastid metabolism, polyamines modulate effectiveness of photosynthesis. The role of polyamines in mature chloroplasts is also related to the photo-adaptation of the photosynthetic apparatus to low and high light intensity and its response to environmental stress. The occurrence of polyamines and enzymes participating in their metabolism at every stage of plastid development indicates that polyamines play a role in plastid differentiation, structure, functioning and senescence.