Hemoglobins, programmed cell death and somatic embryogenesis

Genetic mechanisms of aging in plants: What can we learn from them?

Plants hold all records in longevity. Their aging is a complex process. In the presented review, we analyzed published data on various aspects of plant aging with focus on any inferences that could shed a light on aging in animals and help to fight it in human. Plant aging can be caused by many factors, such as telomere depletion, genomic instability, loss of proteostasis, changes in intercellular interaction, desynchronosis, autophagy misregulation, epigenetic changes and others. Plants have developed a number of mechanisms to increase lifespan. Among these mechanisms are gene duplication ("genetic backup"), the active work of telomerases, abundance of meristematic cells, capacity of maintaining the meristems permanently active and continuous activity of phytohormones. Plant aging usually occurs throughout the whole perennial life, but could be also seasonal senescence. Study of causes for seasonal aging can also help to uncover the mechanisms of plant longevity. The influence of different factors such as microbiome communities, glycation, alternative oxidase activity, mitochondrial dysfunction on plant longevity was also reviewed. Adaptive mechanisms of long-lived plants are considered. Further comparative study of the mechanisms underlying longevity of plants is necessary. This will allow us to reach a potentially new level of understanding of the aging process of plants.

Phytoglobin overexpression promotes barley growth in the presence of enhanced level of atmospheric nitric oxide

To investigate the effect of high atmospheric NO concentrations on crop plants and the role of phytoglobins under these conditions, we performed a long-term study on barley 'Golden Promise' wild type (WT), class 1 phytoglobin knockdown (HvPgb1.1-) and class 1 phytoglobin overexpression (HvPgb1.1+) lines. Plants were cultivated with nitrogen-free nutrient solution during the entire growth period and were fumigated with different NO concentration (ambient, 800, 1500, and 3000 ppb). Analysis of fresh weight, stem number, chlorophyll content, and effective quantum yield of PSII showed that NO fumigation promoted plant growth and tillering significantly in the HvPgb1.1+ line. After 80 d of NO fumigation, dry matter weight, spikes number, kernel number, and plant kernel weight were significantly increased in HvPgb1.1+ plants with increasing NO concentration. In contrast, yield decreased in WT and HvPgb1.1- plants the higher the NO level. Application of atmospheric 15NO and 15NO2 demonstrated NO specificity of phytoglobins. 15N from 15NO could be detected in RNA, DNA, and proteins of barley leaves and the 15N levels were significantly higher in HvPgb1.1+ plants in comparison with HvPgb1.1- and WT plants. Our results demonstrate that overexpression of phytoglobins allows plants to more efficiently use atmospheric NO as N source.

Spatio-temporal expression of phytoglobin: a determining factor in the NO specification of cell fate

Plant growth and development rely on the orchestration of cell proliferation, differentiation, and ultimately death. After varying rounds of divisions, cells respond to positional cues by acquiring a specific fate and embarking upon distinct developmental pathways which might differ significantly from those of adjacent cells exposed to diverse cues. Differential cell behavior is most apparent in response to stress, when some cells might be more vulnerable than others to the same stress condition. This appears to be the case for stem cells which show abnormal features of differentiation and ultimately signs of deterioration at the onset of specific types of stress such as hypoxia and water deficit. A determining factor influencing cell behavior during growth and development, and cell response during conditions of stress is nitric oxide (NO), the level of which can be regulated by phytoglobins (Pgbs), known scavengers of NO. The modulation of NO by Pgbs can be cell, tissue, and/or organ specific, as revealed by the expression patterns of Pgbs dictated by the presence of distinct cis-regulatory elements in their promoters. This review discusses how the temporal and spatial Pgb expression pattern influences NO-mediated responses and ultimately cell fate acquisition in plant developmental processes.

Phytoglobins regulate nitric oxide-dependent abscisic acid synthesis and ethylene-induced program cell death in developing maize somatic embryos

During maize somatic embryogenesis, suppression of phytoglobins (Pgbs) reduced ABA levels leading to ethylene-induced programmed cell death in the developing embryos. These effects modulate embryonic yield depending on the cellular localization of specific phytoglobin gene expression. Suppression of Zea mays phytoglobins (ZmPgb1.1 or ZmPgb1.2) during somatic embryogenesis induces programmed cell death (PCD) by elevating nitric oxide (NO). While ZmPgb1.1 is expressed in many embryonic domains and its suppression results in embryo abortion, ZmPgb1.2 is expressed in the basal cells anchoring the embryos to the embryogenic tissue. Down-regulation of ZmPgb1.2 is required to induce PCD in these anchor cells allowing the embryos to develop further. Exogenous applications of ABA could reverse the effects caused by the suppression of either of the two ZmPgbs. A depletion of ABA, ascribed to a down-regulation of biosynthetic genes, was observed in those embryonic domains where the respective ZmPgbs were repressed. These effects were mediated by NO. Depletion in ABA content increased the transcription of genes participating in the synthesis and response of ethylene, as well as the accumulation of ethylene, which influenced embryogenesis. Somatic embryo number was reduced by high ethylene levels and increased with pharmacological treatments suppressing ethylene synthesis. The ethylene inhibition of embryogenesis was linked to the production of reactive oxygen species (ROS) and the execution of PCD. Integration of ABA and ethylene in the ZmPgb regulation of embryogenesis is proposed in a model where NO accumulates in ZmPgb-suppressing cells, decreasing the level of ABA. Abscisic acid inhibits ethylene biosynthesis and the NO-mediated depletion of ABA relieves this inhibition causing ethylene to accumulate. Elevated ethylene levels trigger production of ROS and induce PCD. Ethylene-induced PCD in the ZmPgb1.1-suppressing line [ZmPgb1.1 (A)] leads to embryo abortion, while PCD in the ZmPgb1.2-suppressing line [ZmPgb1.2 (A)] results in the elimination of the anchor cells and the successful development of the embryos.

Excessive UDPG resulting from the mutation of UAP1 causes programmed cell death by triggering reactive oxygen species accumulation and caspase-like activity in rice

Lesion mimic mutants are valuable to unravel the mechanisms governing the programmed cell death (PCD) process. Uridine 5'-diphosphoglucose-glucose (UDPG) functions as a signaling molecule activating multiple pathways in animals, but little is known about its function in plants. Two novel allelic mutants of spl29 with typical PCD characters and reduced pollen viability were obtained by ethane methyl sulfonate mutagenesis in rice cv Kitaake. The enzymatic analyses showed that UDP-N-acetylglucosamine pyrophosphorylase 1 (UAP1) irreversibly catalyzed the decomposition of UDPG. Its activity was severely destroyed and caused excessive UDPG accumulation, with the lesion occurrence associated with the enhanced caspase-like activities in spl29-2. At the transcriptional level, several key genes involved in endoplasmic reticulum stress and the unfolded protein response were abnormally expressed. Moreover, exogenous UDPG could aggravate lesion initiation and development in spl29-2. Importantly, exogenous UDPG and its derivative UDP-N-acetylglucosamine could induce reactive oxygen species (ROS) accumulation and lesion mimics in Kitaake seedlings. These results suggest that the excessive accumulation of UDPG, caused by the mutation of UAP1, was a key biochemical event resulting in the lesion mimics in spl29-2. Thus, our findings revealed that UDPG might be an important component involved in ROS accumulation, PCD execution and lesion mimicking in rice, which also provided new clues for investigating the connection between sugar metabolism and PCD process.

NO dioxygenase- and peroxidase-like activity of Arabidopsis phytoglobin 3 and its role in Sclerotinia sclerotiorum defense

Phytoglobin 3 appears to be ubiquitous in plants, yet there has been dearth of evidence for their potent physiological functions. Previous crystallographic studies suggest a potential NO dioxygenase like activity of Arabidopsis phytoglobin 3 (AHb3). The present work examined the in vivo function of AHb3 in plant physiology and its role in biotic stress using Arabidopsis- Sclerotinia sclerotorium pathosystem. The gene was found to be ubiquitously expressed in all plant tissues, with moderately increased expression in roots. Its expression was induced upon NO, H₂O₂ and biotic stress. A C-terminal tagged GFP version of the wild type protein revealed its enhanced accumulation in the guard cells. AHb3-GFP was found to be partitioned majorly into the nucleus while residual amounts were present in the cytoplasm. The loss of function AHb3 mutant exhibited reduced root length and fresh weight. AHb3 knockout lines also displayed enhanced susceptibility towards the S. sclerotiorum. Interestingly, these lines displayed enhanced ROS accumulation upon pathogen challenge as suggested by DAB staining. Furthermore, enhanced/decreased NO accumulation in AHb3 knockout/overexpression lines upon treatment with multiple NO donors suggests a potent NO dioxygenase like activity for the protein. Taken together, our data indicate that AHb3 play a crucial role in regulating root length as well as in mediating defense response against S. sclerotiorum, possibly by modulating NO and ROS levels.

Phytoglobin expression influences soil flooding response of corn plants

Background and Aims Excess water is a limiting factor for crop productivity. Under conditions of full submergence or flooding, plants can experience prolonged oxygen depletion which compromises basic physiological and biochemical processes. Severe perturbations of the photosynthetic machinery with a concomitant decline in photosynthetic potential as a result of elevated levels of reactive oxygen species (ROS) are the major consequences of water excess. Phytoglobins (Pgbs) are ubiquitous proteins induced by several types of stress which affect plant response by modulating nitric oxide. Methods Maize plants overexpressing or downregulating two Pgb genes were subjected to soil flooding for 10 d and their performance was estimated by measuring several gas exchange parameters including photosynthetic rate. Above-ground tissue was utilized to localize ROS and to measure the expression and activities of major antioxidant enzymes. Key Results Relative to the wild type, flooded plants overexpressing Pgb genes retained a greater photosynthetic rate and enhanced activity of several antioxidant enzymes. These plants also exhibited high levels of ascorbic acid and reduced ROS staining. This was in contrast to flooded plants downregulating Pgb genes and characterized by the lowest photosynthetic rates and reduced expression and activities of many antioxidant enzymes. Conclusions Induction of Pgb genes alleviates flooding stress by limiting ROS-induced damage and ensuring a sustained photosynthetic rate. This is achieved through improvements of the ascorbate antioxidant status including an enrichment of the ascorbate pool via de novo and recycling mechanisms, and increased activities of several ROS-scavenging enzymes.

Regulation of programmed cell death by phytoglobins

Programmed cell death (PCD) is a fundamental plant process in growth and development and in response to both biotic and abiotic stresses. Nitric oxide (NO) is a central component in determining whether a cell undergoes PCD, either as a direct elicitor of the response or as a factor in signal transduction from various hormones. Both NO and hormones that use NO as a signal transducer are mobile in the plant. Why do one set of cells die while adjacent cells remain alive, if this is the case? There is evidence to suggest that phytoglobins (Pgbs; previously termed non-symbiotic hemoglobins) may act as binary switches to determine plant cellular responses to perturbations. There are anywhere from one to five Pgb genes in plants that are expressed in response to growth and development and to stress. One of their main functions is to scavenge NO. This review will discuss how Pgb modulates cellular responses to auxin, cytokinin, and jasmonic acid during growth and development and in response to stress. The moderation in the production of reactive oxygen species (ROS) by Pgbs and the effects on PCD will also be discussed. An overall mechanism for Pgb involvement will be presented.

Dying with Style: Death Decision in Plant Embryogenesis

Embryogenesis is a fascinating event during the plant life cycle encompassing several steps whereby the zygote develops into a fully developed embryo which, in angiosperms, is composed of an axis separating the apical meristems, and two cotyledons. Recapitulation of embryogenesis can also occur in vitro through somatic embryogenesis, where somatic cells are induced to form embryos, and androgenesis, in which embryos originate from immature male gametophytes. Besides cell division and differentiation, embryo patterning in vivo and in vitro requires the dismantling and selective elimination of cells and tissues via programmed cell death (PCD). While the manifestation of the death program has long been acknowledged in vivo, especially in relation to the elimination of the suspensor during the late phases of embryo development, PCD during in vitro embryogenesis has only been described in more recent years. Independent studies using the gymnosperm Norway spruce and the angiosperm maize have shown that the death program is crucial for the proper formation and further development of immature somatic embryos. This chapter summarizes the recent advances in the field of PCD during embryogenesis and proposes novel regulatory mechanisms activating the death program in plants.

Recent Advances on Genetic and Physiological Bases of In Vitro Somatic Embryo Formation

Somatic embryogenesis involves a broad repertoire of genes, and complex expression patterns controlled by a concerted gene regulatory network. The present work describes this regulatory network focusing on the main aspects involved, with the aim of providing a deeper insight into understanding the total reprogramming of cells into a new organism through a somatic way. To the aim, the chromatin remodeling necessary to totipotent stem cell establishment is described, as the activity of numerous transcription factors necessary to cellular totipotency reprogramming. The eliciting effects of various plant growth regulators on the induction of somatic embryogenesis is also described and put in relation with the activity of specific transcription factors. The role of programmed cell death in the process, and the related function of specific hemoglobins as anti-stress and anti-death compounds is also described. The tools for biotechnology coming from this information is highlighted in the concluding remarks.

Rice ( Oryza) hemoglobins

Hemoglobins (Hbs) corresponding to non-symbiotic (nsHb) and truncated (tHb) Hbs have been identified in rice ( Oryza). This review discusses the major findings from the current studies on rice Hbs. At the molecular level, a family of the nshb genes, consisting of hb1, hb2, hb3, hb4 and hb5, and a single copy of the thb gene exist in Oryza sativa var. indica and O. sativa var. japonica, Hb transcripts coexist in rice organs and Hb polypeptides exist in rice embryonic and vegetative organs and in the cytoplasm of differentiating cells. At the structural level, the crystal structure of rice Hb1 has been elucidated, and the structures of the other rice Hbs have been modeled. Kinetic analysis indicated that rice Hb1 and 2, and possibly rice Hb3 and 4, exhibit a very high affinity for O ₂, whereas rice Hb5 and tHb possibly exhibit a low to moderate affinity for O ₂. Based on the accumulated information on the properties of rice Hbs and data from the analysis of other plant and non-plant Hbs, it is likely that Hbs play a variety of roles in rice organs, including O ₂-transport, O ₂-sensing, NO-scavenging and redox-signaling. From an evolutionary perspective, an outline for the evolution of rice Hbs is available. Rice nshb and thb genes vertically evolved through different lineages, rice nsHbs evolved into clade I and clade II lineages and rice nshbs and thbs evolved under the effect of neutral selection. This review also reveals lacunae in our ability to completely understand rice Hbs. Primary lacunae are the absence of experimental information about the precise functions of rice Hbs, the properties of modeled rice Hbs and the cis-elements and trans-acting factors that regulate the expression of rice hb genes, and the partial understanding of the evolution of rice Hbs.

Membrane transporters mediating root signalling and adaptive responses to oxygen deprivation and soil flooding

This review provides a comprehensive assessment of a previously unexplored topic: elucidating the role that plasma- and organelle-based membrane transporters play in plant-adaptive responses to flooding. We show that energy availability and metabolic shifts under hypoxia and anoxia are critical in regulating membrane-transport activity. We illustrate the high tissue and time dependence of this regulation, reveal the molecular identity of transporters involved and discuss the modes of their regulation. We show that both reduced oxygen availability and accumulation of transition metals in flooded roots result in a reduction in the cytosolic K(+) pool, ultimately determining the cell's fate and transition to programmed cell death (PCD). This process can be strongly affected by hypoxia-induced changes in the amino acid pool profile and, specifically, ϒ-amino butyric acid (GABA) accumulation. It is suggested that GABA plays an important regulatory role, allowing plants to proceed with H2 O2 signalling to activate a cascade of genes that mediate plant adaptation to flooding while at the same time, preventing the cell from entering a 'suicide program'. We conclude that progress in crop breeding for flooding tolerance can only be achieved by pyramiding the numerous physiological traits that confer efficient energy maintenance, cytosolic ion homeostasis, and reactive oxygen species (ROS) control and detoxification.

Plant hemoglobin participation in cell fate determination

Plant hemoglobins (Hbs) have been identified as master regulators in determining the developmental fate of specific cells during maize embryogenesis. Whether an embryogenic cell lives or undergoes programmed cell death (PCD) is modulated by Hbs, through their tight interactions with nitric oxide (NO) and auxin. During maize embryogenesis, Hb-suppressing cells accumulate NO, are depleted of auxin, and are committed to die. We propose that Hbs control cell fate by regulating NO and auxin homeostasis, and that this type of mechanism may influence other hormonal responses modulating plant behavior during development and stress conditions.