Hemoglobin Control of Cell Survival/Death Decision Regulates in Vitro Plant Embryogenesis

Enrichment of two important metabolites D-galacturonic acid and D-glucuronic acid inhibits MdHb1-mediated fruit softening in apple

In apples, fruit firmness is a crucial quality trait influencing fruit storability, transportability, shelf life and consumer preference. However, the genetic network underlying this trait remains unclear. Therefore, the present study investigated the changes in apple fruit at different stages of postharvest storage using a combination of transcriptomic and metabolomic analyses. With prolonged storage, we detected a significant increase in two metabolites, D-galacturonic acid (D-GalUA) and D-glucuronic acid (D-GlcA), which are associated with a key class 1 non-symbiotic haemoglobin (MdHb1). We innovatively found that MdHb1 regulates fruit softening by catalysing the conversion from protopectin to water-soluble pectin. Biochemical analysis demonstrated that MdMYB2/MdNAC14/MdNTL9 transcription factors directly bind to the MdHb1 promoter to activate its transcriptional expression and promote fruit softening. Further injection experiments in apple fruit and histological as well as transmission electron microscopy analyses of the fruit samples revealed that D-GalUA and D-GlcA reduce the transcription of MdHb1, or through the MdMYB2/MdNAC14/MdNTL9-MdHb1 regulatory module, thereby delaying fruit softening. Our study provides novel insights into the role of two important metabolites, D-GalUA and D-GlcA, in the regulation of MdHb1-mediated fruit softening in apples.

The arabidopsis phytoglobin 1 (Pgb1) involvement in somatic embryogenesis is linked to changes in ethylene and the class VII ethylene transcription factor HRE2

MAIN CONCLUSION

Phytoglobin1 promotes Arabidopsis somatic embryogenesis through the mediation of ethylene and the ERFVII HRE2. Generation of somatic embryos in Arabidopsis (Arabidopsis thaliana) is a two-step process, encompassing an induction phase where embryogenic tissue (ET) is formed followed by a developmental phase encouraging the growth of the embryos. Using previously characterized transgenic lines dysregulating the class 1 Phytoglobin (Pgb1) we show that suppression of Pgb1 decreases somatic embryogenesis (SE). Both the formation of ET (SE efficiency) and production of SE (SE productivity) are repressed in explants where Pgb1 is downregulated. The levels of Pgb1 transcripts peak in the middle phase of the induction period coinciding with the formation of the ET. Presence of Pgb1 results in a transcriptional depression of ethylene synthesis and of the class VII ethylene transcription factor (ERFVII) HRE2. Suppression of ethylene after day 3 of induction, or repression of HRE2 are needed for SE efficiency and the decline in HRE2 transcripts appears to be independent from the level of ethylene. Over-expression of HRE2 inhibits SE efficiency regardless of the expression of Pgb1. Furthermore, a functional HRE2 generates a peak in Pgb1 transcripts during the middle induction phase. The expression of another ERFVII, RAP2.12, is not altered by changes in Pgb1 levels, and disruption of RAP2.12 has no effect on SE efficiency although it enhances SE productivity in a Pgb1-independent fashion. Thus, Pgb1 is an important regulator of Arabidopsis SE, and its action is linked to changes in ethylene and the ERFVII HRE2.

Preserving root stem cell functionality under low oxygen stress: the role of nitric oxide and phytoglobins

MAIN CONCLUSION

The preservation of quiescent center stem cell integrity in hypoxic roots by phytoglobins is exercised through their ability to scavenge nitric oxide and attenuate its effects on auxin transport and cell degradation. Under low oxygen stress, the retention or induction of phytoglobin expression maintains cell viability while loss or lack of induction of phytoglobin leads to cell degradation. Plants have evolved unique attributes to ensure survival in the environment in which they must exist. Common among the attributes is the ability to maintain stem cells in a quiescent (or low proliferation) state in unfriendly environments. From the seed embryo to meristematic regions of the plant, quiescent stem cells exist to regenerate the organism when environmental conditions are suitable to allow plant survival. Frequently, plants dispose of mature cells or organs in the process of acclimating to the stresses to ensure survival of meristems, the stem cells of which are capable of regenerating cells and organs that have been sacrificed, a feature not generally available to mammals. Most of the research on plant stress responses has dealt with how mature cells respond because of the difficulty of specifically examining plant meristem responses to stress. This raises the question as to whether quiescent stem cells behave in a similar fashion to mature cells in their response to stress and what factors within these critical cells determine whether they survive or degrade when exposed to environmental stress. This review attempts to examine this question with respect to the quiescent center (QC) stem cells of the root apical meristem. Emphasis is put on how varying levels of nitric oxide, influenced by the expression of phytoglobins, affect QC response to hypoxic stress.

Arabidopsis root apical meristem survival during waterlogging is determined by phytoglobin through nitric oxide and auxin

MAIN CONCLUSION

Over-expression of phytoglobin mitigates the degradation of the root apical meristem (RAM) caused by waterlogging through changes in nitric oxide and auxin distribution at the root tip. Plant performance to waterlogging is ameliorated by the over-expression of the Arabidopsis Phytoglobin 1 (Pgb1) which also contributes to the maintenance of a functional RAM. Hypoxia induces accumulation of ROS and damage in roots of wild type plants; these events were preceded by the exhaustion of the RAM resulting from the loss of functionality of the WOX5-expressing quiescent cells (QCs). These phenotypic deviations were exacerbated by suppression of Pgb1 and attenuated when the same gene was up-regulated. Genetic and pharmacological studies demonstrated that degradation of the RAM in hypoxic roots is attributed to a reduction in the auxin maximum at the root tip, necessary for the specification of the QC. This reduction was primarily caused by alterations in PIN-mediated auxin flow but not auxin synthesis. The expression and localization patterns of several PINs, including PIN1, 2, 3 and 4, facilitating the basipetal translocation of auxin and its distribution at the root tip, were altered in hypoxic WT and Pgb1-suppressing roots but mostly unchanged in those over-expressing Pgb1. Disruption of PIN1 and PIN2 signal in hypoxic roots suppressing Pgb1 initiated in the transition zone at 12 h and was specifically associated to the absence of Pgb1 protein in the same region. Exogenous auxin restored a functional RAM, while inhibition of the directional auxin flow exacerbated the degradation of the RAM. The regulation of root behavior by Pgb1 was mediated by nitric oxide (NO) in a model consistent with the recognized function of Pgbs as NO scavengers. Collectively, this study contributes to our understanding of the role of Pgbs in preserving root meristem function and QC niche during conditions of stress, and suggests that the root transition zone is most vulnerable to hypoxia.

Plant stem cells under low oxygen: metabolic rewiring by phytoglobin underlies stem cell functionality

Root growth in maize (Zea mays L.) is regulated by the activity of the quiescent center (QC) stem cells located within the root apical meristem. Here, we show that despite being highly hypoxic under normal oxygen tension, QC stem cells are vulnerable to hypoxic stress, which causes their degradation with subsequent inhibition of root growth. Under low oxygen, QC stem cells became depleted of starch and soluble sugars and exhibited reliance on glycolytic fermentation with the impairment of the TCA cycle through the depressed activity of several enzymes, including pyruvate dehydrogenase (PDH). This finding suggests that carbohydrate delivery from the shoot might be insufficient to meet the metabolic demand of QC stem cells during stress. Some metabolic changes characteristic of the hypoxic response in mature root cells were not observed in the QC. Hypoxia-responsive genes, such as PYRUVATE DECARBOXYLASE (PDC) and ALCOHOL DEHYDROGENASE (ADH), were not activated in response to hypoxia, despite an increase in ADH activity. Increases in phosphoenolpyruvate (PEP) with little change in steady-state levels of succinate were also atypical responses to low-oxygen tensions. Overexpression of PHYTOGLOBIN 1 (ZmPgb1.1) preserved the functionality of the QC stem cells during stress. The QC stem cell preservation was underpinned by extensive metabolic rewiring centered around activation of the TCA cycle and retention of carbohydrate storage products, denoting a more efficient energy production and diminished demand for carbohydrates under conditions where nutrient transport may be limiting. Overall, this study provides an overview of metabolic responses occurring in plant stem cells during oxygen deficiency.

Interplay between the Brassica napus phytoglobin (BnPgb1), folic acid, and antioxidant responses enhances plant tolerance to waterlogging

Oxygen deprivation by waterlogging reduces the productivity of several crop species, including the oil-producing crop Brassica napus L., which is highly sensitive to excess moisture. Among factors induced by oxygen deficiency are phytoglobins (Pgbs), heme-containing proteins known to ameliorate the response of plants to the stress. This study examined the early responses to waterlogging in B. napus plants over-expressing or down-regulating the class 1 (BnPgb1) and class 2 (BnPgb2) Pgbs. The depression of gas exchange parameters and plant biomass was exacerbated by the suppression of BnPgb1, while suppression of BnPgb2 did not evoke any changes. This suggests that natural occurring levels of BnPgb1 (but not BnPg2) are required for the response of the plants to waterlogging. Typical waterlogging symptoms, including the accumulation of reactive oxygen species (ROS) and the deterioration of the root apical meristem (RAM) were attenuated by over-expression of BnPgb1. These effects were associated with the activation of antioxidant system and the transcriptional induction of folic acid (FA). Pharmacological treatments revealed that high levels of FA were sufficient to revert the inhibitory effect of waterlogging, suggesting that the interplay between BnPgb1, antioxidant responses and FA might contribute to plant tolerance to waterlogging stress.

KNOX transcription factor VvHB63 affects grape seed development by interacting with protein VvHB06

The fast-growing demand for seedless table grapes has attracted the attention of scientists for the development of new seedless cultivars. Various genes and pathways have been identified which affect seedlessness. However, the detail of the mechanism(s) regulating seedless traits in grape is still unclear, and genes related to seedlessness in grape require further study. Transcriptomic and genomic analyses of Homeobox (HB) transcription factors have suggested the involvement of HB genes, especially of HB-KNOX members, in grape seed development. Here, we functionally characterize VvHB63 gene in grape and report its role in fruit and seed development. VvHB63 showed higher expressions levels in the chalaza and integument of ovules in seedless grapes, than in seeded ones. However, no differences were observed in the sequences of seedless and seeded grape cultivars. In situ hybridization (ISH) analysis showed that VvHB63 gene was expressed in the episperm cells and ovules of 'Thompson Seedless'. Conserved domains KNOX1 and KNOX2 were important for the interaction of VvHB63 with VvHB06. Heterologous over-expression of VvHB63 (35 S::VvHB63-OE) in tomato induced smaller fruits and seeds than in wild type or SlTkn1-KO. The synergistic cooperation between VvHB63 and related proteins play an important role in ovule development.

Specificity in root domain accumulation of Phytoglobin1 and nitric oxide (NO) determines meristematic viability in water-stressed Brassica napus roots

BACKGROUND AND AIMS

Drought reduces plant productivity, especially in the susceptible species Brassica napus. Water stress, mimicked by applications of 10 % polyethylene glycol (PEG), elevates nitric oxide (NO) in root cells after a few hours, contributing to degradation of the root apical meristems (RAMs), the function of which relies on auxin and brassinosteroids (BRs). Phytoglobins (Pgbs) are effective NO scavengers induced by this stress. This study examines the effects of BnPgb1 dysregulation in dehydrating B. napus roots, and the spatiotemporal relationship between Pgb1 and activities of auxin and BRs in the regulation of the RAM.

METHODS

Brassica napus lines over-expressing [BnPgb1(S)] or down-regulating [BnPgb1(RNAi)] BnPgb1 were exposed to PEG-induced water stress. The localization of BnPgb1, NO, auxin and PIN1 were analysed during the first 48 h, while the expression level of biosynthetic auxin and BR genes was measured during the first 24 h. Pharmacological treatments were conducted to assess the requirement of auxin and BR in dehydrating roots.

KEY RESULTS

During PEG stress, BnPgb1 protein accumulated preferentially in the peripheral domains of the root elongation zone, exposing the meristem to NO, which inhibits polar auxin transport (PAT), probably by interfering with PIN1 localization and the synthesis of auxin. Diminished auxin at the root tip depressed the synthesis of BR and caused the degradation of the RAMs. The strength of BnPgb1 signal in the elongation zone was increased in BnPgb1(S) roots, where NO was confined to the most apical cells. Consequently, PAT and auxin synthesis were retained, and the definition of RAMs was maintained. Auxin preservation of the RAM required BRs, although BRs alone was not sufficient to fully rescue drought-damaged RAMs in auxin-depleted environments.

CONCLUSIONS

The tissue-specific localization of BnPgb1 and NO determine B. napus root responses to water stress. A model is proposed in which auxin and BRs act as downstream components of BnPgb1 signalling in the preservation of RAMs in dehydrating roots.

Over-expression of the barley Phytoglobin 1 (HvPgb1) evokes leaf-specific transcriptional responses during root waterlogging

Oxygen deprivation (hypoxia) in the root due to waterlogging causes profound metabolic changes in the aerial organs depressing growth and limiting plant productivity in barley (Hordeum vulgare L.). Genome-wide analyses in waterlogged wild type (WT) barley (cv. Golden Promise) plants and plants over-expressing the phytoglobin 1 HvPgb1 [HvPgb1(OE)] were performed to determine leaf specific transcriptional responses during waterlogging. Normoxic WT plants outperformed their HvPgb1(OE) counterparts for dry weight biomass, chlorophyll content, photosynthetic rate, stomatal conductance, and transpiration. Root waterlogging severely depressed all these parameters in WT plants but not in HvPgb1(OE) plants, which exhibited an increase in photosynthetic rate. In leaftissue, root waterlogging repressed genes encoding photosynthetic components and chlorophyll biosynthetic enzymes, while induced those of reactive oxygen species (ROS)-generating enzymes. This repression was alleviated in HvPgb1(OE) leaves which also exhibited an induction of enzymes participating in antioxidant responses. In the same leaves, the transcript levels of several genes participating in nitrogen metabolism were also higher relative to WT leaves. Ethylene levels were diminished by root waterlogging in leaves of WT plants, but not in HvPgb1(OE), which were enriched in transcripts of ethylene biosynthetic enzymes and ethylene response factors. Pharmacological treatments increasing the level or action of ethylene further suggested the requirement of ethylene in plant response to root waterlogging. In natural germplasm an elevation in foliar HvPgb1 between 16h and 24h of waterlogging occurred in tolerant genotypes but not in susceptible ones. By integrating morpho-physiological parameters with transcriptome data, this study provides a framework defining leaf responses to root waterlogging and indicates that the induction of HvPgb1 may be used as a selection tool to enhance resilience to excess moisture.

Conformational Dynamics of Phytoglobin BvPgb1.2 from Beta vulgaris ssp. vulgaris

Plant hemoglobins, often referred to as phytoglobins, play important roles in abiotic stress tolerance. Several essential small physiological metabolites can be bound to these heme proteins. In addition, phytoglobins can catalyze a range of different oxidative reactions in vivo. These proteins are often oligomeric, but the degree and relevance of subunit interactions are largely unknown. In this study, we delineate which residues are involved in dimer formation of a sugar beet phytoglobin type 1.2 (BvPgb1.2) using NMR relaxation experiments. E. coli cells harboring a phytoglobin expression vector were cultivated in isotope-labeled (²H, ¹³C and ¹⁵N) M9 medium. The triple-labeled protein was purified to homogeneity using two chromatographic steps. Two forms of BvPgb1.2 were examined, the oxy-form and the more stable cyanide-form. Using three-dimensional triple-resonance NMR experiments, sequence-specific assignments for CN-bound BvPgb1.2 were achieved for 137 backbone amide cross-peaks in the ¹H-¹⁵N TROSY spectrum, which amounts to 83% of the total number of 165 expected cross-peaks. A large proportion of the non-assigned residues are located in α-helixes G and H, which are proposed to be involved in protein dimerization. Such knowledge around dimer formation will be instrumental for developing a better understanding of phytoglobins' roles in planta.

Anaerobiosis modulation of two phytoglobins in barley (Hordeum vulgare L.), and their regulation by gibberellin and abscisic acid in aleurone cells

The transcript levels of the phytoglobin (Pgb) genes Pgb1 and Pgb3, and the protein content of Pgb1 were responsive to anaerobiosis in several tissues of barley (Hordeum vulgare L.). Oxygen deficiency induced the level of both Pgb transcripts and protein in aleurone layers and coleoptiles, as well as up-regulated both Pgb1 and Pgb3 in leaves, apexes and more strongly in roots of barley seedlings. In O₂-depleted aleurone cells the induction of the Pgb transcript-protein pair was reversed by re-supplying O₂. Based on this observation, it is suggested that Pgb1 and Pgb3 are inducible in all tissues. In aleurone cells, gibberellic acid (GA) induced Pgb1 and Pgb3 together with α-amylase, whereas abscisic acid (ABA) eliminated the GA stimulating effects on both α-amylase and Pgb1 and Pgb3 expression. While GA had no effects on alcohol dehydrogenase (Adh1, Adh2 and Adh3) transcripts, ABA induced all three Adh genes. It is concluded that Pgb and α-amylase in seeds are regulated reciprocally with the ethanolic fermentation pathway, and that Pgb induction is mediated by GA. Nitric oxide turnover and scavenging mediated by Pgb represents an important alternative to fermentation under anoxia.

Transduction of Signals during Somatic Embryogenesis

Somatic embryogenesis (SE) is an in vitro biological process in which bipolar structures (somatic embryos) can be induced to form from somatic cells and regenerate into whole plants. Acquisition of the embryogenic potential in culture is initiated when some competent cells within the explants respond to inductive signals (mostly plant growth regulators, PRGs), and de-differentiate into embryogenic cells. Such cells, "canalized" into the embryogenic developmental pathway, are able to generate embryos comparable in structure and physiology to their in vivo counterparts. Genomic and transcriptomic studies have identified several pathways governing the initial stages of the embryogenic process. In this review, the authors emphasize the importance of the developmental signals required for the progression of embryo development, starting with the de-differentiation of somatic cells and culminating with tissue patterning during the formation of the embryo body. The action and interaction of PGRs are highlighted, along with the participation of master regulators, mostly transcription factors (TFs), and proteins involved in stress responses and the signal transduction required for the initiation of the embryogenic process.

Selenium restores mitochondrial dysfunction to reduce Cr-induced cell apoptosis in Chinese cabbage (Brassica campestris L. ssp. Pekinensis) root tips

Chromium (Cr) disrupts the growth and physiology of plants. Selenium (Se) is considered as a promising option to help plants ameliorate Cr toxicity. To investigate the effects of exogenous Se on reactive oxygen species (ROS) burst and programmed cell death (PCD) in root tip cells under Cr stress, hydroponic experiments were carried out with Chinese cabbage seedlings grown in Hoagland solution containing 1 mg L^-1 Cr and 0.1 mg L^-1 Se. Results showed that Se scavenged the overproduction of H₂O₂ and O₂^－·, and alleviated the level of lipid peroxidation in root tips stressed by Cr. Moreover, Se effectively prevented DNA degradation and reduced the number of apoptotic cells in root tips. Compared with Cr treatment, Se supplementation reduced the content of ROS and malondialdehyde in mitochondria by 38.23% and 17.52%, respectively. Se application decreased the opening degree of mitochondrial permeability transition pores by 32.30%, increased mitochondrial membrane potential by 40.91%, alleviated the release of cyt c from mitochondria into cytosol by 18.42% and caused 57.40% decrease of caspase 3-like protease activity, and thus restored mitochondrial dysfunction caused by Cr stress. In addition, the alteration of Se on mitochondrial physiological properties maintained calcium homeostasis between mitochondria and cytosol, which further contributed to reducing the appearance of Cr-induced PCD. Findings suggested that Se restored mitochondrial dysfunction, which further rescued root tip cells from PCD, consequently activating defense strategies to protect plants from Cr toxicity and maintaining plant growth.

Cold stress in maize (Zea mays) is alleviated by the over-expression of Phytoglobin 1 (ZmPgb1.1)

Maize (Zea mays) plants over-expressing or suppressing the class 1 Phytoglobin (ZmPgb1.1) were evaluated for their ability to cope with low temperature stress. Cold treatment (10 °C day/4 °C night) depressed several gas exchange parameters including photosynthetic rate, stomatal conductance and transpiration, while elevated the levels of reactive oxygen species (ROS) and ROS-induced damage. These effects were attenuated by the over-expression of ZmPgb1.1, and aggravated when the level of the same gene was suppressed. Combination of transcriptomic and pharmacological studies revealed that over-expression of ZmPgb1.1 suppressed the level of nitric oxide (NO), which lowers the transcription of several Brassinosteroid (BR) biosynthetic and response genes. Cellular BR was required to induce the expression of ZmMPK5, a component of the mitogen-activated protein kinase (MAPK) cascade, which is known to be involved in the regulation of ROS-producing pathways. Experimental reduction of NO content, suppression of BR or inhibition of ZmMPK5 reverted the beneficial effects of ZmPgb1.1 over-expression, and increased plant susceptibility to cold stress through accumulation of ROS. Conversely, tolerance to cold was augmented in the ZmPgb1.1 down-regulating line when the levels of NO or BR were elevated. Together, this study demonstrates a novel role of ZmPgb1.1 in modulating plant performance to cold stress, and integrates the ZmPgb1.1 response in a model requiring NO and BR to alleviate oxidative stress through ZmMPK5.

Mitochondrial bioenergetics and enzymatic antioxidant defense differ in Paraná pine cell lines with contrasting embryogenic potential

Araucaria angustifolia is classified as a critically endangered species by the International Union for Conservation of Nature. This threat is worsened by the inefficiency of methods for ex-situ conservation and propagation. In conifers, somatic embryogenesis (SE) associated with cryopreservation is an efficient method to achieve germplasm conservation and mass clonal propagation. However, the efficiency of SE is highly dependent on genotype responsivity to the artificial stimulus used in vitro during cell line proliferation and later during somatic embryo development. In this study, we evaluated the activity of antioxidant enzymes and characterized mitochondrial functions during the proliferation of embryogenic cells of A. angustifolia responsive (SE1) and non-responsive (SE6) to the development of somatic embryos. The activities of the antioxidant enzymes GR (EC 1.6.4.2), MDHAR (EC 1.6.5.4), and POX (EC 1.11.1.7) were increased in SE1 culture, while in SE6 culture, only the activity of DHAR (EC 1.8.5.1) was significantly higher. Additionally, SE6 culture presented a higher number of mitochondria, which agreed with the increased rate of oxygen consumption compared to responsive SE1 culture; however, the mitochondrial volume was lower. Although the ATP levels did not differ, the NAD(P)H levels were higher in SE1 cells. NDs, AOX, and UCP were less active in responsive SE1 than in non-responsive cells. Our results show significant differences between SE1 and SE6 embryogenic cells regarding mitochondrial functions and antioxidant enzyme activities, which may be intrinsic to the in vitro proliferation phase of both cell lines, possessing a crucial role for the induction of in vitro maturation process.

Tolerance to excess moisture in soybean is enhanced by over-expression of the Glycine max Phytoglobin (GmPgb1)

Excess moisture in the form of waterlogging or full submergence can cause severe conditions of hypoxia or anoxia compromising several physiological and biochemical processes. A decline in photosynthetic rate due to accumulation of ROS and damage of leaf tissue are the main consequences of excess moisture. These effects compromise crop yield and quality, especially in sensitive species, such as soybean (Glycine max.). Phytoglobins (Pgbs) are expressed during hypoxia and through their ability to scavenge nitric oxide participate in several stress-related responses. Soybean plants over-expressing or suppressing the Pgb1 gene GmPgb1 were generated and their ability to cope with waterlogging and full submergence conditions was assessed. Plants over-expressing GmPgb1 exhibited a higher retention of photosynthetic rate during waterlogging and survival rate during submergence relative to wild type plants. The same plants also had lower levels of ROS due to a reduction in expression of Respiratory Burst Oxidase Homologs (RBOH), components of the NADPH oxidase enzyme, and enhanced antioxidant system characterized by higher expression of catalases (CAT) and superoxide dismutase (SOD), as well as elevated expression and activity of ascorbate peroxidase (APX). Plants over-expressing GmPgb1 also exhibited an expression pattern of aquaporins typical of excess moisture resilience. This was in contrast to plants downregulating GmPgb1 which were characterized by the lowest photosynthetic rates, higher ROS signal, and reduced expression and activities of many antioxidant enzymes. Results from these studies suggest that GmPgb1 exercises a protective role during conditions of excess moisture with similar mechanisms operating during waterlogging and submergence.

Over-expression of the Zea mays phytoglobin (ZmPgb1.1) alleviates the effect of water stress through shoot-specific mechanisms

Water deficit limits plant growth and development by interfering with several physiological and molecular processes both in root and shoot tissues. Through their ability to scavenge nitric oxide (NO), phytoglobins (Pgbs) exercise a protective role during several conditions of stress. While their action has been mainly documented in roots, it is unclear whether Pgb exercises a specific and direct role in shoot tissue. We used a Zea mays root-less system to assess how over-expression or down-regulation of ZmPgb1.1 influences the behavior of shoots exposed to polyethylene glycol (PEG)-simulated water deficit. Relative to their WT and ZmPgb1.1 down-regulating counterparts, PEG-treated shoots over-expressing ZmPgb1.1 exhibited a reduced accumulation of ROS and lipid peroxidation. These effects were ascribed to lower transcript levels of Respiratory Burst Oxidase Homolog (RBOH) genes encoding the ROS generating enzyme complex NADPH oxidase, and a more active antioxidant system. Furthermore, over-expression of ZmPgb1.1 attenuated the reduction in osmotic potential and relative water content experienced during water stress, an observation also demonstrated at a whole plant level, possibly through the retention of the expression of three aquaporins involved in water transfer and implicated in drought tolerance. Pharmacological treatments modulating NO and ethylene levels revealed that the ZmPgb1.1 action was mediated by ethylene synthesis and response, with NO acting as an upstream intermediate. Collectively we provide substantial evidence that ZmPgb1.1 exercises a direct role in shoot tissue, independent from that previously reported in roots, which confers tolerance to water stress.

Plant hemoglobins: a journey from unicellular green algae to vascular plants

Globins (Glbs) are widely distributed in archaea, bacteria and eukaryotes. They can be classified into proteins with 2/2 or 3/3 α-helical folding around the heme cavity. Both types of Glbs occur in green algae, bryophytes and vascular plants. The Glbs of angiosperms have been more intensively studied, and several protein structures have been solved. They can be hexacoordinate or pentacoordinate, depending on whether a histidine is coordinating or not at the sixth position of the iron atom. The 3/3 Glbs of class 1 and the 2/2 Glbs (also called class 3 in plants) are present in all angiosperms, whereas the 3/3 Glbs of class 2 have been only found in early angiosperms and eudicots. The three Glb classes are expected to play different roles. Class 1 Glbs are involved in hypoxia responses and modulate NO concentration, which may explain their roles in plant morphogenesis, hormone signaling, cell fate determination, nutrient deficiency, nitrogen metabolism and plant-microorganism symbioses. Symbiotic Glbs derive from class 1 or class 2 Glbs and transport O₂ in nodules. The physiological roles of class 2 and class 3 Glbs are poorly defined but could involve O₂ and NO transport and/or metabolism, respectively. More research is warranted on these intriguing proteins to determine their non-redundant functions.

Genome-Wide Association Study Dissects the Genetic Architecture of Maize Husk Tightness

The husk is a leafy outer tissue that encloses a maize ear. Previously, we identified the optimum husk structure by measuring the husk length, husk layer number, husk thickness and husk width. Husk tightness (HTI) is a combined trait based on the above four husk measurements. Unveiling the genetic basis of HTI will aid in guiding the genetic improvement of maize for mechanical harvesting and for protecting the ear from pest damage and pathogen infection. Here, we used a maize associate population of 508 inbred lines with tropical, subtropical and temperate backgrounds to analyze the genetic architecture of HTI. Evaluating the phenotypic diversity in three different environments showed that HTI exhibited broad natural variations and a moderate heritability level of 0.41. A diversity analysis indicated that the inbred lines having a temperate background were more loosely related than those having a tropical or subtropical background. HTI showed significant negative correlations with husk thickness and width, which indicates that thicker and wider husks wrapped the ear tighter than thinner and slimmer husks. Combining husk traits with ∼1.25 million single nucleotide polymorphisms in a genome-wide association study revealed 27 variants that were significantly associated with HTI above the threshold of P < 7.26 × 10^-6. We found 27 candidate genes for HTI that may participate in (1) husk senescence involving lipid peroxidation (GRMZM2G017616) and programmed cell death (GRMZM2G168898 and GRMZM2G035045); (2) husk morphogenesis involving cell division (GRMZM5G869246) and cell wall architecture (GRMZM2G319798); and (3) cell signal transduction involving protein phosphorylation (GRMZM2G149277 and GRMZM2G004207) and the ABSISIC ACID INSENSITIVE3/VIVIPAROUS1 transcription factor (GRMZM2G088427). These results provide useful information for understanding the genetic basis of husk development. Further studies of identified candidate genes will help elucidate the molecular pathways that regulate HTI in maize.

Directing Trophic Divergence in Plant-Pathogen Interactions: Antagonistic Phytohormones With NO Doubt?

A fundamental process culminating in the mechanisms of plant-pathogen interactions is the regulation of trophic divergence into biotrophic, hemibiotrophic, and necrotrophic interactions. Plant hormones, of almost all types, play significant roles in this regulatory apparatus. In plant-pathogen interactions, two classical mechanisms underlying hormone-dependent trophic divergence are long recognized. While salicylic acid dominates in the execution of host defense response against biotrophic and early-stage hemibiotrophic pathogens, jasmonic acid, and ethylene are key players facilitating host defense response against necrotrophic and later-stage hemibiotrophic pathogens. Evidence increasingly suggests that trophic divergence appears to be modulated by more complex signaling networks. Acting antagonistically or agonistically, other hormones such as auxins, cytokinins, abscisic acid, gibberellins, brassinosteroids, and strigolactones, as well as nitric oxide, are emerging candidates in the regulation of trophic divergence. In this review, the latest advances in the dynamic regulation of trophic divergence are summarized, emphasizing common and contrasting hormonal and nitric oxide signaling strategies deployed in plant-pathogen interactions.

Suppression of the maize phytoglobin ZmPgb1.1 promotes plant tolerance against Clavibacter nebraskensis

Suppression of the maize phytoglobin ZmPgb1.1 enhances tolerance against Clavibacter nebraskensis by promoting hypersensitive response mechanisms mediated by ethylene and reactive oxygen species. Suppression of the maize phytoglobin, ZmPgb1.1, reduced lesion size and disease severity in leaves following inoculation with Clavibacter nebraskensis, the causal agent of Goss's bacterial wilt disease of corn. These effects were associated with an increase of the transcriptional levels of ethylene biosynthetic and responsive genes, which resulted in the accumulation of reactive oxygen species (ROS) and TUNEL-positive nuclei in the proximity of the inoculation site. An in vitro system, in which maize cells were treated with induced xylem sap, was employed to define the cause-effect relationship of these events. Phytoglobins (Pgbs) are hemoglobins able to scavenge nitric oxide (NO). Suppression of ZmPgb1.1 elevated the level of NO in cells exposed to the induced xylem sap causing a rise in the transcript levels of ethylene biosynthesis and response genes, as well as ethylene. Accumulation of ethylene in the same cells was sufficient to elevate the amount of reactive oxygen species (ROS), through the activation of the respiratory burst oxidase homologs (Rboh) genes, and trigger programmed cell death (PCD). The sequence of these events was demonstrated by manipulating the content of NO and ethylene in culture through pharmacological treatments. Collectively, our results illustrated that suppression of ZmPgb1.1 evokes tolerance against C. nebraskensis culminating in the execution of PCD, a key step of the hypersensitive response.

Phytoglobins in the nuclei, cytoplasm and chloroplasts modulate nitric oxide signaling and interact with abscisic acid

Symbiotic hemoglobins provide O₂ to N₂ -fixing bacteria within legume nodules, but the functions of non-symbiotic hemoglobins or phytoglobins (Glbs) are much less defined. Immunolabeling combined with confocal microscopy of the Glbs tagged at the C-terminus with green fluorescent protein was used to determine their subcellular localizations in Arabidopsis and Lotus japonicus. Recombinant proteins were used to examine nitric oxide (NO) scavenging in vitro and transgenic plants to show S-nitrosylation and other in vivo interactions with NO and abscisic acid (ABA) responses. We found that Glbs occur in the nuclei, chloroplasts and amyloplasts of both model plants, and also in the cytoplasm of Arabidopsis cells. The proteins show similar NO dioxygenase activities in vitro, are nitrosylated in Cys residues in vivo, and scavenge NO in the stomatal cells. The Cys/Ser mutation does not affect NO dioxygenase activity, and S-nitrosylation does not significantly consume NO. We demonstrate an interaction between Glbs and ABA on several grounds: Glb1 and Glb2 scavenge NO produced in stomatal guard cells following ABA supply; plants overexpressing Glb1 show higher constitutive expression of the ABA responsive genes Responsive to ABA (RAB18), Responsive to Dehydration (RD29A) and Highly ABA-Induced 2 (HAI2), and are more tolerant to dehydration; and ABA strongly upregulates class 1 Glbs. We conclude that Glbs modulate NO and interact with ABA in crucial physiological processes such as the plant's response to dessication.

CRISPR/Cas9 knockout of leghemoglobin genes in Lotus japonicus uncovers their synergistic roles in symbiotic nitrogen fixation

Legume nodules contain high concentrations of leghemoglobins (Lbs) encoded by several genes. The reason for this multiplicity is unknown. CRISPR/Cas9 technology was used to generate stable mutants of the three Lbs of Lotus japonicus. The phenotypes were characterized at the physiological, biochemical and molecular levels. Nodules of the triple mutants were examined by electron microscopy and subjected to RNA-sequencing (RNA-seq) analysis. Complementation studies revealed that Lbs function synergistically to maintain optimal N₂ fixation. The nodules of the triple mutants overproduced superoxide radicals and hydrogen peroxide, which was probably linked to activation of NADPH oxidases and changes in superoxide dismutase isoforms expression. The mutant nodules showed major ultrastructural alterations, including vacuolization, accumulation of poly-β-hydroxybutyrate and disruption of mitochondria. RNA-seq of c. 20 000 genes revealed significant changes in expression of carbon and nitrogen metabolism genes, transcription factors, and proteinases. Lb-deficient nodules had c. 30-50-fold less heme but similar transcript levels of heme biosynthetic genes, suggesting a post-translational regulatory mechanism of heme synthesis. We conclude that Lbs act additively in nodules and that the lack of Lbs results in early nodule senescence. Our observations also provide insight into the reprogramming of the gene expression network associated with Lb deficiency, probably as a result of uncontrolled intracellular free O₂ concentration.

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.

Spatial identification of transcripts and biological processes in laser micro-dissected sub-regions of waterlogged corn roots with altered expression of phytoglobin

Over-expression of the corn phytoglobin ZmPgb1.2 increases tolerance to waterlogging, while suppression of ZmPgb1.2 compromises plant growth. To unravel compartment-specific transcriptional changes evoked by ZmPgb1.2 during hypoxia, laser micro-dissected sub-regions from waterlogged roots of WT and ZmPgb1.2 overexpressing [ZmPgb1.2(S)] plants were probed for global transcriptional analysis using next generation RNA sequencing. These sub-regions included compartments within the meristematic, elongation, and maturation zone. Of the 149 genes differentially expressed by the up-regulation of ZmPgb1.2, 78 occurred within the meristematic region and included genes involved in jasmonic acid synthesis and response, ascorbic acid metabolism, and ethylene signalling. The ZmPgb1.2 regulation of these genes, discussed in the context of known functions of Pgbs, was further validated by monitoring their expression in meristematic cells of waterlogged roots suppressing ZmPgb1.2. Of the 27 genes differentially expressed by the over-expression of ZmPgb1.2 in the elongation zone, pyruvate kinase and alcohol dehydrogenase showed an expression pattern correlated to the level of ZmPgb1.2 in the tissue. The transcriptional induction of these two enzymes in hypoxic domains of the elongation zone over-expressing ZmPgb1.2 suggests the activation of the fermentation pathway which might be required to sustain metabolic flux and production of ATP in support of cell elongation.

Metacaspase gene family in Rosaceae genomes: Comparative genomic analysis and their expression during pear pollen tube and fruit development

Metacaspase (MC), which is discovered gene family with distant caspase homologs in plants, fungi, and protozoa, may be involved in programmed cell death (PCD) processes during plant development and respond abiotic and biotic stresses. To reveal the evolutionary relationship of MC gene family in Rosaceae genomes, we identified 8, 7, 8, 12, 12, and 23 MC genes in the genomes of Fragaria vesca, Prunus mume, Prunus persica, Pyrus communis, Pyrus bretschneideri and Malus domestica, respectively. Phylogenetic analysis suggested that the MC genes could be grouped into three clades: Type I*, Type I and Type II, which was supported by gene structure and conserved motif analysis. Microsynteny analysis revealed that MC genes present in the corresponding syntenic blocks of P. communis, P. bretschneideri and M. domestica, and further suggested that large-scale duplication events play an important role in the expansion of MC gene family members in these three genomes than other Rosaceae plants (F. vesca, P. mume and P. persica). RNA-seq data showed the specific expression patterns of PbMC genes in response to drought stress. The expression analysis of MC genes demonstrated that PbMC01 and PbMC03 were able to be detected in all four pear pollen tubes and seven fruit development stages. The current study highlighted the evolutionary relationship and duplication of the MC gene family in these six Rosaceae genomes and provided appropriate candidate genes for further studies in P. bretschneideri.

In vitro differentiation of tracheary elements is induced by suppression of Arabidopsis phytoglobins

Differentiation of tracheary elements (TEs) in vitro was affected by the expression level of the Arabidopsis thaliana Col-0 phytoglobins (Pgbs). Over-expression of Pgb1 or Pgb2 (35S:Pgb1 or 35S:Pgb2 lines) reduced the differentiation process while suppression of either Pgb (Pgb1-RNAi or pgb2 lines) enhanced the production of TEs. The inductive effect of Pgb suppression on TE differentiation was linked to the reduced expression of the transcription factor MYC2. Suppression of this gene, observed under conditions of high NO levels or low Pgb expression, was sufficient to promote TE differentiation, while its over-expression abolished the promotive effect of Pgb suppression on the differentiation process. Cells in which MYC2 was mutated accumulated ethylene which induced the expression of the homeodomain-leucine zipper (HD-Zip) III ATHB8. Production of ethylene was reduced in cells over-expressing MYC2 in a WT or a pgb mutant background. While stabilizing procambial cell specification, ATHB8 in known to activate downstream components triggering programmed cell death (PCD) and modifications of cell wall components, required steps of the TE differentiation process. Collectively, we provide evidence that in addition to their recognised participation in stress responses, Pgbs may play a key role in the specification of cell fate during development.

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.

Elevation of cytosolic Ca2+ in response to energy deficiency in plants: the general mechanism of adaptation to low oxygen stress

Ca2+ can be released from cell compartments to the cytosol during stress conditions. We discuss here the causes of Ca2+ release under conditions of ATP concentration decline that result in the suppression of ATPases and activation of calcium ion channels. The main signaling and metabolic consequences of Ca2+ release are considered for stressed plant cells. The signaling function includes generation and spreading of calcium waves, while the metabolic function results in the activation of particular enzymes and genes. Ca2+ is involved in the activation of glutamate decarboxylase, initiating the γ-aminobutyric acid shunt and triggering the formation of alanine, processes which play a role, in particular, in pH regulation. Ca2+ activates the transcription of several genes, e.g. of plant hemoglobin (phytoglobin, Pgb) which scavenges nitric oxide and regulates redox and energy balance through the Pgb–nitric oxide cycle. This cycle involves NADH and NADPH oxidation from the cytosolic side of mitochondria, in which Ca2+- and low pH-activated external NADH and NADPH dehydrogenases participate. Ca2+ can also activate the genes of alcohol dehydrogenase and pyruvate decarboxylase stimulating hypoxic fermentation. It is concluded that calcium is a primary factor that causes the metabolic shift under conditions of oxygen deficiency.

Protection of root apex meristem during stress responses

By regulating the levels of nitric oxide (NO) in a cell and tissue specific fashion, Phytoglobins (Pgbs), plant hemoglobin-like proteins, interfere with many NO-mediated pathways participating in developmental and stress-related responses. Recent evidence reveals that one of the functions of Pgbs is to protect the root apical meristem (RAM) from stress conditions by retaining the viability and function of the quiescent center (QC), required to maintain the stem cells in an undifferentiated state and ensure proper tissue patterning and root viability. Based on this and other evidence, it is suggested that Pgbs regulate cell fate by modulating NO homeostasis.

Expression of Arabidopsis class 1 phytoglobin (AtPgb1) delays death and degradation of the root apical meristem during severe PEG-induced water deficit

Maintenance of a functional root is fundamental to plant survival in response to some abiotic stresses, such as water deficit. In this study, we found that overexpression of Arabidopsis class 1 phytoglobin (AtPgb1) alleviated the growth retardation of polyethylene glycol (PEG)-induced water stress by reducing programmed cell death (PCD) associated with protein folding in the endoplasmic reticulum (ER). This was in contrast to PEG-stressed roots down-regulating AtPgb1 that exhibited extensive PCD and reduced expression of several attenuators of ER stress, including BAX Inhibitor-1 (BI-1). The death program experienced by the suppression of AtPgb1 in stressed roots was mediated by reactive oxygen species (ROS) and ethylene. Suppression of ROS synthesis or ethylene perception reduced PCD and partially restored root growth. The PEG-induced cessation of root growth was preceded by structural changes in the root apical meristem (RAM), including the loss of cell and tissue specification, possibly as a result of alterations in PIN1- and PIN4-mediated auxin accumulation at the root pole. These events were attenuated by the overexpression of AtPgb1 and aggravated when AtPgb1 was suppressed. Specifically, suppression of AtPgb1 compromised the functionality of the WOX5-expressing quiescent cells (QCs), leading to the early and premature differentiation of the adjacent columella stem cells and to a rapid reduction in meristem size. The expression and localization of other root domain markers, such as SCARECROW (SCR), which demarks the endodermis and QCs, and WEREWOLF (WER), which specifies the lateral root cap, were also most affected in PEG-treated roots with suppressed AtPgb1. Collectively, the results demonstrate that AtPgb1 exercises a protective role in roots exposed to lethal levels of PEG, and suggest a novel function of this gene in ensuring meristem functionality through the retention of cell fate specification.

Determining Cellular Responses: Phytoglobins May Direct the Traffic

How stem cells retain their undifferentiated state or how differentiated cells are capable of having dissimilar responses to perturbations are major open questions in plant biology. Cell-specific phytoglobin expression may be one mechanism determining cell fate by the modulation of nitric oxide (NO), affecting cellular hormonal responses and processes such as cell differentiation.

Learning To Breathe: Developmental Phase Transitions in Oxygen Status

Plants are developmentally disposed to significant changes in oxygen availability, but our understanding of the importance of hypoxia is almost entirely limited to stress biology. Differential patterns of the abundance of oxygen, nitric oxide (^•NO), and reactive oxygen species (ROS), as well as of redox potential, occur in organs and meristems, and examples are emerging in the literature of mechanistic relationships of these to development. We describe here the convergence of these cues in meristematic and reproductive tissues, and discuss the evidence for regulated hypoxic niches within which oxygen-, ROS-, ^•NO-, and redox-dependent signalling curate developmental transitions in plants.

Does Early Embryogenesis in Eudicots and Monocots Involve the Same Mechanism and Molecular Players?

A comparison of eudicot and monocot model plants explores recent advances and open questions on gene regulatory networks during zygote development, parental influences on early embryogenesis, zygotic genome activation, and cell fate determination.

Phytoglobins Improve Hypoxic Root Growth by Alleviating Apical Meristem Cell Death

Hypoxic root growth in maize (Zea mays) is influenced by the expression of phytoglobins (ZmPgbs). Relative to the wild type, suppression of ZmPgb1.1 or ZmPgb1.2 inhibits the growth of roots exposed to 4% oxygen, causing structural abnormalities in the root apical meristems. These effects were accompanied by increasing levels of reactive oxygen species (ROS), possibly through the transcriptional induction of four Respiratory Burst Oxidase Homologs TUNEL-positive nuclei in meristematic cells indicated the involvement of programmed cell death (PCD) in the process. These cells also accumulated nitric oxide and stained heavily for ethylene biosynthetic transcripts. A sharp increase in the expression level of several 1-aminocyclopropane synthase (ZmAcs2, ZmAcs6, and ZmAcs7), 1-aminocyclopropane oxidase (Aco15, Aco20, Aco31, and Aco35), and ethylene-responsive (ZmErf2 and ZmEbf1) genes was observed in hypoxic ZmPgb-suppressing roots, which overproduced ethylene. Inhibiting ROS synthesis with diphenyleneiodonium or ethylene perception with 1-methylcyclopropene suppressed PCD, increased BAX inhibitor-1, an effective attenuator of the death programs in eukaryotes, and restored root growth. Hypoxic roots overexpressing ZmPgbs had the lowest level of ethylene and showed a reduction in ROS staining and TUNEL-positive nuclei in the meristematic cells. These roots retained functional meristems and exhibited the highest growth performance when subjected to hypoxic conditions. Collectively, these results suggest a novel function of Pgbs in protecting root apical meristems from hypoxia-induced PCD through mechanisms initiated by nitric oxide and mediated by ethylene via ROS.

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.

Genome-wide identification and expression analysis of the metacaspase gene family in Hevea brasiliensis

Metacaspases, a family of cysteine proteases, have been suggested to play important roles in programmed cell death (PCD) during plant development and stress responses. To date, no systematic characterization of this gene family has been reported in rubber tree (Hevea brasiliensis). In the present study, nine metacaspase genes, designated as HbMC1 to HbMC9, were identified from whole-genome sequence of rubber tree. Multiple sequence alignment and phylogenetic analyses suggested that these genes were divided into two types: type I (HbMC1-HBMC7) and type II (HbMC8 and HbMC9). Gene structure analysis demonstrated that type I and type II HbMCs separately contained four and two introns, indicating the conserved exon-intron organization of HbMCs. Quantitative real-time PCR analysis revealed that HbMCs showed distinct expression patterns in different tissues, suggesting the functional diversity of HbMCs in various tissues during development. Most of the HbMCs were regulated by drought, cold, and salt stress, implying their possible functions in regulating abiotic stress-induced cell death. Of the nine HbMCs, HbMC1, HbMC2, HbMC5, and HbMC8 displayed a significantly higher relative transcript accumulation in barks of tapping panel dryness (TPD) trees compared with healthy trees. In addition, the four genes were up-regulated by ethephon (ET) and methyl jasmonate (MeJA), indicating their potential involvement in TPD resulting from ET- or JA-induced PCD. In summary, this work provides valuable information for further functional characterization of HbMC genes in rubber tree.

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.

Molecular dynamics simulations indicate that tyrosineB10 limits motions of distal histidine to regulate CO binding in soybean leghemoglobin

Myoglobin (Mb) uses strong electrostatic interaction in its distal heme pocket to regulate ligand binding. The mechanism of regulation of ligand binding in soybean leghemoglobin a (Lba) has been enigmatic and more so due to the absence of gaseous ligand bound atomic resolution three-dimensional structure of the plant globin. While the 20-fold higher oxygen affinity of Lba compared with Mb is required for its dual physiological function, the mechanism by which this high affinity is achieved is only emerging. Extensive mutational analysis combined with kinetic and CO-FT-IR spectroscopic investigation led to the hypothesis that Lba depended on weakened electrostatic interaction between distal HisE7 and bound ligand achieved by invoking B10Tyr, which itself hydrogen bonds with HisE7 thus restricting it in a single conformation detrimental to Mb-like strong electrostatic interaction. Such theory has been re-assessed here using CO-Lba in silico model and molecular dynamics simulation. The investigation supports the presence of at least two major conformations of HisE7 in Lba brought about by imidazole ring flip, one of which makes hydrogen bonds effectively with B10Tyr affecting the former's ability to stabilize bound ligand, while the other does not. However, HisE7 in Lba has limited conformational freedom unlike high frequency of imidazole ring flips observed in Mb and in TyrB10Leu mutant of Lba. Thus, it appears that TyrB10 limits the conformational freedom of distal His in Lba, tuning down ligand dissociation rate constant by reducing the strength of hydrogen bonding to bound ligand, which the freedom of distal His of Mb allows.

Genetic and molecular characterization of submergence response identifies Subtol6 as a major submergence tolerance locus in maize

Maize is highly sensitive to short term flooding and submergence. Early season flooding reduces germination, survival and growth rate of maize seedlings. We aimed to discover genetic variation for submergence tolerance in maize and elucidate the genetic basis of submergence tolerance through transcriptional profiling and linkage analysis of contrasting genotypes. A diverse set of maize nested association mapping (NAM) founder lines were screened, and two highly tolerant (Mo18W and M162W) and sensitive (B97 and B73) genotypes were identified. Tolerant lines exhibited delayed senescence and lower oxidative stress levels compared to sensitive lines. Transcriptome analysis was performed on these inbreds to provide genome level insights into the molecular responses to submergence. Tolerant lines had higher transcript abundance of several fermentation-related genes and an unannotated Pyrophosphate-Dependent Fructose-6-Phosphate 1-Phosphotransferase gene during submergence. A coexpression network enriched for CBF (C-REPEAT/DRE BINDING FACTOR: C-REPEAT/DRE BINDING FACTOR) genes, was induced by submergence in all four inbreds, but was more activated in the tolerant Mo18W. A recombinant inbred line (RIL) population derived from Mo18W and B73 was screened for submergence tolerance. A major QTL named Subtol6 was mapped to chromosome 6 that explains 22% of the phenotypic variation within the RIL population. We identified two candidate genes (HEMOGLOBIN2 and RAV1) underlying Subtol6 based on contrasting expression patterns observed in B73 and Mo18W. Sources of tolerance identified in this study (Subtol6) can be useful to increase survival rate during flooding events that are predicted to increase in frequency with climate change.

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.