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 hypoxiainduced PCD through mechanisms initiated by nitric oxide and mediated by ethylene via ROS.Oxygen deficiency (hypoxia), experienced by plants grown in poorly drained soils or subjected to flooding, impairs plant growth and results in heavy crop losses (Dennis et al., 2000). Submergence or flooding reduces oxygen availability for plant cells, inhibiting the gas exchange required for basic physiological processes (Bailey-Serres and Voesenek, 2008). Both roots and shoots are affected by hypoxia, regardless of whether the plant is submerged or only the root is exposed to the condition. The consequences to shoots of prolonged root hypoxia include reduced photosynthetic rate and stomatal conductance, decreased leaf growth and senescence, wilting of the aboveground organs, and alterations in plant water relations (Mustroph and Albrecht, 2003). Ethylene accumulates rapidly in flooded Rumex palustris root cells (Visser et al., 1996), and in some species, ethylene affects the selective death of cortical cells generating lysogenous aerenchyma (Drew et al., 1979(Drew et al., , 2000; Drew, 1997) Precursors of ethylene have been shown to induce changes in Brassica napus growth behavior and root architecture (Patrick et al., 2009). Depending upon the concentration and species, ethylene can either stimulate or inhibit root growth (Konings and Jackson, 1979). Ethylene regulation of programmed cell death (PCD) is not restricted to hypoxia but, rather, is observed in ...

HighlightSuppression of the phytoglobin GLB2 enhances jasmonic acid which promotes the accumulation of auxin and the formation of somatic embryos in Arabidopsis

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.

Retardation of growth induced by water stress is attenuated by the overexpression of AtPgb1 and aggravated when it is suppressed. Expression of AtPgb1 is required to delay the death of root tips and to prevent the premature differentiation of meristematic cells.

Overexpression of phytoglobin retains stem cell fate specification and function in hypoxic root apical meristems

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HighlightDuring Arabidopsis somatic embryogenesis phytoglobin 2 exercises its effects only when targeted to the nucleus, while when cytoplasmic it mimics the effects of its mutant form.

Plants respond to hypoxic stress through either acclimation to the stress or avoidance of it, as they do to most environmental stresses. The hypothesis that has general consensus among the community is that ethylene response factors (ERFs) are central elements that control both types of responses to hypoxia. Recent studies suggest that this may not be the case for all cells experiencing hypoxic stress. Mature maize root cells undergoing hypoxic stress were found to undergo acclimation and avoidance mechanisms involving ERFs, whereas meristematic root cells and cells still undergoing differentiation acclimated to the response without the involvement of ethylene synthesis or ERFs. Phytoglobins (PGBs) and NO were demonstrated to be components critical to the acclimation response. These findings are discussed relative to the possibility that PGBs may be acting as molecular switches controlling cellular stress responses and hormonal changes and responses in cells.