Halophytes are defined as plants that are adapted to live in soils containing high concentrations of salt and benefiting from it, and thus represent an ideal model to understand complex physiological and genetic mechanisms of salinity stress tolerance. It is also known that oxidative stress signalling and reactive oxygen species (ROS) detoxification are both essential components of salinity stress tolerance mechanisms. This paper comprehensively reviews the differences in ROS homeostasis between halophytes and glycophytes in an attempt to answer the questions of whether stress-induced ROS production is similar between halophytes and glycophytes; is the superior salinity tolerance in halophytes attributed to higher antioxidant activity; and is there something special about the specific 'pool' of enzymatic and non-enzymatic antioxidants in halophytes. We argue that truly salt-tolerant species possessing efficient mechanisms for Na(+) exclusion from the cytosol may not require a high level of antioxidant activity, as they simply do not allow excessive ROS production in the first instance. We also suggest that H2O2 'signatures' may operate in plant signalling networks, in addition to well-known cytosolic calcium 'signatures'. According to the suggested concept, the intrinsically higher superoxide dismutase (SOD) levels in halophytes are required for rapid induction of the H2O2 'signature', and to trigger a cascade of adaptive responses (both genetic and physiological), while the role of other enzymatic antioxidants may be in decreasing the basal levels of H2O2, once the signalling has been processed. Finally, we emphasize the importance of non-enzymatic antioxidants as the only effective means to prevent detrimental effects of hydroxyl radicals on cellular structures.

Enhanced salinity tolerance in the halophyte species studied here is not related to the constitutively higher AHA transcript levels in the root epidermis, but to the plant's ability to rapidly upregulate plasma membrane H(+)-ATPase upon salinity treatment. This is necessary for assisting plants to maintain highly negative membrane potential values and to exclude Na(+), or enable better K(+) retention in the cytosol under saline conditions.

ABSTRACT

Recently, reactive oxygen species (ROS) have emerged as important signaling molecules mediating a broad range of plant adaptive and developmental responses. In plant cells, both photosynthetic and respiratory electron-transport chains, as well as the NADPH oxidases and peroxidases are involved in ROS generation. 1 ROS have been shown to regulate gene expression and signaling transduction pathways and, as such, can control numerous processes, like root gravitropism, hypersensitive response to pathogens, stomatal closure and cell expansion and development. 1-5 During pathogen attack, programmed cell death (PCD) is induced in order to isolate cells and therefore avoiding pathogen spread. ROS play a crucial role in this signaling network. 6 In many cases, ROS production is genetically programmed and is induced during development. Generation of singlet oxygen induces controlled PCD in aleurone cells, leaf senescence, tracheary elements maturation or trichome development. 4 While ROS control over numerous adaptive and developmental responses is absolutely essential, a controlled balance of ROSproducing and ROS-scavenging systems must be kept to ensure an accurate execution of signaling without provoking toxicity. Several types of ROS may be formed in plant cells. These forms can be poorly reactive (non-radicals, such as H 2 O 2 or O 3 ) or can Transition metals such as iron (Fe) and Copper (Cu) are essential for plant cell development. At the same time, due their capability to generate hydroxyl radicals they can be potentially toxic to plant metabolism. Recent works on hydroxyl-radical activation of ion transporters suggest that hydroxyl radicals generated by transition metals could play an important role in plant growth and adaptation to imbalanced environments. in this mini-review, the relation between transition metals uptake and utilization and oxidative stress-activated ion transport in plant cells is analyzed, and a new model depicting both apoplastic and cytosolic mode of ROS signaling to plasma membrane transporters is suggested.