▪ Abstract Many plants increase in freezing tolerance upon exposure to low nonfreezing temperatures, a phenomenon known as cold acclimation. In this review, recent advances in determining the nature and function of genes with roles in freezing tolerance and the mechanisms involved in low temperature gene regulation and signal transduction are described. One of the important conclusions to emerge from these studies is that cold acclimation includes the expression of certain cold-induced genes that function to stabilize membranes against freeze-induced injury. In addition, a family of Arabidopsis transcription factors, the CBF/DREB1 proteins, have been identified that control the expression of a regulon of cold-induced genes that increase plant freezing tolerance. These results along with many of the others summarized here further our understanding of the basic mechanisms that plants have evolved to survive freezing temperatures. In addition, the findings have potential practical applications as freezing te...

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PLANT COLD ACCLIMATION: Freezing Tolerance Genes and Regulatory Mechanisms

Plant cold acclimation: Freezing tolerance genes and regulatory mechanisms

Abscisic acid (ABA) regulates numerous developmental processes and adaptive stress responses in plants. Many ABA signaling components have been identified, but their interconnections and a consensus on the structure of the ABA signaling network have eluded researchers. Recently, several advances have led to the identification of ABA receptors and their three-dimensional structures, and an understanding of how key regulatory phosphatase and kinase activities are controlled by ABA. A new model for ABA action has been proposed and validated, in which the soluble PYR/PYL/RCAR receptors function at the apex of a negative regulatory pathway to directly regulate PP2C phosphatases, which in turn directly regulate SnRK2 kinases. This model unifies many previously defined signaling components and highlights the importance of future work focused on defining the direct targets of SnRK2s and PP2Cs, dissecting the mechanisms of hormone interactions (i.e., cross talk) and defining connections between this new negative regulatory pathway and other factors implicated in ABA signaling.

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Abscisic Acid: Emergence of a Core Signaling Network

Molecular studies of drought stress in plants use a variety of strategies and include different species subjected to a wide range of water deficits. Initial research has by necessity been largely descriptive, and relevant genes have been identified either by reference to physiological evidence or by differential screening. A large number of genes with a potential role in drought tolerance have been described, and major themes in the molecular response have been established. Particular areas of importance are sugar metabolism and late-embryogenesis-abundant (LEA) proteins. Studies have begun to examine mechanisms that control the gene expression, and putative regulatory pathways have been established. Recent attempts to understand gene function have utilized transgenic plants. These efforts are of clear agronomic importance.

The molecular basis of dehydration tolerance in plants

Abscisic acid (ABA) regulates many agronomically important aspects of plant development, including the synthesis of seed storage proteins and lipids, the promotion of seed desiccation tolerance and dormancy, and the inhibition of the phase transitions from embryonic to germinative growth and from

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Abscisic Acid Signaling in Seeds and Seedlings

The phytohormone abscisic acid (ABA) regulates various physiological processes in plants. The molecular mechanisms by which this is achieved are not fully understood. Genetic approaches have characterized several downstream components of ABA signalling, but a receptor for ABA has remained elusive. Although studies indicate that several ABA response genes encode RNA-binding or RNA-processing proteins, none has been found to be functional in binding ABA. Here we show that FCA, an RNA-binding protein involved in flowering, binds ABA with high affinity in an interaction that is stereospecific and follows receptor kinetics. The interaction between FCA and ABA has molecular effects on downstream events in the autonomous floral pathway and, consequently, on the ability of the plant to undergo transition to flowering. We further show that ABA binding exerts a direct control on the FCA-mediated processing of precursor messenger RNA. Our results indicate that FCA is an ABA receptor involved in RNA metabolism and in controlling flowering time.

The RNA-binding protein FCA is an abscisic acid receptor

Transgenic alfalfa root cultures expressing sense and antisense barley hemoglobin transcripts were examined under varying levels of atmospheric oxygen. Root cultures overexpressing the hemoglobin gene (Hb+) maintained root growth when placed under 3% oxygen, whereas control cultures or cultures underexpressing hemoglobin (Hb-) experienced 30-70% declines in growth under the same conditions. ATP levels and ATP/ADP ratios for Hb+ lines did not significantly differ in 40 and 3% oxygen, whereas the ATP levels and ATP/ADP ratios in control and Hb- lines were significantly lower under 3% oxygen. Large increases in the production of nitric oxide (NO) were measured in root cultures grown under hypoxic conditions compared to aerobic conditions. The amount of NO accumulated in an Hb- line was 2.5-fold higher than that in the Hb+ line. Treatment of transgenic root lines under 40% oxygen with NO resulted in significant declines in the ATP levels and ATP/ADP ratio of an Hb- line and the control line, with no significant change in an Hb+ line. The root cell structure of an Hb- line showed evidence of cell breakdown under hypoxic growth, whereas an Hb+ line had no evidence of cell breakdown under similar growth conditions. These results lead us to hypothesize that NO is involved in the response of plants to hypoxia and that hemoglobin modulates the levels of NO in the hypoxic cell.

Expression of a stress-induced hemoglobin affects NO levels produced by alfalfa root cultures under hypoxic stress.

The role of nitrate reduction to produce nitric oxide (NO) and its subsequent oxidation by oxyhaemoglobin as a mechanism to maintain plant cell energetics during hypoxia is examined. Nitrate reduction in hypoxic conditions can be considered as an alternative respiratory pathway, with nitrate as an intermediate electron acceptor, contributing to the oxidation of NADH. NO, produced in the reaction, does not accumulate due to the induction of hypoxia-induced (class 1) haemoglobins. These haemoglobins remain in the oxyhaemoglobin form, even at oxygen tensions two orders of magnitude lower than necessary to saturate cytochrome c oxidase. They act, probably in conjunction with a flavoprotein, as NO dioxygenases converting NO back to nitrate, consuming NAD(P)H in the process. The overall system oxidizes 2.5 moles of NADH per one mole of nitrate recycled during the reaction, leading to the maintenance of redox and energy status during hypoxia and resulting in the reduced production of ethanol and lactic acid.

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Nitrate, NO and haemoglobin in plant adaptation to hypoxia: an alternative to classic fermentation pathways

Plant Haemoglobins, Nitric Oxide and Hypoxic Stress

Mitochondria isolated from the roots of barley (Hordeum vulgare L) and rice (Oryza sativa L) seedlings were capable of oxidizing external NADH and NADPH anaerobically in the presence of nitrite The reaction was linked to ATP synthesis and nitric oxide (NO) was a measurable product The rates of NADH and NADPH oxidation were in the range of 12-16 nmol min i1 mg i1 protein for both species The anaerobic ATP synthesis rate was 7-9 nmol min i1 mg i1 protein for barley and 15-17 nmol min i1 mg i1 protein for rice The rates are of the same order of magnitude as glycolytic ATP production during anoxia and about 3-5% of the aerobic mitochondrial ATP synthesis rate NADH/NADPH oxidation and ATP synthesis were sensitive to the mitochondrial inhibitors myxothiazol, oligomycin, diphenyleneiodonium and insensitive to rotenone and antimycin A The uncoupler FCCP com- pletely eliminated ATP production Succinate was also capable of driving ATP synthesis We conclude that plant mitochondria, under anaerobic conditions, have a capacity to use nitrite as an electron acceptor to oxidize cytosolic NADH/NADPH and generate ATP

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Robert D. Hill

Papers

The RNA-binding protein FCA is an abscisic acid receptor

Expression of a stress-induced hemoglobin affects NO levels produced by alfalfa root cultures under hypoxic stress.

Nitrate, NO and haemoglobin in plant adaptation to hypoxia: an alternative to classic fermentation pathways

Plant Haemoglobins, Nitric Oxide and Hypoxic Stress

Nitrite-driven anaerobic ATP synthesis in barley and rice root mitochondria