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The purpose of this work was to further investigate the regulatory interplay between pyrophosphate:fructose 6-phosphate 1-phosphotransferase (PFP) and its positive effector, fructose 2,6-bisphosphate (Fru-2,6-P 2 ), in the storage organs of cold- and drought-stressed plants. Since there is no detectable cytoplasmic fructose-1,6-bisphosphatase (cytFBPase) activity in the taproots of carrot plants, PFP is the only enzyme that can replace its function when stored starch is converted to transportable sucrose. The working hypothesis was that PFP is likely to be involved in the mobilisation of energy reserves and might have a special role in storage organs such as carrot taproots upon stress. Both cold and drought stress resulted in a marked increase in the endogenous Fru-2,6-P 2 levels. It is suggested that the significant changes in photosynthate allocation are the direct results of the stimulation of PFP activity by elevated Fru-2,6-P 2 levels. PFP stimulated by Fru-2,6-P 2 operated in the gluconeogenic direction in the taproots of stressed carrot plants, whereas the glycolytic direction was dominant in the non-stressed controls. This suggests that the metabolic status determining the net activity of PFP depends on the physiological stress situation, making PFP an important sensor of environmental changes. The experimental data indicated that PFP is involved in the mobilisation of energy reserves during unfavourable environmental changes by promoting the re-synthesis of transportable sucrose in taproots.

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Acta Agronomica Hungarica
Authors:
J. Szarka
,
O. Toldi
,
E. Szarka
,
J. Remenyik
, and
et al.

The fact that production is often unsuccessful even when resistant varieties are selected on the basis of the hypersensitive reaction can be attributed to the lack of adequate knowledge on plant disease resistance. In addition to specific plant responses to pathogen species, plants also possess an aspecific defense reaction which, instead of causing rapid tissue destruction, is based on the opposite strategy, protecting the plant against attack by microbes through tissue compaction achieved by cell enlargement and cell division. Genetic analyses carried out in pepper revealed that the general defense reaction was inherited as a monogenic recessive trait (gds). Pathophysiological observations indicate that the stimulus threshold is lower and the reaction rate faster than for specific defense reactions. Biochemical analyses suggest that, unlike plants exhibiting rapid tissue destruction, plants containing the gds gene do not require an oxidative burst elicited by hydrogen peroxide to stimulate the defense mechanism. It was also found that the regulation of the general defense system involves metabolic pathways that are independent of salicylic acid. The general and specific plant reactions form an integrated system of plant defense.

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What is remarkable about resurrection plants is the ability of vegetative tissues (root, shoot, stem, leaves) to tolerate dehydration of the tissues and then return as functional units on rehydration. This phenomenon made resurrection plants exciting targets for molecular analysis of the poikilohydric ability and drought tolerance. The protective mechanisms of vegetative desiccation tolerance appear to involve three major components, sugars, proteins and antioxidants. According to the recent scientific consensus all three are postulated to be involved in maintaining cellular integrity during the drying phases. The aim of this review is to establish a provisional hierarchy among these stress avoiding mechanisms that are associated with desiccation tolerance. The main reason for ranking these signal metabolites and protective agents is their potential importance in practical applications. Although vegetative desiccation tolerance is a complex trait both genetically and physiologically, there are already examples where outcomes of targeted studies in resurrection plants are going to be directly utilized to engineer crop plants genetically. Here we also show that conventional genetic transformation techniques, via in vitro plant regeneration systems, still represent an unavoidable part of the high-throughput technology chain of molecular breeding.

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