The Physiology of Flowering Plants - KHAM PHA MOI

WATER-DEFICIT STRESS 353whilst those coding for enzymes of proline degradation are repressed.Transgenic individuals have been produced,which acquire the capabilityto synthesize a compatible solute not normally formed by the species,and which then gain increased water stress tolerance (Fig. 13.6).The way in which such solutes function is still not completelyclear. They are known as compatible solutes, since they are compatiblewith normal functioning of enzymes and other proteins:having no net electric charge, they do not disturb the ionic bondsmaintaining tertiary and quaternary protein structure. The loweringof cellular C p , termed osmotic adjustment, enables the cell to holdwater against a low external C. Some species can maintain turgorin the face of an external C of 4 MPa by this process. Compatiblesolutes, being in the cytoplasm, also maintain osmotic balancebetween the cytoplasm and the vacuolar sap, which becomes moreconcentrated as water is withdrawn from the cell. But sometimes theconcentrations of these solutes are too low for a significant osmoticeffect and other activities have therefore been proposed. Polyols canact as removers of oxidative radicals and some compatible solutescan stabilize molecular structures. Sugars can to some extent replacethe water of hydration of proteins and prevent denaturation by waterloss. In highly dehydrated plant tissues, sucrose participates in theformation of a glassy matrix, with tight H-bonding between molecules,in which proteins are protected from damage.Protection against oxidative damageThere is an increase in transcription of genes coding for enzymesinvolved in the removal of ROS, e.g. SOD (superoxide dismutase) andcatalase. The protective value of such enzymes is indicated by the factthat the water stress tolerance of plants can be improved by producingtransgenic individuals which overexpress SOD. There are alsoreports of increases in the levels of antioxidants – b-carotene, ascorbateand a-tocopherol (vitamin E).Fig: 13:6 Increased water stressresistance in transgenic tobacco(Nicotiana tabacum) expressing ayeast gene for synthesis of the sugartrehalose, not normally synthesizedby tobacco. Both control andtransgenic plants were deprived ofirrigation for 15 days and the extentof stress injury was estimated interms of the percentage of leavesthat were dead, damaged, orsurviving fairly undamaged. Openbars = control plants; black bars =transgenic plants; lines indicate SE.The concentrations of trehalose inthe transformed plants were low,indicating that the effect in thisinstance was not osmotic. FromRomero et al. (1997). ª Springer-Verlag GmbH & Co. KG.LEA genes and protein stabilizationThere are also many genes activated coding for proteins for whichthe precise function is not elucidated. This includes LEA genes (‘lateembryogenesis abundant’), which are expressed during the normaldesiccation occurring when seeds mature (Section 11.14.1), particularlythe group of LEA genes coding for dehydrins (the D11 familyof LEA proteins). Sequencing suggests that the majority of the LEAproteins should be highly hydrophilic. They have consequently beenassigned protective functions in binding water and stabilizing membranes.Some are suggested to act as chaperones, proteins which bindto unfolded protein molecules and thereby prevent incorrect (denatured)folding and aggregation. Chaperone action is important inensuring the correct folding of newly synthesized polypeptides inunstressed plants. In stressed cells, chaperones could react withproteins that have unfolded owing to loss of water of hydration,though direct proof of this is lacking.