The Physiology of Flowering Plants - KHAM PHA MOI

352 RESISTANCE TO STRESSswitched off or down-regulated, with a corresponding change in thepattern of protein synthesis. One experiment with Arabidopsis identifiedover 44 up-regulated genes, for instance (Seki et al. 2001). Genes for thesynthesis of the hormone ABA are up-regulated, leading to increasedlevels of this hormone. As discussed in Chapters 2 and 3, ABA transportedinto leaves induces stomatal closure in the early stages of waterstress. But ABA also forms a part of the signalling chain for the controllingof other genes responsive to water stress (Fig. 13.4). Artificiallyapplied ABA can substitute for water stress in the induction of manywater-stress-responsive genes. But not all water-stress-responsivegenes are induced by ABA. There obviously are several signal transductionchains involved in the response; in Arabidopsis at least fourare inferred to exist, of which two involve ABA. Functions in combatingthe effects of water deficit can be assigned to quite a number,although far from all, of the coded proteins, as descibed below.Compatible solutes and osmotic adjustmentA decrease in cellular C p (osmotic potential) typically occurs as aresponse to water deficit; this decrease is not just a concentration effectbut results from synthesis of organic solutes, which accumulate in thecytoplasm (not the vacuole). The main solutes include sugars (sucrose,trehalose and umbelliferose); fructans (soluble polysaccharides of fructoseunits); polyols (sugar alcohols); cyclic polyols; amino acids, especiallyproline; quaternary ammonium compounds (QAC), often glycinebetaine; and tertiary sulphonium compounds (Fig. 13.5). The accumulationof these solutes is controlled at the genetic level; e.g. genesencoding enzymes involved in the synthesis of proline are activated,Fig: 13:5 The structure of some nitrogenous compatible solutes. These chemicals havecharged atoms (shown in heavy type), but no net charge. Glycine betaine and proline betaineare examples of quaternary ammonium compounds (QAC), characterized by a fullymethylated N atom with a permanent positive charge; other common plant QAC are alaninebetaine, with an extra CH 2 group, also 3-hydroxyproline betaine and 4-hydroxyprolinebetaine, with an OH group on C atom 3 or 4. -dimethylsulphoniopropionate (DMSP) is themost common plant tertiary sulphonium compound; this group of chemicals has a fullymethylated S atom with a positive charge.