Osmotic Adjustment

Most organisms increase the cellular concentration of osmotically active compounds, termed compatible solutes (Table 19.1), when in danger of becoming desiccated by either drought conditions or external lowering of the osmotic potential.17-20 The accumulating compounds are "compatible" with normal cellular metabolism at high concentrations.21 Often these are hydrophilic, which gave rise to the view that they could replace water at the surface of proteins, protein complexes, or membranes. The biochemical mechanisms through which compatible solutes protect are still unknown, but this does not necessarily preclude working on application strategies.

Our focus has been on physiological mechanisms of compatible solute action. Also, we wish to understand how compatible solutes are integrated into a whole plant stress response that includes maintenance of ion homeostasis and water relations, C/N partitioning, reserve allocation and management of reducing power.22-25 There may be more than one function for a particular solute (see Shen6-7) and, based on results from in vitro experiments,26-28 different compatible solutes may have different functions. The importance of solute accumulation, interpreted as "osmotic adjustment", had been recognized long ago (e.g., refs. 21, 29-30).

A correlation between solute amount and tolerance has been documented. Plant transformation had to be developed before experiments could be designed to replace correlative relationships by proofs. The logical next step has been the engineering of plants to express enzymes that lead to the synthesis of such solutes and subsequent physiological analysis of these plants.

Different compounds can function as compatible solutes. Potassium, if available, serves this function. Also, amino acids and some amino acid derivatives, sugars, acyclic and cyclic polyols, fructans, and quaternary amino and sulfonium compounds frequently have been identified.19-20,30-32 Recently, genes have been characterized leading to ectoine (1,4,5,6-tetrahydro-2-methyl-4pyrimidine-carboxylic acid), a zwitterionic metabolite found in a number of halobacteria which

Table 19.1. Breeding objectives and mechanisms for enhanced stress tolerance

Plant breeding term

Physiological term

Mechanistic term

Suggested gene complex

Vigor

Growth

Meristem activity

Cell cycle, cyclins

conductance metabolism

Photorespiration

ROS scavenging Stomatal

One-carbon

Signal transduction

Protein kinases/ Phosphatases

Osmotic adjustment

Osmolyte accumulation

Internal osmotic pressure decrease Water transport

Polyol,proline Gly-bet synthesis Water channels Metabolite facilitators

Osmo-protection

Osmolyte accumulation

Radical oxygen scavenging

SOD, ASX, Cat, Asc/GS cycle

Protective proteins

Protein solvation Membrane integrity Enzyme complex stabilization

LEA/ dehydrins Lipid saturation

Ion homeostasis

Sodium exclusion Sodium partitioning

Proton pumping

Vacuolar sink size Na/H-antiport

HKT, Cation activitychannels P-ATPase; V-ATPase PPiase;

shows exceptional protection of protein function in in vitro assays.33-34 Typically, pathways leading to osmolyte synthesis are connected to pathways in general metabolism with high flux rates.32 Examples are the proline biosyn-thetic pathway,20 glycinebetaine synthesis,19 and the pathway leading to the methylated inositol, D-pinitol.2,32,35-38 We present a description of pinitol biosynthesis, which highlights essential features that seem to characterize what is required of a compatible solute.

Biosynthesis of D-pinitol in the halophyte Mesembryanthemum crystallinum L. (common ice plant) requires an increased flux of carbon from glucose 6-phosphate to myo-inositol

1-phosphate and then myo-inositol.36 The first gene in the pathway, encoding inositol-lP synthase, is transcriptionally upregulated, and increased protein amounts can be detected.36-37 The second enzyme, inositol monopos phatase, is not regulated under stress conditions in the ice plant (D. E. Nelson, personal communication). Utilizing increased amounts of inositol following stress, the enzyme myo-inositol O-methyltransferase (IMT) generates D-ono-nitol.35 In the ice plant, IMT is only expressed following salt stress, i.e., the protein is virtually absent in unstressed plants and increases dramatically within one to two days of stress.35,37 Finally, D-ononitol is converted into D-pinitol by an epimerization reaction which may include more than one enzyme. This activity, which we term OEP (ononitol epime-rase), has not yet been characterized biochemically or genetically. There are two signature features of this pathway. First, the pathway is connected to inositol synthesis and phospho-lipid biosynthesis, pathways which are tightly controlled in organisms in which they have been studied,39 and beyond that the synthesis of the methylated inositols is connected to the major flux of carbon in photosynthetic cells. The second feature is that the pathway includes additional enzymes which remove the product from general metabolism. D-pinitol is an extremely stable end product.40 The activities of the IMT and OEP enzymes have not been detected in tobacco and Arabidopsis.36'41-42 In fact, genes for these enzymes may be missing in these species.

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