Elucidation of the role of osmoprotective compounds and osmoregulatory genes: The key role of bacteria

Abstract

A series of compounds, including proline and several betaines (glycine betaine, proline betaine), known to accumulate in plants during osmotic stress, have been found to function as osmoprotective compounds for bacteria such as enteric bacteria and Rhizobium meliloti. These molecules can be accumulated in massive amounts in the bacterial cytoplasm without precluding normal cellular functions. Accumulation of glycine betaine, one of the most efficient osmoprotec-tants, contributes to maintaining the proper osmotic strength of the cytoplasm, and thus prevents osmotic dehydration. For the accumulation of glycine betaine, the bacteria need an external supply of this compound or its precursor choline. Uptake systems for both molecules have been characterized. Escherichia coli has both a high-affinity and a low-affinity transport system for choline, and glycine betaine uptake is also mediated by two transport systems (low-affinity and high-affinity). Within the cells, choline never accumulates but is converted into glycine betaine by two dehydrogenases. In E. coli glycine betaine serves only in osmoregulation and cannot be catabolized, whereas in R. meliloti glycine betaine can function as a carbon, nitrogen and energy source, as well as an osmoprotectant. When choline and glycine betaine are not available, the bacteria can also achieve a moderate level of osmotic tolerance by endogenous synthesis of other compatible solutes, such as glutamate and the non-reducing disaccharide trehalose.

Recently, several genes involved in osmoregulation in E. coli (uptake systems and biosynthetic enzymes) have been cloned and sequenced. The bet genes, mapped at 7.5 min, encode the osmoregulatory choline-glycine betaine pathway. Four open reading frames are identified: betA encoding choline dehydrogenase; betB encoding glycine betaine aldehyde dehydrogenase; betT encoding a proton motive force driven, high-affinity transport system for choline; and betl responsible for choline regulation of betB. The proP and proU genes encode two glycine betaine transport systems: proP encodes a constitutive, low-affinity transport system, wdproU encodes a high-affinity system that is strongly induced at the transcriptional level by high osmolarity. The pro U region has been shown to contain three genes, proV, proW, emdproX in one operon. The proV and the proW genes encode a hydrophilic and a hydrophobic protein, respectively, whereas proX encodes a periplasmic glycine betaine-binding protein. Two osmotically regulated genes, otsA and otsB, governing synthesis of trehalose-6-phosphate synthase have also been identified and mapped to 42 min. Trehalose can be degraded by a periplasmic trehalase encoded by the osmotically regulated gene treA which maps at 26 min.

The discovery of all these new osmoregulatory genes in bacteria may provide useful models for understanding turgor mechanisms and osmoresponsive genes in plants. For example, overexpression of the choline-glycine betaine pathway in legumes using Rhizobium as vector may provide clues as to their functions and might even produce plants with increased water stress tolerance. In addition, nucleotide probes for these genes may contribute to selection schemes in breeding programs for drought and salinity. To achieve this challenging goal, the emerging technology must be closely integrated with established procedures of plant improvement.