Regulation of Levels of Proline as an Osmolyte in Plants under Water Stress

Abstract

Compatible osmolytes are potent osmoprotectants that play a role in counteracting the effects of osmotic stress. Proline (Pro) is one of the most common compatible osmolytes in water-stressed plants. The accumulation of Pro in dehydrated plants is caused both by activation of the biosynthesis of Pro and by inactivation of the degradation of Pro. In plants, L-Pro is synthesized from L-glutamic acid (L-Glu) via delta(1)-pyrroline-5-carboxylate (P5C) by two enzymes, P5C synthetase (P5CS) and P5C reductase (P5CR). L-Pro is metabolized to L-Glu via P5C by two enzymes, proline dehydrogenase (oxidase) (ProDH; EC 1.5.99.8) and P5C dehydrogenase (P5CDH; EC 1.5.1.12). Such metabolism of Pro is inhibited when Pro accumulates during dehydration and it is activated when rehydration occurs. Under dehydration conditions, when expression of the gene for P5CS is strongly induced, expression of the gene for ProDH is inhibited. By contrast, under rehydration conditions, when the expression of the gene for ProDH is strongly induced, the expression of the gene for P5CS is inhibited. Thus, P5CS, which acts during the biosynthesis of Pro, and ProDH, which acts during the metabolism of Pro, appear to be the rate-limiting factors under water stress. Therefore, it is suggested that levels of Pro are regulated at the level of transcriptional the genes of these two enzymes during dehydration and rehydration. Moreover, it has been demonstrated that Pro acts as an osmoprotectant and that overproduction of Pro results in increased tolerance to osmotic stress of transgenic tobacco plants. Genetically engineered crop plants that overproduce Pro might, thus, acquire osmotolerance, namely, the ability to tolerate environmental stresses such as drought and high salinity.

The AtProT Family. Compatible Solute Transporters with Similar Substrate Specificity But Differential Expression Patterns

“…Intracellular transport of Pro is also necessary, as Pro is synthesized in the cytosol but, under stress conditions, accumulates both in the cytosol and in the chloroplasts (Bü ssis and Heineke, 1998). Furthermore, Pro degradation takes place in the mitochondria (Yoshiba et al, 1997). The AtProTs show no obvious targeting signal directing protein import into mitochondria or chloroplasts (ARAMEMNON database; Schwacke et al, 2003).…”

“…In general, cellular concentration of compatible solutes can be regulated by increasing biosynthesis, decreasing degradation, and/or modifying rates of uptake or release of these compounds. In plants, research has focused on the metabolism of Pro and Gly betaine and their importance for stress tolerance (Yoshiba et al, 1997;McNeil et al, 1999). However, only limited information on the significance of transport of compatible solutes is available.…”

Role of proline and leaf expansion rate in the recovery of stressed white clover leaves with increased phosphorus concentration

“…Proline is a nonprotein amino acid that forms in leaf tissue as leaves are subjected to water stress, and together with sugar is readily metabolized in leaves upon recovery from water stress (Wyn Jones et al, 1979 ;Kameli & Losel, 1993). Recently proline has been considered as an osmoprotectant, with overproduction, as in the high-P clover plants, resulting in increased tolerance to osmotic and environmental stresses (Yoshiba et al, 1997). The role of proline and other solutes such as fructans (Pilon-Smits et al, 1995) in drought resistance might not necessarily be osmotic because of their relatively small contributions to the osmolality of expressed leaf sap, unless they are largely restricted to small compartments such as the cytoplasm or organelles where their presence could become osmotically highly significant.…”

DROUGHT RESPONSES ON GROWTH, PROLINE CONTENT AND ROOT ANATOMY OF Acacia auriculiformis Cunn., Tectona grandis L., Alstonia spectabilis Br., AND Cedrela odorata L.

“…Pada tanaman yang toleran cekaman kekeringan, mekanisme untuk mempertahankan turgor dilakukan dengan cara meningkatkan kadar senyawa osmotik seperti prolin dan asam-asam organik untuk penyesuaian osmotik sehingga tidak terjadi plasmolisis. Kandungan prolin pada tanaman yang toleran terhadap kekeringan terlihat meningkat akumulasinya dibandingkan tanaman yang peka terhadap kekeringan (Yoshiba et al, 1997). Kadar prolin bisa digunakan sebagai salah satu indikator sifat ketahanan terhadap cekaman kekeringan (Ashraf dan Foolad, 2007) …”