Abscisic acid-independent and abscisic acid-dependent regulation of proline biosynthesis following cold and osmotic stresses in Arabidopsis thaliana

Abstract

The role of the phytohormone abscisic acid (ABA) in the regulation of proline synthesis was investigated by following the expression of the At-P5S and At-P5R proline biosynthesis genes in Arabidopsis thaliana wild type, in an ABA-deficient aba1-1 mutant as well as in ABA-insensitive abi1-1 and abi2-1 mutants after ABA, cold and osmotic stress treatments. In wild-type and in ABA mutant seedlings, 50 microM ABA or osmotic stress treatment triggered expression of At-P5S, whereas At-P5R accumulation was scarcely detectable. Expression of either gene was mediated by endogenous ABA since transcript levels were similar in wild-type and in ABA-deficient mutant plants. Proline accumulated to a greater extent after osmotic stress than upon ABA or cold treatment. Thus, ABA-treated abi1-1 mutant plants accumulated less proline than the ABA-treated wild type. Upon salt stress, proline accumulated to a lesser extent in aba1-1 and abi1-1 mutant plants, suggesting an indirect role of ABA on proline accumulation during salt adaptation of the plant. These results indicate that the expression of the genes of the proline biosynthetic pathway is ABA independent upon cold and osmotic treatments, although their expression can be triggered by exogenously applied ABA. However, the endogenous ABA content may affect proline accumulation upon salt stress, suggesting post-transcriptional control of proline biosynthesis in response to NaCl.

Dissecting the roles of osmolyte accumulation during stress

“…Furthermore, if flux through the proline biosynthetic pathway is a more important determinant of stress tolerance than free proline content, this may account both for conflicting evidence that absolute proline levels are a reliable measure of stress tolerance and the observation that proline accumulation is frequently only evident after the onset of stress-induced injury. Since the prompt accumulation of transcripts encoding P5C synthetase (Savouré et al 1997) and proline dehydrogenase (Kiyosue et al 1996) within 1 h of the imposition of severe stress precedes proline accumulation by several hours, we favour the view that elevated levels of proline accumulated after stress may simply be symptomatic of a metabolic adjustment of benefit only in the early stages of acclimation. As has frequently been suggested by others, proline degradation upon relief from stress might provide carbon, nitrogen and energy for recovery .…”

“…Likewise, observations that the Arabidopsis sos1 (salt overly sensitive) mutant has higher levels of salinity-induced proline and P5C synthetase expression than the wild-type (Liu & Zhu 1997) do not invalidate protective roles for free proline or proline synthesis in salinity tolerance in wild-type plants, but merely indicate that in this mutant, which is impaired in low-affinity Na + uptake and high-affinity K + uptake, proline synthesis does not limit salt tolerance. Similar caution is warranted in concluding that free proline does not play an important role in cold adaptation since coldinduced proline accumulation in an ABA-deficient mutant of Arabidopsis incapable of cold acclimation is comparable to levels observed in the wild type (Savouré et al 1997). Secondly, many attempts to improve stress tolerance by enhancing osmolyte accumulation are based on the assumption that maintenance of cell turgor is a critical limitation to productivity under stresses that cause dehydration.…”

“…Although our understanding of the intricate network of signalling events that regulates osmolyte accumulation is rudimentary, any attempt at formulating a map of these effects must at least incorporate light, chloroplastic redox poise, growth regulator action, nutrient-sensing mechanisms and the developmental state of the plant. Exactly how changes in the long-distance transport of growth regulators at the whole-plant level or their participation in signal cross-talk at the cell level achieve these effects is still poorly understood (Ishitani et al 1995;Savouré et al 1997;Strizhov et al 1997;Zhang et al 1997). Superimposed on these effects may be the relatively new discoveries concerning the signalling roles of reactive oxygen species and intracellular antioxidants ( Fig.…”

Proline Metabolism and Its Implications for Plant-Environment Interaction

“…ABA is well known to be required for the up regulation of many stress-induced genes (Shinozaki and Yamaguchi-Shinozaki, 2007). However, conflicting data have indicated either ABA-dependent (Yoshiba et al, 1995;Strizhov et al, 1997) or ABA-independent regulation of P5CS1 (Savoure et al, 1997). Recently, quantitative comparison of P5CS1 expression to known ABA-induced genes found largely ABA-independent regulation of P5CS1, implying that the underlying mechanisms controlling P5CS1 expression are distinct from that of many commonly studied stress marker genes (Sharma and Verslues, 2010).…”

24-epibrassinolide induced antioxidative defense system of Brassica juncea L. under Zn metal stress

“…The ROS such as hydrogen peroxide, superoxide radical and hydroxyl radical are highly reactive and induce lipid peroxidation, thereby affecting the structural integrity and permeability of cellular membranes. ROS can also cause protein denaturation and DNA damage (Savoure et al 1997). To prevent accumulation of these reactive molecules, plants have developed a highly efficient antioxidative defense system, including low-molecular-weight antioxidants, such as ascorbate and glutathione and protective enzymes, such as SOD, CAT and the enzymes of the ascorbate-glutathione cycle.…”