Elsevier

Plant Science

Volume 176, Issue 3, March 2009, Pages 334-341
Plant Science

The role of the bifunctional enzyme, fructose-6-phosphate-2-kinase/fructose-2,6-bisphosphatase, in carbon partitioning during salt stress and salt tolerance in Rice (Oryza sativa L.)

https://doi.org/10.1016/j.plantsci.2008.11.009Get rights and content

Abstract

A fructose-6-phosphate-2-kinase/fructose-2,6-bisphosphatase (F6P2K/F26BPase) gene was found to exhibit different transcript expression levels after 48 h of salt treatment between the salt-tolerant rice line, LPT123-TC171, and its original cultivar, LPT123, by differential RNA display and confirmed by RT-PCR. DNA sequence similarity analysis suggests this is the F6P2K/F26BPase gene located on chromosome 5. During 9 days of salt stress induced by inclusion of 85 mM NaCl in the nutrient solution, the growth of LPT123, but not LPT123-TC171, was inhibited with reduced dry and fresh weights, suggesting the higher salt tolerance of LPT123-TC171. Under these saline conditions, both rice cultivars significantly increased leaf sucrose levels and decreased the proportion of carbon assimilated to starch, but this was significantly more marked in the salt-tolerant LPT123-TC171 cultivar. Under normal culture conditions F6P2K/F26BPase transcripts could not be detected in LPT123 leaves, but were readily detected in LPT123-TC171. Salt stress for 72 h induced and up-regulated F6P2K/F26BPase expression levels in LPT123 and LPT123-TC171 cultivars, respectively, with higher levels of F6P2K/F26BPase transcripts being found in LPT123-TC171. Correspondingly, an increase in F6P2K and F26BPase enzymatic activities were observed in LPT123, but in LPT123-TC171, only a significant increase in F26BPase activity was seen, leading to a significant reduction in the F6P2K/F26BPase activity ratio after 9 days of salt treatment in LPT123-TC171. In correlation with the observed F6P2K/F26BPase activity ratios, the average leaf F26BP levels significantly decreased after 9 days salt-stress while the sucrose levels increased. These data support the role of a decrease in F26BP levels leading to an increased partition of carbon to sucrose, and suggest that this may contribute to salt-stress tolerance in rice (Oryza sativa L.).

Introduction

Adverse environmental conditions such as drought and saline soils can reduce a variety of activities essential for respiration [1] and photosynthesis [2] in NaCl sensitive plants. Plants exposed to these stresses induce the differential expression of a wide diversity of genes [3], [4], presumably in order to cope with the deleterious effects resulting from both dehydration, which can denature many proteins and membranes, and ion displacement, in which the accumulating chemical compounds displace inorganic cofactors needed for some enzymes to work efficiently.

Some of the proteins made in response to such stresses are enzymes that synthesize compounds believed to serve as osmoprotectants [5], [6], [7], including proline, polyamines, glycine, betaine and soluble sugars. Soluble sugars accumulate in the leaves of many plant species when they experience water stress [8], [9], [10]. In the case of rice subjected to osmotic stress it has been suggested that trehalose or carbohydrates might be more important than proline [11]. However, in contrast, proline accumulation during water stress, as well as soluble sugars, was proposed to be one of the mechanisms for drought tolerance in the indica rice variety [12].

Here, we report the differential expression of the fructose-6-phosphate-2-kinase/fructose-2,6-bisphosphatase (F6P2K/F26BPase) gene transcripts in two cultivars of rice (Oryza sativa L.), the wild type cultivar, LPT123, and the salt-resistant line, LPT123-TC171 [13].

F6P2K/F26BPase is a bifunctional enzyme which regulates the cellular level of fructose-2,6-bisphosphate (F26BP), itself a regulatory metabolite as both an inhibitor of fructose-1,6-bisphosphatase and an activator of 6-phosphofructo-1-kinase. Thus, F26BP (and consequentially F6P2K/F26BPase) plays an important role in controlling carbohydrate metabolism in eukaryote cells [14]. In plant leaves, F26BP contributes to the coordination of sucrose synthesis with the rate of CO2 fixation and the regulation of photosynthetically assimilated carbon partitioning between sucrose and starch [15]. In transgenic plants with a decreased foliar F26BP expression level, the synthesis of sucrose is promoted, whereas an increase in the level of F26BP stimulates the flux towards starch [16], [17], [18].

The differential expression of F6P2K/F26BPase in rice leaves between two closely related rice cultivars that possess a different salt tolerance reported in this manuscript may suggest a role for this enzyme in the regulation of carbohydrate metabolism during saline/osmotic stress. Therefore, we also investigated the (i) sucrose and starch accumulation responses, (ii) the F6P2K/F26BPase gene expression levels and their two associated enzyme activities, and (iii) the levels of F26BP in the LPT123 and LPT123-TC171 rice lines during growth under normal and salt-stress conditions. The results obtained and presented in this manuscript reveal the changes in the expression of F6P2K/F26BPase during salt stress that reflected the increased sucrose/starch ratios, which supports of a role for carbohydrate metabolism based adaptation in salt tolerance in rice.

Section snippets

Plant materials and growing conditions

The wild type rice cultivar, LPT123, and its derived salt-tolerant mutant line, LPT123-TC171 [13], used in this study were obtained as seed stock from the Rice Department, Ministry of Agriculture and Cooperatives, Thailand. Seeds of each of the two rice cultivars were planted in moist sand for a week. Germinated seeds were transferred to modified WP nutrient solution [19] for 1 week in a greenhouse under natural light (93–99 μmol photons m−2 s−1) and a relative humidity of between 74% and 81%. The

Characterization of salt tolerance in LPT123 and LPT123-TC171 indica rice varieties (O. sativa L.)

The indica Thai rice (O. sativa L.) cultivar, LPT123, and its derived salt-tolerant line, LPT123-TC171, were used to identify potentially altered gene expression levels that might be involved in salt-tolerance in rice. When grown in standard nutrient solution supplemented with 85 mM NaCl as a salt-stress condition, both LPT123 and LPT123-TC171 showed a reduction in fresh weight after 2 days of salt stress, compared to the plants grown without saline supplementation (Fig. 1). However, a more

Conclusion

The F6P2K/F26BPase is one of the genes whose transcriptional regulation level correlates with the salt-stress response in rice (O. sativa L.) and shows transcriptional up-regulation in response to both salt stress and salt tolerance. However, post-translational or other interacting biochemical controls may be the key steps to regulate any salt-tolerant ability gained by sucrose accumulation via controlling the ratio of F6P2K/F26BPase enzymatic activity, resulting in a decrease in the F26BP

Acknowledgements

This research was supported by Ministry of University Affairs, and The 90th Anniversary of Chulalongkorn University Fund. TU was supported by DPST, Thailand.

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