The upregulation of thiamine (vitamin B1) biosynthesis in Arabidopsis thaliana seedlings under salt and osmotic stress conditions is mediated by abscisic acid at the early stages of this stress response.
BACKGROUND
Recent reports suggest that vitamin B1 (thiamine) participates in the processes underlying plant adaptations to certain types of abiotic and biotic stress, mainly oxidative stress. Most of the genes coding for enzymes involved in thiamine biosynthesis in Arabidopsis thaliana have been identified. In our present study, we examined the expression of thiamine biosynthetic genes, of genes encoding thiamine diphosphate-dependent enzymes and the levels of thiamine compounds during the early (sensing) and late (adaptation) responses of Arabidopsis seedlings to oxidative, salinity and osmotic stress. The possible roles of plant hormones in the regulation of the thiamine contribution to stress responses were also explored.


RESULTS
The expression of Arabidopsis genes involved in the thiamine diphosphate biosynthesis pathway, including that of THI1, THIC, TH1 and TPK, was analyzed for 48 h in seedlings subjected to NaCl or sorbitol treatment. These genes were found to be predominantly up-regulated in the early phase (2-6 h) of the stress response. The changes in these gene transcript levels were further found to correlate with increases in thiamine and its diphosphate ester content in seedlings, as well as with the enhancement of gene expression for enzymes which require thiamine diphosphate as a cofactor, mainly α-ketoglutarate dehydrogenase, pyruvate dehydrogenase and transketolase. In the case of the phytohormones including the salicylic, jasmonic and abscisic acids which are known to be involved in plant stress responses, only abscisic acid was found to significantly influence the expression of thiamine biosynthetic genes, the thiamine diphosphate levels, as well as the expression of genes coding for main thiamine diphosphate-dependent enzymes. Using Arabidopsis mutant plants defective in abscisic acid production, we demonstrate that this phytohormone is important in the regulation of THI1 and THIC gene expression during salt stress but that the regulatory mechanisms underlying the osmotic stress response are more complex.


CONCLUSIONS
On the basis of the obtained results and earlier reported data, a general model is proposed for the involvement of the biosynthesis of thiamine compounds and thiamine diphosphate-dependent enzymes in abiotic stress sensing and adaptation processes in plants. A possible regulatory role of abscisic acid in the stress sensing phase is also suggested by these data.
Department of Analytical Biochemistry, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Kraków, Poland. maria.rapala-kozik@uj.edu.pl
Maria
M
Rapala-Kozik
Natalia
N
Wolak
Marta
M
Kujda
Agnieszka K
AK
Banas
D000040:Abscisic Acid / Q000378:metabolism*; D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D007506:Iron-Sulfur Proteins / Q000235:genetics / Q000378:metabolism; D009995:Osmosis; D018749:RNA, Plant / Q000235:genetics; D036226:Seedlings / Q000378:metabolism*; D012965:Sodium Chloride / Q000378:metabolism; D013312:Stress, Physiological; D013831:Thiamine / Q000096:biosynthesis*; D013997:Time Factors; D015854:Up-Regulation
D029681:Arabidopsis Proteins; D007506:Iron-Sulfur Proteins; D018749:RNA, Plant; C437085:THI1 protein, Arabidopsis; C525883:THIC protein, Arabidopsis; D012965:Sodium Chloride; D000040:Abscisic Acid; D013831:Thiamine