A regulatory role for proline metabolism in stimulating Arabidopsis thaliana seed germination

Analysis of the rate-limiting dehydrogenases of the oxidative pentose phosphate pathway (OPPP), as well as changes in the contributions of C-14(1)- and C-14(6)-labelled glucose to respired CO2 during germination, indicated activation of the OPPP during Arabidopsis seed germination. An approximately four-fold increase in free proline (Pro) was also observed prior to radicle emergence in most seeds. Delayed radicle emergence in transgenic Arabidopsis seeds which expressed an antisense copy of the gene encoding the Pro biosynthetic enzyme delta(1)-pyrroline-5-carboxylate synthetase correlated with an approximately 35% reduction in the maximal concentration of Pro accumulated by the antisense lines during germination. A dose-dependent inhibition of Arabidopsis seed germination by millimolar concentrations of Pro capable of feedback inhibition of Pro synthesis reinforced the role of Pro synthesis in promoting germination. The ability of the artificial oxidants methylene blue and phenazine ethosulphate to overcome the inhibitory effects of Pro suggests a functional link between elevated Pro synthesis and increased OPPP activity and the importance of coupling of both pathways in stimulating germination.