Proline accumulation induces the production of total phenolics in transgenic tobacco plants under water deficit without increasing the G6PDH activity

Proline metabolism has been proposed to play an important role in activating the shikimic acid pathway for increased secondary metabolites production in plants. Here we evaluated the role of proline per se and in combination with water deficit on the production of phenolic compounds in leaves of transgenic tobacco plants (Nicotiana tabacum) with increased proline content. For this, an experiment was carried out under greenhouse conditions using three genotypes (transgenic events E1 and E2 constitutively expressing the mutant gene VaP5CSF129A and an untransformed control) and two levels of water regimes (100% and 30% of soil field capacity). Total phenolics, proline, activities of glucose-6-phosphate dehydrogenase (G6PDH) and phenylalanine ammonia-lyase (PAL), and lignin were measured in the leaves. As expected, a reduction in biomass was observed in all genotypes under drought conditions and the maximum accumulation of proline occurred in transgenic plants in both water regimes. Under irrigated conditions, phenolics were only slightly superior in leaves of the transgenic event E2, while both events presented a noticeable increment in total phenolics and PAL activity under water deficit. Lignin biosynthesis also increased in all genotypes in response to drought stress, but it was more noticeable in the transgenic plants. On the other hand, the activity of the G6PDH enzyme was consistently lower in the transgenic plants in both irrigated and water-deficit condition. Our results do not confirm the association between de novo proline synthesis and the stimulation of G6PDH to produce phenolics.