Proline controls the level of polyamines in common sage plants under normal conditions and at UV-B irradiation

Common sage (Salvia officinalis L.) plants grown in water culture to the stage of 4-5 true leaves were treated for 12, 24, 36, or 48 h with proline added to nutrient medium to a final concentration of 5 mM, or irradiated with UV-B light (12.3 kJ/m(2) for 10 min), or subjected to combined action of these factors. In these plants, activity of proline dehydrogenase (PDH), the content of proline, and the contents of free and conjugated polyamines were determined in the leaves and roots. It was shown that, in control plants, the content of endogenous proline was close to zero. In the presence of proline in medium, its total content in the roots was 9 mu mol/g fr wt in 12 h of exposure, whereas in the leaves the content of proline increased only in 24 h and achieved only 1 mu mol/g fr wt. The content of free putrescine increased in the leaves and especially in the roots after 10-min irradiation with UV-B light. The biosynthesis of putrescine was induced in the presence of proline in medium and was observed earlier than after UV-B irradiation. UV-B irradiation affected not only the synthesis of putrescine but also that of spermidine and spermine; it also induced accumulation of their soluble conjugates. Exogenous proline enhanced putrescine synthesis but inhibited the formation of polyamine soluble conjugates. At combined treatment of the two factors, the content of free putrescine in the leaves displayed a tendency to the rise and in the roots, to the decrease. At the same time, the content of polyamine free conjugates increased in both leaves and roots. All these facts could be considered as an indirect indication of relationship between proline and polyamine biosyntheses. We can also state that an artificially created high proline concentration in common sage tissues characterized of its low constitutive level resulted in disturbances in the homeostasis of low-molecular cell metabolites and induced a requirement in its restoration by diverse ways. This agrees with activation of PDH, a key enzyme of proline degradation. Induction of polyamine biosynthesis and changes in the content of their soluble conjugates might be one of the ways for such restoration. Under stress conditions, the high proline concentration is not toxic for plants because polyamines and proline are the components of the plant defense system, thus weakening damaging effects of abiotic stressors.