SAMDC Activity Is Involved in the Melatonin-Mediated Tolerance of Barley (Hordeum vulgare L.) Seedlings to Osmotic Stress

Exogenous melatonin can elevate the tolerance of plants to drought stress. The mechanism of melatonin-mediated tolerance has been extensively studied. Numerous studies have shown that melatonin treatment can increase contents of osmotic adjustment substances, alleviate oxidative damage, and maintain photosynthetic characteristics of plants under drought stress. However, a research gap remains, i.e., whether melatonin plays the above-mentioned functions by regulating polyamine metabolism. In the present study, S-adenosylmethionine decarboxylase (SAMDC) activity and the levels of spermine and spermidine were detected in melatonin-treated barley (Hordeum vulgare L.) seedlings under polyethylene glycol-6000, which was used to simulate drought stress. The aim was to probe whether polyamine metabolism was involved in the process of melatonin-mediated osmotic stress tolerance. The results indicated that exogenous melatonin improved the tolerance of barley seedlings to osmotic stress, coupled with increasing content of proline, alleviating oxidative damage and maintaining the photosynthetic characteristics of barley seedlings. More importantly, melatonin elevated the activity of SAMDC, and thereby the contents of spermine and spermidine. Thus, it was speculated that melatonin functioned by regulating polyamine metabolism. The hypothesis was further reinforced by the results of inhibitor experiments. Inhibitor methylglyoxal-bis (guanylhydrazone), which can inhibit SAMDC activity strongly and specifically, not only inhibited the melatonin-induced increases in the activity of SAMDC and the levels of spermine and spermidine, but also inhibited melatonin-induced increases in antioxidant ability, proline content, and photosynthetic capacity. The relationship between SAMDC activity and the effects of melatonin on proline content, antioxidant ability, and leaf photosynthetic characteristics was further explored and is discussed below. Therefore, it could be concluded that SAMDC might be involved in the melatonin-mediated tolerance of barley (Hordeum vulgare L.) seedlings to osmotic stress.