Huai-Pan Liu

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.

Exogenous melatonin can alleviate cold injury in post-harvested fruit during fruit refrigeration. The mechanism of melatonin-mediated cold tolerance is increasingly being focused on in studies. However, in cold + melatonin-treated fruit, the changes in the activities of S-adenosylmethionine decarboxylase (SAMDC) and 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, the key enzymes that catalyze polyamine and ethylene biosynthesis, respectively, are still unclear. In this study, the typical symptoms of cold injury, the contents of polyamines and ethylene, and the activities of SAMDC and ACC synthase are elucidated in melatonin-treated peach fruit under cold conditions. The results show that exogenous melatonin alleviated cold injury in peach fruit and elevated SAMDC activity and, thereby, the levels of spermidine and spermine. More importantly, ACC synthase activity, the ACC level and the ethylene emission rate decreased. These results hint that exogenous melatonin enhanced the tolerance of peach fruit to cold stress by elevating SAMDC and decreasing ACC activities. The results of the inhibitor experiments supplied additional evidence for this hypothesis. The inhibitor methylglyoxal-bis (guanylhydrazone) could inhibit SAMDC activity, decrease the levels of spermidine and spermine, increase the ACC level and ethylene emission rate, and thereby decrease the fruit’s cold tolerance. Meanwhile, the inhibitor aminoethoxyvinylglycine could decrease the ACC synthase activity, the ACC level, and the ethylene emission rate, and also increase the fruit’s cold tolerance. Furthermore, the proposed competitive relationship between SAMDC and ACC synthase for substrate S-adenosyl-L-Met is also discussed in this paper.

Treatment with exogenous melatonin can raise postharvest fruit cold tolerance and alleviate cold injury. However, the mechanism, by which melatonin mediate cold tolerance, is still to be explored. So in this paper, the conjugated polyamines in tonoplast and vacuole structure were illuminated in the flesh cells of apple fruit subjected to cold stress, which were pretreated with exogenous melatonin. The results showed that under cold condition, fruit firmness decreased and flesh browning area increased, and vacuole structure changed. The cold injury could be alleviated by pretreatment with exogenous melatonin, as judged by stabilization of the vacuole structure, increased fruit firmness and reduced flesh browning area. Furthermore, the cold alleviation was coupled with the increases of the levels of covalently conjugated putrescine and non-covalently conjugated spermidine and spermine in the tonoplast. The results suggested that these conjugated polyamines in tonoplast and vacuole structure might be involved in melatonin-mediated cold tolerance. The suggestion was further verified by applications with inhibitors, phenanthroline and methylglyoxal-bis (guanylhydrazone) (MGBG). Phenanthroline and MGBG could restrain the melatonin-induced increases of covalently and non-covalently conjugated polyamines mentioned above in the tonoplast, respectively, and increase flesh browning area, decrease fruit firmness and vacuole structure stabilization. So, we can conclude that exogenous melatonin alleviates cold injury in apple fruit via elevating the contents of the polyamines conjugated to the tonoplast and thereby maintaining vacuole structure.

Exogenous polyamines can enhance postharvest fruit cold resistance and alleviate cold injury. However, the mechanism underlying polyamine-mediated cold resistance remains to be eluciated. Therefore, in this research, polyamines conjugated to plasma membrane (PLM) and membrane conformation were detected in the putrescine pretreated peach fruits under cold condition. The results showed that cold stress resulted in the changes in the PLM conformation and cause the cold injury as judged by the increased plasma membrane permeability and the decreased sulphydryl content. Pretreatment with putrescine mitigated the cold injury of the fruits and resulted in the stabilization of PLM, coupled with the increased contents of non-covalently (NC)-conjugated spermidine (Spd) and spermine (Spm), and covalently-conjugated (C) putrescine (Put) and Spd in PLM. From the results, it could be inferred that polyamines conjugated to PLM and membrane conformation stabilization might be involved in the putrescine-mediated resistance of the harvested peach fruits to cold stress. This inference was confirmed by the complementary experiments with methylglyoxal-bis(guanylhydrazone) (MGBG) and phenanthroline, which inhibited S-adenosylmethionine decarboxylase and transglutaminase activities, respectively. MGBG restrained the putrescine-induced increases in NC-conjugated Spd and Spm and phenanthroline declined the increases in C-conjugated putrescine in PLM induced by exogenous putrescine. Furthermore, both MGBG and phenanthroline reversed the mitigation effects of putrescine on fruit cold injury, in parallel with the decrease of membrane conformation stabilization. Collectively, we could conclude that putrescine-mediated fruit cold resistance involved in the increases in the polyamines conjugated to PLM and the maintenance of membrane conformation. The relationship between the conjugated polyamines and membrane conformation stabilization was discussed in the research.

Fusarium head blight (FHB), or scab, caused by Fusarium species, is an extremely destructive fungal disease in wheat worldwide. In recent decades, researchers have made unremitting efforts in genetic breeding and control technology related to FHB and have made great progress, especially in the exploration of germplasm resources resistant to FHB; identification and pathogenesis of pathogenic strains; discovery and identification of disease-resistant genes; biochemical control, and so on. However, FHB burst have not been effectively controlled and thereby pose increasingly severe threats to wheat productivity. This review focuses on recent advances in pathogenesis, resistance quantitative trait loci (QTLs)/genes, resistance mechanism, and signaling pathways. We identify two primary pathogenetic patterns of Fusarium species and three significant signaling pathways mediated by UGT, WRKY, and SnRK1, respectively; many publicly approved superstar QTLs and genes are fully summarized to illustrate the pathogenetic patterns of Fusarium species, signaling behavior of the major genes, and their sophisticated and dexterous crosstalk. Besides the research status of FHB resistance, breeding bottlenecks in resistant germplasm resources are also analyzed deeply. Finally, this review proposes that the maintenance of intracellular ROS (reactive oxygen species) homeostasis, regulated by several TaCERK-mediated theoretical patterns, may play an important role in plant response to FHB and puts forward some suggestions on resistant QTL/gene mining and molecular breeding in order to provide a valuable reference to contain FHB outbreaks in agricultural production and promote the sustainable development of green agriculture.

It has been well documented that polyamine is associated closely with environmental stresses and melatonin pretreatment enhances the resistance of fruit to cold stress. However, mechanism underlining melatonin-mediated chilling resistance remains to be explored. Therefore, in the research, flesh browning degree, relative plasma membrane permeability, malondialdehyde and protein sulfhydryl content in plasma membrane were used to assess the resistance of apple fruit to chilling stress. The polyamine conjugated to plasma membrane of melatonin-pretreated apple (Malus pumila Mill.) fruit flesh cell was elucidated under chilling stress. The results showed that melatonin pretreatment led to obvious increases in the contents of conjugated non-covalently spermidine and spermine, and conjugated covalently putrescine and spermine in plasma membrane of apple fruit under chilling stress, compared with the apple fruit without melatonin pretreatment. Methylglyoxyl-bis (guanylhydrazone) pretreatment could inhibit the melatonin-induced increases of conjugated non-covalently spermidine and spermine by inhibiting the activity of S-adenosylmethionine decarboxylase and free spermidine and spermine contents in flesh, coupled with the decrease in chilling resistance. Similarly, phenanthrolin pretreatment could inhibit the melatonin-induced increases in putrescine and spermidine covalently conjugated to plasma membrane by inhibiting transglutaminase activity in flesh, simultaneously aggravating chilling damage. From the results collectively, it should be suggested the melatonin pretreatment could enhance chilling resistance by increasing non-covalently conjugated spermidine and spermine, and covalently conjugated putrescine and spermine in plasma membrane of apple fruit.