| Trehalose (α-D- glucopyranosylα-D- glucopyranosyl)is a disaccharide composed of two molecules of glucose. It was first described in the early 19th century as a component of the ergot of rye, later it was discovered in a great variety of species such as bacteria, fungi, eukaryotic microorganisms, insects and invertebrates. Its presence in plants has long been thought to be confined to resurrection plants, in which it serves as a stress protectant. Recently, trehalose metabolism has been thought to be common in plant species. The absence of reducing ends renders trehalose highly resistant to heat, pH and Maillard's reaction (a reaction between carbohydrates and amino acids). Moreover, trehalose has a strong stabilizing effect on biological structures, forming a glass-like structure after dehydration. Because of these characteristics, trehalose is predicated to become a useful stabilizer in foods and an additive in cosmetics and pharmaceutical. In the past, most studies on trehalose was focused upon its protection for biological structures of microorganisms, but less attention was paid to its protective role for plants.The global mean temperature tends to increase in the future with the increase of atmospheric CO2. High temperature negatively affects plant growth and survival and hence crop yield. Although many factors may contribute to the negative correlation between crop yield and average temperature, direct high temperature effects on physiological processes probably play an important role and heat-stress-induced reductions of photosynthesis can limit overall yield. Therefore, studies aimed at better understanding how heat affects photosynthesis are, in the context of global warming, of increasing importance. Considering the situation above, we focus our research interest, in this study, on the mechanism by which trehalose improved the heat tolerance in wheat plants, especially on the effect of trehalose on photosynthesis process. Heat stress treatments were 40℃for 12 h, 24 h and 36 h, then recovery, the trehalose concentrations used in the experiments were 0.5 mmol/L, 1.0 mmol/L and 1.5 mmol/L. Our conclusions were as follows:1 Optimum trehalose level is the basis for normal growth of wheat plants. Under normal growth condition, 1.5mM trehalose pretreatment did not increase trehalose content in wheat leaves. Moreover, low concentration of trehalose pretreatment promoted plant growth, while higher concentration of trehalose inhibited plant growth. Under 12 h and 24 h heat stress conditions, trehalose had no significant effects on plant growth while plant growth was inhibited during severe 36 h heat with the trehalose concentrations investigated growing. During recovery from heat, trehalose pretreatment had a certain promotion on wheat growth. However, with the heat time increasing, the wheat plant growth could not be recovered any more. Trehalose pretreatment inhibited root elongation especially during 36 h heat and its recovery.2 Trehalose pretreatment increased leaf water content during both 36 h heat and recovery from 12 h- and 24 h- heat, which may be associated with its protection for aquaporins.3 During recovery from 36 h- high temperature of 40℃, trehalose pretreatment ameliated photoinhibition expressed as Fv/Fm to a certain extent.4 Trehalose pretreatment might help to protect photosynthetic apparatus and enhance heat tolerance by depressing energy absorption, enhancing energy dissipation as heat. Heat decreased chlorophyll content and the ratio of (A+Z)/(V+A+Z) but had hardly effect onβ-carotene content. After recovery from heat, both chlorophyll andβ-carotene content continued to decrease, while the ratio of (A+Z)/(V+A+Z) had a marked increase. However, if seedlings pretreated with trehalose for 3 d were confronted with heat,β-carotene content and (A+Z)/(V+A+Z) had a striking increase when compared with control, whereas chlorophyll content had a marked decrease. After recovery from heat, the relative content of chlorophyll andβ-carotene in trehalose-pretreated seedlings is higher and (A+Z)/(V+A+Z) ratio is lower than no trehalose treatment.5 24 h 40℃heat stress damaged chloroplast ultrastructure. However, trehalose pretreatment may protect chloroplast ultrastructure from damage by heat stress. In particular, during recovery from heat, trehalose pretreatment formed functional chloroplast more quickly than without trehalose.6 Heat stress damaged some thylakoid proteins, while trehalose pretreatment may protect 20-97kDa polypeptides in thylakoid membrane from dissociating during heat and recovery from heat. Especially, trehalose pretreatment protected D1 protein during heat and recovery, prevented its degrading and maintained the higher PSII photochemical activity.7 Trehalose pretreatment had some effect on lipid and fatty acid composition and level during heat and recovery. 24 h 40℃heat led to a remarkable increase of total IUFA compared with control. However, compared with no-trehalose treatment, IUFA in trehalose-pretreated seedlings had a marked decrease during heat and a marked increase during de-heat, showing an evident connection between the level of unsaturation of fatty acids with environmental temperature. During recovery from heat, contents of PC and PG in the presence of trehalose were higher than in the absence of trehalose. These may be related with their significant roles to stabilize the photosynthetic machinery against heat induced photoinhibition and accelerate the recovery of the photosystem II protein complex.8 Heat stress caused membrane lipid peroxidation and damaged membrane integrity. While trehalose pretreatment alleviated heat induced damage to membrane, which was indicative of the decrease in electrolyte leakage, MDA content and the generation of O2.- and H2O2. The protection of trehalose for membrane was related to it induced decline in LOX activity, protection for antioxidant enzymes and scavenging ROS. Trehalose, as a reactive oxygen species scavenger, may play a direct role in eliminating ROS, which is an important role in protecting photosynthetic apparatus, thylakoid membrane and proteins during heat and recovery.
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