Conjugated Polyamines in Root Plasma Membrane Enhanced the Tolerance of Plum Seedling to Osmotic Stress by Stabilizing Membrane Structure and Therefore Elevating H+-ATPase Activity

Polyamines are small positively charged molecules in plants and play important functions in many biological processes under various environmental stresses. One of the most confounding problems relating to polyamines (PAs) in stresses is the lack of understanding of the mechanisms underlying their function(s). Furthermore, a limited number of studies have addressed this issue at the sub-cellular level, especially in tree plants under drought stress. Therefore, in this research, by simulating natural drought stress with polyethylene glycol (PEG) osmotic stress, the relationship between the levels of conjugated polyamines and the activity of H+-ATPase in the plasma membrane was elucidated with the roots of two plum (Prunus salicina L.) cultivars, which were different in drought tolerance, as experimental materials. Furthermore, free PA levels and the activities of S-adenosylmethionine decarboxylase (SAMDC) and transglutaminase (TGase), which were closely associated with the levels of free and conjugated PAs, were also detected. Results showed that under osmotic stress, the increases of the levels of non-covalently conjugated (non-CC) spermidine (Spd) and spermine (Spm), covalently conjugated (CC) putrescine (Put) and Spd in the plasma membrane of drought-tolerant Ganli No. 5 were more significant than those of drought-sensitive Suli No. 3, indicating that these conjugated PAs might be involved in the tolerance of plum seedlings to stress. Furthermore, the conjugated PAs were closely correlated with plum seedling growth, water retention capacity, plasma membrane damage degree, and hydrogen (H+)-ATPase activity in the plasma membrane. To get more complementary pieces of evidence, we subjected plum seedlings to combined treatments of PEG and exogenous PA (Spd and Spm), and an inhibitor of SAMDC [methylglyoxal-bis (guanylhydrazone), (MGBG)] or TGase (o-phenanthroline). These results collectively suggested that non-CC Spd and Spm, CC Put and Spd in plasma membrane might function in enhancing the tolerance of plum seedlings to osmotic stress by stabilizing membrane structure and therefore elevating H+-ATPase activity.

Effects of Se Application on Polyamines and Carbon–Nitrogen Metabolism of Pepper Plants Suffering from Cd Toxicity

Previous studies have shown that the application of selenium (Se) can efficiently mitigate the toxic effects of cadmium (Cd) on various crops. The objective of the present work is to decipher the mechanisms responsible for the efficiency of Se against the effects of Cd in pepper plants, with respect to the carbon and nitrogen metabolism. The following were analyzed: the concentrations of anions related with this metabolism, such as nitrates, nitrites, and ammonium, the activities of different enzymes such as nitrate reductase, nitrite reductase, and glutamate synthase, polyamines in their different forms, organic acid salts, amino acids, and sugars in the leaf and root tissues of the pepper plants grown in a hydroponics system. Four different treatments were applied: plants without Cd or Se applied (−Cd/−Se); plants grown with Cd added to the nutrient solution (NS) but without Se (+Cd/−Se); plants grown with Cd in the NS, and with the foliar application of Se (+CD/+SeF); and lastly, plants grown with Cd in the NS, and with Se applied to the root (+Cd/+SeR). The metabolites and enzymes related with carbon and nitrogen metabolism were analyzed 15 days after the application. The results showed the superiority of the +Cd/+SeR treatment with respect to the +Cd/+SeF treatment, as shown by an increase in the conjugated polyamines, the decrease in glutamate and phenylalanine, and the increase of malate and chlorogenic acid. The results indicated that SeR decreased the accumulation and toxicity of Se as polyamine homeostasis improved, defense mechanisms such as the phenylpropanoid increased, and the entry of Cd into the plants was blocked.