Hydrogen peroxide (H2O2), as a reactive oxygen species, is widely generated in many biological systems. It has been considered as an important signaling molecule that mediates various physiological and biochemical processes in plants. Normal metabolism in plant cells results in H2O2 generation, from a variety of sources. Also, it is now clear that nitric oxide (NO) and calcium (Ca2+) function as signaling molecules in plants. Both H2O2 and NO are involved in plant development and abiotic responses. A wide range of evidences suggest that NO could be generated under similar stress conditions and with similar kinetics as H2O2. The interplay between H2O2 and NO has important functional implications to modulate transduction processes in plants. Moreover, close interaction also exists between H2O2 and Ca2+ in response to development and abiotic stresses in plants. Cellular responses to H2O2 and Ca2+ signaling systems are complex. There is quite a bit of interaction between H2O2 and Ca2+ signaling in responses to several stimuli. This review aims to introduce these evidences in our understanding of the crosstalk among H2O2, NO and Ca2+ signaling which regulates plant growth and development, and other cellular and physiological responses to abiotic stresses.

5-Aminolevulinic acid (ALA) is a common precursor of tetrapyrroles as well as a crucial growth regulator in higher plants. ALA has been proven to be effectively in improving photosynthesis and alleviating the adverse effects of various abiotic stresses in higher plants. However, little is known about the mechanism of ALA in ameliorating the photosynthesis of plant under abiotic stress. In this paper, we studied the effects of exogenous ALA on salinity-induced damages of photosynthesis in cucumber (Cucumis sativus L.) seedlings. We found that the morphology (plant height, leave area), light utilization capacity of PS Ⅱ (qL, Y(Ⅱ)) and gas exchange capacity (Pn, gs, Ci and Tr) were significantly retarded under NaCl stress, but these parameters were all recovered by the foliar application of 25 mg L-1 ALA. Besides, salinity caused heme accumulation and up-regulation of gene expression of ferrochelatase (HEMH) with suppression of other genes involved in chlorophyll synthesis pathway. Exogenously application of ALA under salinity down-regulated the heme content and HEMH expression, but increased the gene expression levels of glutamyl-tRNA reductase (HEMA1), Mg-chelatase (CHLH) and protochlorophyllide oxidoreductase (POR). Moreover, the contents of intermediates involved in chlorophyll branch were increased by ALA, including protoporphyrin IX (Proto IX), Mg-protoporphyrin IX (Mg-Proto Ⅸ), protochlorophyllide (Pchlide) and chlorophyll (Chl a and Chl b) under salt stress. Ultrastructural observation of mesophyll cell showed that the damages of photosynthetic apparatus under salinity were fixed by ALA. Collectively, the chlorophyll biosynthesis pathway was enhanced by exogenous ALA to improve the tolerance of cucumber under salinity.

Crop productivity is restricted by various abiotic stresses such as drought, salinity, heat, and cold. Many efforts have been taken to decrease the inhibition of plant growth by alleviating the abiotic stresses. Exogenous applications of hormones, plant growth regulators, and/or small signaling molecules have been reported as a means to enhance plant resistance to stress. One of the small signaling molecules utilized is 5-aminolevulinic acid (ALA) that has been shown to enhance plant growth under abiotic stress. As a metabolic intermediate in higher plants, ALA is a precursor of all tetrapyrroles such as chlorophyll, heme and siroheme. The pathway towards biosynthesis upstream and the metabolism downstream of ALA contains multiple regulatory points that are affected by positive/negative factors. However, report about the regulatory aspects of the ALA metabolic pathway and the role of ALA in stimulating physiochemical processes in higher plants under stress have not been collated and summarized systematically. In this regard, we summarize recent developments in understanding the mechanisms of plant responses to abiotic stress which are affected by ALA as well as new information on the metabolic pathway of ALA. We find that exogenous application of ALA can enhance some key physiological and biochemical processes in plants such as photosynthesis, nutrient uptake, antioxidant characteristics and osmotic equilibrium, however, more in-depth research on the specific mechanisms are needed.