Rice (Oryza sativa L.) frequently suffers in late spring from severe damage due to cold spells, which causes the block of chlorophyll biosynthesis during early rice seedling greening. However, the inhibitory mechanism by which this occurs is still unclear. To explore the responsive mechanism of rice seedlings to low temperatures during greening, the effects of chilling stress on chlorophyll biosynthesis and plastid development were studied in rice seedlings. Chlorophyll biosynthesis was obviously inhibited and chlorophyll accumulation declined under low temperatures during greening. The decrease in chlorophyll synthesis was due to the inhibited synthesis of δ-aminolevulinic acid (ALA) and the suppression of conversion from protochlorophyllide (Pchlide) into chlorophylls (Chls). Meanwhile, the activities of glutamate-1-semialdehyde transaminase (GSA-AT), Mg-chelatase, and protochlorophyllide oxidoreductase (POR) were downregulated under low temperatures. Further investigations showed that chloroplasts at 18 °C had loose granum lamellae, while the thylakoid and lamellar structures of grana could hardly develop at 12 °C after 48 h of greening. Additionally, photosystem II (PSII) and photosystem I (PSI) proteins obviously declined in the stressed seedlings, to the point that the PSII and PSI proteins could hardly be detected after 48 h of greening at 12 °C. Furthermore, the accumulation of reactive oxygen species (ROS) and malondialdehyde (MDA) and cell death were all induced by low temperature. Chilling stress had no effect on the development of epidermis cells, but the stomata were smaller under chilling stress than those at 28 °C. Taken together, our study promotes more comprehensive understanding in that chilling could inhibit chlorophyll biosynthesis and cause oxidative damages during greening.

Wheat stripe rust (Puccinia striiformis f. sp. tritici) is one of the most serious diseases of wheat (Triticum aestivum L.) worldwide. To gain a better understanding of the protective mechanism against stripe rust at the adult plant stage, this study investigated the differences in photosystem II (PSII) and antioxidant enzymatic systems between susceptible and resistant wheat in response to stripe rust disease caused by Puccinia striiformis. We found that chlorophyll fluorescence and the activities of the antioxidant enzymes were higher in resistant wheat than in susceptible wheat after stripe rust infection. Compared with the susceptible wheat, the resistant wheat accumulated a higher content of D1 protein and a lower level of reactive oxygen species (ROS) after infection. Further, this is the first study to demonstrate that D1 and LHCII phosphorylation are involved in the resistance to stripe rust in wheat. In addition, we found that CP29 was phosphorylated under stripe rust infection; CP29 is also phosphorylated in monocots under other environmental stresses. Furthermore, the thylakoid structure showed more extensive damage in the susceptible wheat compared with that observed in the resistant wheat. Therefore, these findings provide evidence that thylakoid protein phosphorylation and antioxidant enzyme systems play an important role in plant responses and defense mechanisms in wheat cultivars subject to this type of biotic stress.

To study the effects of application of salicylic acid (SA) on the antioxidant system and photosystem II (PS II) in wheat seedlings we used two different experiments. The first method was carried out by immersing roots in Hoagland’s nutrient solution containing 0, 0.25, or 2.5 mM SA, and the second method was performed by spraying two-week-old seedlings with the same SA concentrations. After 24 h, chlorophyll fluorescence, thylakoid membrane proteins, antioxidant enzyme activities, and reactive oxygen species were measured. The low concentration of SA caused a significant increase in the antioxidant enzyme activities. However, the treatment with 2.5 mM SA resulted in an increase in the non-photochemical quenching coefficient and a decrease in the antioxidant enzyme activities, the quantum yield of photosystem II photochemistry, and the photochemical quenching, especially in the first method of application. All these results indicate that the effects of SA on PS II and the antioxidative defense system were dependent on the concentration used and the method of application.

There are numerous studies on enhancing plant resistance to stress using melatonin, but few studies about its effect on photosynthesis. Herein, we summarized the role of melatonin in photosynthesis. Melatonin regulates chlorophyll synthesis and degradation through the transcription of related genes and hormone signals. It protects photosynthetic proteins and maintains the photosynthetic process through improving the transcription of photosystem genes, activating the antioxidant system, and promoting the xanthophyll cycle. Melatonin potentially regulates plant stomatal movement through CAND2/PMTR1. Finally, it controls the photosynthetic carbon cycle by regulating the metabolism of sugar, the gluconeogenesis pathway, and the degradation and transport of transient starch.