Very high sensitivity of African rice to artificial ultraviolet-B radiation caused by genotype and quantity of cyclobutane pyrimidine dimer photolyase

Poaceae plants transfer cyclobutane pyrimidine dimer photolyase to chloroplasts for ultraviolet-B resistance

Photoreactivation enzyme that repairs cyclobutane pyrimidine dimer (CPD) induced by ultraviolet-B radiation, commonly called CPD photolyase (PHR) is essential for plants living under sunlight. Rice (Oryza sativa) PHR (OsPHR) is a unique triple-targeting protein. The signal sequences required for its translocation to the nucleus or mitochondria are located in the C-terminal region but have yet to be identified for chloroplasts. Here, we identified sequences located in the N-terminal region, including the serine-phosphorylation site at position 7 of OsPHR, and found that OsPHR is transported/localized to chloroplasts via a vesicle transport system under the control of serine-phosphorylation. However, the sequence identified in this study is only conserved in some Poaceae species, and in many other plants, PHR is not localized to the chloroplasts. Therefore, we reasoned that Poaceae species need the ability to repair CPD in the chloroplast genome to survive under sunlight and have uniquely acquired this mechanism for PHR chloroplast translocation.

The recent relationship between ultraviolet-B radiation and biotic resistance in plants: a novel non-chemical strategy for managing biotic stresses

Ultraviolet-B radiation (UVB; 280-315 nm) is a significant environmental factor that alters plant development, changes interactions between species, and reduces the prevalence of pests and diseases. While UVB radiation has negative effects on plant growth and performance at higher doses, at lower and ambient doses, UVB radiation acts as a non-chemical method for managing biotic stresses by having positive effects on disease resistance and genes that protect plants from pests. Understanding the recent relationship between UVB radiation and plants' biotic stresses is crucial for the development of crops that are resistant to UVB and biotic stresses. However, little is known about the recent interactions between UVB radiation and biotic stresses in plants. This review discusses the most recent connections between UVB radiation and biotic stresses in crops, including how UVB radiation affects a plant's resistance to disease and pests. The interaction of UVB radiation with pathogens and herbivores has been the subject of the most extensive research of these. This review also discusses additional potential strategies for conferring multiple UVB-biotic stress resistance in crop plants, such as controlling growth inhibition, miRNA 396 and 398 modulations, and MAP kinase. This study provides crucial knowledge and methods for scientists looking to develop multiple resistant crops that will improve global food security.

High ultraviolet-B sensitivity due to lower CPD photolyase activity is needed for biotic stress response to the rice blast fungus, Magnaporthe oryzae

Sensitivity to ultraviolet-B (UVB, 280-315 nm) radiation varies widely among rice (Oryza sativa) cultivars due to differences in the activity of cyclobutane pyrimidines dimer (CPD) photolyase. Interestingly, cultivars with high UVB sensitivity and low CPD photolyase activity have been domesticated in tropical areas with high UVB radiation. Here, we investigated how differences in CPD photolyase activity affect plant resistance to the rice blast fungus, Magnaporthe oryzae, which is one of the other major stresses. We used Asian and African rice cultivars and transgenic lines with different CPD photolyase activities to evaluate the interaction effects of CPD photolyase activity on resistance to M. oryzae. In UVB-resistant rice plants overexpressing CPD photolyase, 12 h of low-dose UVB (0.4 W m^-2) pretreatment enhanced sensitivity to M. oryzae. In contrast, UVB-sensitive rice (transgenic rice with antisense CPD photolyase, A-S; and rice cultivars with low CPD photolyase activity) showed resistance to M. oryzae. Several defense-related genes were upregulated in UVB-sensitive rice compared to UVB-resistant rice. UVB-pretreated A-S plants showed decreased multicellular infection and robust accumulation of reactive oxygen species. High UVB-induced CPD accumulation promoted defense responses and cross-protection mechanisms against rice blast disease. This may indicate a trade-off between high UVB sensitivity and biotic stress tolerance in tropical rice cultivars.

β-Sitosterol differentially regulates key metabolites for growth improvement and stress tolerance in rice plants during prolonged UV-B stress

BACKGROUND

Elevated ultraviolet-B (UV-B) radiation is potentially deleterious to many organisms specifically crop plants and has become a global challenge. Rice is an exceptionally important staple food which is grown worldwide, and many efforts have been done recently to improve rice varieties against UV-B stress. This current study aims to investigate the effects of exogenous application of β-sitosterol (βSito) on growth improvement and tolerance level of rice plants against prolonged UV-B stress. The physiological and metabolic responses were evaluated in rice plants not supplemented with βSito (Nβ) and those supplemented with βSito (Sβ).

RESULTS

The Nβ and Sβ plants were grown under non-stress (ns) and under prolonged UV-B stress (uvs) conditions and termed as Nβ^ns, Sβ^ns and Nβ^uvs, Sβ^uvs, respectively. The application of βSito contributes positively under non-stress and specifically to UV-B stress in terms of improving numerous physiological parameters associated with growth and development such as shoot and root length, RWC, whole plant biomass, chlorophyll pigments, and photosynthetic-related parameters (Pn, Gs, Tr, WUEi, Fv/Fm, and NPQ) in Sβ compared with Nβ plants. Moreover, enhanced oxidative stress tolerance of Sβ^uvs vs. Nβ^uvs plants under stress was attributed to low levels of ROS and substantial trigger in activities of antioxidant enzymes (SOD, POD, CAT, and APX). Metabolic analysis was performed using GC-TOFMS, which revealed higher accumulation of several key metabolites including organic acids, sugars, amino acids, and others in Sβ^uvs vs. Nβ^uvs plants, which were mainly reduced in Nβ plants under stress vs. non-stress conditions.

CONCLUSION

These results provide useful data regarding the important role of βSito on growth maintenance and modulation of several metabolites associated with osmotic and redox adjustments during UV-B stress tolerance in rice plants. Importantly, βSito-regulated plasticity could further be explored specifically in relation to different environmental stresses in other economically useful crop plants.

UV-B signalling in rice: Response identification, gene expression profiling and mutant isolation

Responses of rice seedlings to UV-B radiation (UV-B) were investigated, aiming to establish rice as a model plant for UV-B signalling studies. The growth of japonica rice coleoptiles, grown under red light, was inhibited by brief irradiation with UV-B, but not with blue light. The effective UV-B fluences (10^-1 -10³ μmol m^-2 ) were much lower than those reported in Arabidopsis. The response was much less in indica rice cultivars and its extent varied among Oryza species. We next identified UV-B-specific anthocyanin accumulation in the first leaf of purple rice and used this visible phenotype to isolate mutants. Some isolated mutants were further characterized, and one was found to have a defect in the growth response. Using microarrays, we identified a number of genes that are regulated by low-fluence-rate UV-B in japonica coleoptiles. Some up-regulated genes were analysed by real-time PCR for UV-B specificity and the difference between japonica and indica. More than 70% of UV-B-regulated rice genes had no homologs in UV-B-regulated Arabidopsis genes. Many UV-B-regulated rice genes are related to plant hormones and especially to jasmonate biosynthetic and responsive genes in apparent agreement with the growth response. Possible involvement of two rice homologs of UVR8, a UV-B photoreceptor, is discussed.

Transgenic rice Oryza glaberrima with higher CPD photolyase activity alleviates UVB-caused growth inhibition

The ultraviolet B (UVB) sensitivity of rice cultivated in Asia and Africa varies greatly, with African rice cultivars (Oryza glaberrima Steud. and O. barthii A. Chev.) being more sensitive to UVB because of their low cyclobutane pyrimidine dimer (CPD) photolyase activity, which is a CPD repair enzyme, relative to Asian rice cultivars (O. sativa L.). Hence, the production of UVB-resistant African rice with augmented CPD photolyase activity is of great importance, although difficulty in transforming the African rice cultivars to this end has been reported. Here, we successfully produced overexpressing transgenic African rice with higher CPD photolyase activity by modifying media conditions for callus induction and regeneration using the parental line (PL), UVB-sensitive African rice TOG12380 (O. glaberrima). The overexpressing transgenic African rice carried a single copy of the CPD photolyase enzyme, with a 4.4-fold higher level of CPD photolyase transcripts and 2.6-fold higher activity than its PL counterpart. When the plants were grown for 21 days in a growth chamber under visible radiation or with supplementary various UVB radiation, the overexpressing transgenic plants have a significantly increased UVB resistance index compared to PL plants. These results strongly suggest that CPD photolyase remains an essential factor for tolerating UVB radiation stress in African rice. As a result, African rice cultivars with overexpressed CPD photolyase may survive better in tropical areas more prone to UVB radiation stress, including Africa. Collectively, our results provide strong evidence that CPD photolyase is a useful biotechnological tool for reducing UVB-induced growth inhibition in African rice crops of O. glaberrima.