Photosynthetic and biochemical activities in flag leaves of a newly developed superhigh-yield hybrid rice (Oryza sativa) and its parents during the reproductive stage

Nitrogen application enhances yield, yield-attributes, and physiological characteristics of dryland wheat/maize under strip intercropping

Intercropping has been acknowledged as a sustainable practice for enhancing crop productivity and water use efficiency under rainfed conditions. However, the contribution of different planting rows towards crop physiology and yield is elusive. In addition, the influence of nitrogen (N) fertilization on the physiology, yield, and soil water storage of rainfed intercropping systems is poorly understood; therefore, the objective of this experiment was to study the contribution of different crop rows on the physiological, yield, and related traits of wheat/maize relay-strip intercropping (RSI) with and without N application. The treatments comprised of two factors viz. intercropping with three levels (sole wheat, sole maize, and RSI) and two N application rates, with and without N application. Results showed that RSI significantly improved the land use efficiency and grain yield of both crops under rainfed conditions. Intercropping with N application (+N treatment) resulted in the highest wheat grain yield with 70.37 and 52.78% increase as compared with monoculture and without N application in 2019 and 2020, respectively, where border rows contributed the maximum followed by second rows. The increase in grain yield was attributed to higher values of the number of ears per square meter (10-25.33% more in comparison to sole crop without N application) during both study years. The sole wheat crop without any N application recorded the least values for all yield-related parameters. Despite the absence of significant differences, the relative decrease in intercropped maize under both N treatments was over 9% compared to the sole maize crop, which was mainly ascribed to the border rows (24.65% decrease compared to the sole crop) that recorded 12 and 13% decrease in kernel number and thousand-grain weight, respectively than the sole crop. This might be attributed to the reduced photosynthesis and chlorophyll pigmentation in RSI maize crop during the blended growth period. In a nutshell, it can be concluded that wheat/maize RSI significantly improved the land use efficiency and the total yield compared to the sole crops' yield in arid areas in which yield advantages were mainly ascribed to the improvement in wheat yield.

Improved photosynthetic characteristics correlated with enhanced biomass in a heterotic F1 hybrid of maize (Zea mays L.)

Heterosis is a phenomenon wherein F₁ hybrid often displays phenotypic superiority and surpasses its parents in terms of growth and agronomic traits. Investigations on the physiological and biochemical properties of the heterotic F₁ hybrid are important to uncover the mechanisms underlying heterosis in plants. In the present study, the photosynthetic capacity of a heterotic F₁ hybrid of Zea mays L. (DHM 117) that exhibited a higher growth rate and increased biomass was compared with its parental inbreds at vegetative and reproductive stages in the field during 2017 and 2018. The net photosynthetic rate (P_n), stomatal conductance (g_s), transpiration rate (E) as well as foliar carbohydrates were higher in F₁ hybrid than parental inbreds at vegetative and reproductive stages. An increase in total chlorophyll content along with better chlorophyll a fluorescence characteristics including effective quantum yield of photosystem II (ΔF/F_m'), maximum quantum yield of PSII (F_v/F_m), photochemical quenching (q_p) and decreased non-photochemical quenching (NPQ) was observed in F₁ hybrid than the parental inbreds. Further, the expression of potential genes related to C₄ photosynthesis was considerably upregulated in F₁ hybrid than the parental inbreds during vegetative and reproductive stages. Moreover, the F₁ hybrid exhibited distinct heterosis in yield with 63% and 62% increase relative to parental inbreds during 2017 and 2018. We conclude that improved photosynthetic efficiency associated with increased foliar carbohydrates could have contributed to higher growth rate, biomass and yield in the F₁ hybrid.

Leaf growth in early development is key to biomass heterosis in Arabidopsis

Arabidopsis thaliana hybrids have similar properties to hybrid crops, with greater biomass relative to the parents. We asked whether the greater biomass was due to increased photosynthetic efficiency per unit leaf area or to overall increased leaf area and increased total photosynthate per plant. We found that photosynthetic parameters (electron transport rate, CO2 assimilation rate, chlorophyll content, and chloroplast number) were unchanged on a leaf unit area and unit fresh weight basis between parents and hybrids, indicating that heterosis is not a result of increased photosynthetic efficiency. To investigate the possibility of increased leaf area producing more photosynthate per plant, we studied C24×Landsberg erecta (Ler) hybrids in detail. These hybrids have earlier germination and leaf growth than the parents, leading to a larger leaf area at any point in development of the plant. The developing leaves of the hybrids are significantly larger than those of the parents, with consequent greater production of photosynthate and an increased contribution to heterosis. The set of leaves contributing to heterosis changes as the plant develops; the four most recently emerged leaves make the greatest contribution. As a leaf matures, its contribution to heterosis attenuates. While photosynthesis per unit leaf area is unchanged at any stage of development in the hybrid, leaf area is greater and the amount of photosynthate per plant is increased.

Enhancing xanthine dehydrogenase activity is an effective way to delay leaf senescence and increase rice yield

Xanthine dehydrogenase (XDH) is an important enzyme in purine metabolism. It is involved in regulation of the normal growth and non-biological stress-induced ageing processes in plants. The present study investigated XDH's role in regulating rice leaf senescence. We measured physical characteristics, chlorophyll content and fluorescence parameters, active oxygen metabolism, and purine metabolism in wild-type Kitaake rice (Oryza sativa L.), an OsXDH over-expression transgenic line (OE9), and an OsXDH RNA interference line (Ri3) during different growth stages. The expression patterns of the OsXDH gene confirmed that XDH was involved in the regulation of normal and abiotic stress-induced ageing processes in rice. There was no significant difference between the phenotypes of transgenic lines and wild type at the seedling stage, but differences were observed at the full heading and maturation stages. The OE9 plants were taller, with higher chlorophyll content, and their photosystems had stronger light energy absorption, transmission, dissipation, and distribution capacity, which ultimately improved the seed setting rate and 1000-seed weight. The opposite effect was found in the Ri3 plants. The OE9 line had a strong ability to remove reactive oxygen species, with increased accumulation of allantoin and alantoate. Experimental spraying of allantoin on leaves showed that it could alleviate chlorophyll degradation and decrease the content of H₂O₂ and malonaldehyde (MDA) in rice leaves after the full heading stage. The urate oxidase gene (UO) expression levels in the interference line were significantly lower than those in the over-expression line and wild-type lines. The allantoinase (ALN) and allantoate amidinohydrolase (AAH) genes had significantly higher expression in the Ri3 plants than the in OE9 or wild-type plants, with OE9 plants showing the lowest levels. The senescence-related genes ACD1, WRKY23, WRKY53, SGR, XERO1, and GH27 in Ri3 plants had the highest expression levels, followed by those in the wild-type plants, with OE9 plants showing the lowest levels. These results suggest that enhanced activity of XDH can regulate the synthesis of urea-related substances, improve plant antioxidant capacity, effectively delay the ageing process in rice leaves, and increase rice yield.

Senescence and Defense Pathways Contribute to Heterosis

Hybrids are used extensively in agriculture due to their superior performance in seed yield and plant growth, yet the molecular mechanisms underpinning hybrid performance are not well understood. Recent evidence has suggested that a decrease in basal defense response gene expression regulated by reduced levels of salicylic acid (SA) may be important for vigor in certain hybrid combinations. Decreasing levels of SA in the Arabidopsis (Arabidopsis thaliana) accession C24 through the introduction of the SA catabolic enzyme salicylate1 hydroxylase (NahG) increases plant size, phenocopying the large-sized C24/Landsberg erecta (Ler) F1 hybrids. C24♀ × Ler♂ F1 hybrids and C24 NahG lines shared differentially expressed genes and pathways associated with plant defense and leaf senescence including decreased expression of SA biosynthetic genes and SA response genes. The expression of TL1 BINDING TRANSCRIPTION FACTOR1, a key regulator in resource allocation between growth and defense, was decreased in both the F1 hybrid and the C24 NahG lines, which may promote growth. Both C24 NahG lines and the F1 hybrids showed decreased expression of the key senescence-associated transcription factors WRKY53, NAC-CONTAINING PROTEIN29, and ORESARA1 with a delayed onset of senescence compared to C24 plants. The delay in senescence resulted in an extension of the photosynthetic period in the leaves of F1 hybrids compared to the parental lines, potentially allowing each leaf to contribute more resources toward growth.

Epigenetic regulation of agronomical traits in Brassicaceae

Epigenetic regulation, covalent modification of DNA and changes in histone proteins are closely linked to plant development and stress response through flexibly altering the chromatin structure to regulate gene expression. In this review, we will illustrate the importance of epigenetic influences by discussing three agriculturally important traits of Brassicaceae. (1) Vernalization, an acceleration of flowering by prolonged cold exposure regulated through epigenetic silencing of a central floral repressor, FLOWERING LOCUS C. This is associated with cold-dependent repressive histone mark accumulation, which confers competency of consequence vegetative-to-reproductive phase transition. (2) Hybrid vigor, in which an F₁ hybrid shows superior performance to the parental lines. Combination of distinct epigenomes with different DNA methylation states between parental lines is important for increase in growth rate in a hybrid progeny. This is independent of siRNA-directed DNA methylation but dependent on the chromatin remodeler DDM1. (3) Self-incompatibility, a reproductive mating system to prevent self-fertilization. This is controlled by the S-locus consisting of SP11 and SRK which are responsible for self/non-self recognition. Because self-incompatibility in Brassicaceae is sporophytically controlled, there are dominance relationships between S haplotypes in the stigma and pollen. The dominance relationships in the pollen rely on de novo DNA methylation at the promoter region of a recessive allele, which is triggered by siRNA production from a flanking region of a dominant allele.

Growth Properties and Biomass Production in the Hybrid C4 Crop Sorghum bicolor

Hybrid vigor (heterosis) has been used as a breeding technique for crop improvement to achieve enhanced biomass production, but the physiological mechanisms underlying heterosis remain poorly understood. In this study, to find a clue to the enhancement of biomass production by heterosis, we systemically evaluated the effect of heterosis on the growth rate and photosynthetic efficiency in sorghum hybrid [Sorghum bicolor (L.) Moench cv. Tentaka] and its parental lines (restorer line and maintainer line). The final biomass of Tentaka was 10-14 times greater than that of the parental lines grown in an experimental field, but the relative growth rate during the vegetative growth stage did not differ. Tentaka exhibited a relatively enlarged leaf area with lower leaf nitrogen content per leaf area (Narea). When the plants were grown hydroponically at different N levels, daily CO2 assimilation per leaf area (A) increased with Narea, and the ratio of A to Narea (N-use efficiency) was higher in the plants grown at low N levels but not different between Tentaka and the parental lines. The relationships between the CO2 assimilation rate, the amounts of photosynthetic enzymes, including ribulose-1,5-bisphosphate carboxylase/oxygenase, phosphoenolpyruvate carboxylase and pyruvate phosphate dikinase, Chl and Narea did not differ between Tentaka and the parental lines. Thus, Tentaka tended to exhibit enlargement of leaf area with lower N content, leading to a higher N-use efficiency for CO2 assimilation, but the photosynthetic properties did not differ. The greater biomass in Tentaka was mainly due to the prolonged vegetative growth period.

Photosynthesis performance, antioxidant enzymes, and ultrastructural analyses of rice seedlings under chromium stress

The present study was conducted to examine the effects of increasing concentrations of chromium (Cr(6+)) (0, 25, 50, 100, and 200 μmol) on rice (Oryza sativa L.) morphological traits, photosynthesis performance, and the activities of antioxidative enzymes. In addition, the ultrastructure of chloroplasts in the leaves of hydroponically cultivated rice (O. sativa L.) seedlings was analyzed. Plant fresh and dry weights, height, root length, and photosynthetic pigments were decreased by Cr-induced toxicity (200 μM), and the growth of rice seedlings was starkly inhibited compared with that of the control. In addition, the decreased maximum quantum yield of primary photochemistry (Fv/Fm) might be ascribed to the decreased the number of active photosystem II reaction centers. These results were confirmed by inhibited photophosphorylation, reduced ATP content and its coupling factor Ca(2+)-ATPase, and decreased Mg(2+)-ATPase activities. Furthermore, overtly increased activities of antioxidative enzymes were observed under Cr(6+) toxicity. Malondialdehyde and the generation rates of superoxide (O2̄) also increased with Cr(6+) concentration, while hydrogen peroxide content first increased at a low Cr(6+) concentration of 25 μM and then decreased. Moreover, transmission electron microscopy showed that Cr(6+) exposure resulted in significant chloroplast damage. Taken together, these findings indicate that high Cr(6+)concentrations stimulate the production of toxic reactive oxygen species and promote lipid peroxidation in plants, causing severe damage to cell membranes, degradation of photosynthetic pigments, and inhibition of photosynthesis.

Genome-wide analysis of DNA methylation in photoperiod- and thermo-sensitive male sterile rice Peiai 64S

Background

Epigenetic modifications play important roles in the regulation of plant development. DNA methylation is an important epigenetic modification that dynamically regulates gene expression during developmental processes. However, little studies have been reported about the methylation profiles of photoperiod- and thermo-sensitive genic male sterile (PTGMS) rice during the fertility transition.

Results

In this study, using methylated DNA immunoprecipitation sequencing (MeDIP-seq), the global DNA methylation patterns were compared in the rice PTGMS line PA64S under two different environments (different temperatures and day lengths). The profiling of the DNA methylation under two different phenotypes (sterility and fertility) revealed that hypermethylation was observed in PA64S (sterility), and 1258 differentially methylated regions (DMRs) were found between PA64S (sterility) and PA64S (fertility). Twenty differentially methylated genes of them were further validated through bisulfite sequencing, and four of these genes were analyzed by qRT-PCR. Especially, a differentially methylated gene (LOC_Os08g38210), which encoded transcription factor BIM2, is a component of brassinosteroid signaling in rice. The hypermethylated BIM2 gene may suppress some downstream genes in brassinosteroid signaling pathway, and thus affect the male fertility in PA64S.

Conclusions

The results presented here indicated that hypermethylation was observed in PA64S (sterility). Gene Ontology (GO) analysis and KEGG analysis revealed that flavone and flavonol biosynthrsis, circadian rhythm, photosynthesis and oxidative phosphorylation pathways were involved in sterility-fertility transition of PA64S.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1317-7) contains supplementary material, which is available to authorized users.

Photosynthetic changes of flag leaves during senescence stage in super high-yield hybrid rice LYPJ grown in field condition

Photosynthetic activities and thylakoid membrane protein patterns as well as the ultrastructure of chloroplasts in flag leaves were investigated during the senescence processes in high-yield hybrid rice LYPJ under field condition. The earlier decrease of PS I activity than PS II in LYPJ was primarily due to the significant degradation of PS I chlorophyll-protein complex. The degradation rate for each chlorophyll-protein complex was different and the order for the stability of thylakoid membrane complexes during flag leaf senescence in rice LYPJ was: LHCII > OEC > PSII core antenna > PSII core > PSI core > LHCI, which was partly supported by the BN-PAGE gel combined with immunoblot analysis. A decrease in the chlorophyll a/b ratio at the senescence stage was observed to coincide with stability of the LHCII subunits. Ultrastructural investigations revealed that the chloroplasts have large loosen stacking grana without interconnecting stroma thylakoids during the senescence processes. It was hypothesized that the stability of grana thylakoids harboring the major LHCII under high radiation condition in summer might played a key role in the dissipation of excess light energy. This alternative strategy would protect photosynthetic apparatus from photodamage and might be causally related to the high yield of this rice cultivar.

Can we learn from heterosis and epigenetics to improve photosynthesis?

Heterosis is the increase in fitness and yield of F1 hybrids derived from a cross between distantly related genotypes. The use of heterosis is one of the most successful crop breeding strategies, but the underlying molecular mechanisms are still poorly defined. There is ample evidence that heterosis is associated with increased rates of photosynthesis and recent analyses have shed light on the underlying biochemical principles. In parallel, the importance of epigenetic chromatin modifications in heterosis has now been established. The first direct links between epigenetic changes and improved photosynthesis have also been demonstrated. As epigenetic engineering is now possible, we discuss the feasibility of altering the epigenetic code to enhance photosynthesis.

The xanthophyll cycle and antioxidative defense system are enhanced in the wheat hybrid subjected to high light stress

Although the wheat hybrids have often shown higher grain yields, the physiological basis of the higher yields remains unknown. Previous studies suggest that tolerance to photoinhibition in the hybrid may be one of the physiological bases (Yang et al., 2006, Plant Sci 171:389-97). The objective of this study was to further investigate the possible mechanism responsible for tolerance to photoinhibition in the hybrid. Photosystem II (PSII) photochemistry, the xanthophyll cycle, and antioxidative defense system were compared between the hybrid and its parents subjected to high light stress (1500μmolm(-2)s(-1)). The analyses of oxygen-evolving activity, chlorophyll fluorescence, and protein blotting demonstrated that the higher tolerance in the hybrid than in its parents was associated with its higher tolerance of PSII to photoinhibition. High light induced an increase in non-photochemical quenching, and this increase was greater in the hybrid than in its parents. There were no differences in the pool size of the xanthophyll cycle between the hybrid and its parents. The content of violaxanthin decreased significantly, whereas the content of zeaxanthin+antherxanthin increased considerably during high light treatments. However, the decrease in violaxanthin content and the increase in zeaxanthin+antherxanthin content were greater in the hybrid than in its parents. High light resulted in a significant accumulation of H(2)O(2), O(2)(-) and catalytic Fe, and this accumulation was less in the hybrid than in its parents. High light induced a significant increase in the activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase, and monodehydroascorbate reductase, and these increases were greater in the hybrid than its parents. These results suggest that the higher tolerance to photoinhibition in the hybrid may be associated with its higher capacity for antioxidative defense metabolism and the xanthophyll cycle.

Comparative transcriptional profiling and preliminary study on heterosis mechanism of super-hybrid rice

Heterosis is a biological phenomenon whereby the offspring from two parents show improved and superior performance than either inbred parental lines. Hybrid rice is one of the most successful apotheoses in crops utilizing heterosis. Transcriptional profiling of F(1) super-hybrid rice Liangyou-2186 and its parents by serial analysis of gene expression (SAGE) revealed 1183 differentially expressed genes (DGs), among which DGs were found significantly enriched in pathways such as photosynthesis and carbon-fixation, and most of the key genes involved in the carbon-fixation pathway exhibited up-regulated expression in F(1) hybrid rice. Moreover, increased catabolic activity of corresponding enzymes and photosynthetic efficiency were also detected, which combined to indicate that carbon fixation is enhanced in F(1) hybrid, and might probably be associated with the yield vigor and heterosis in super-hybrid rice. By correlating DGs with yield-related quantitative trait loci (QTL), a potential relationship between differential gene expression and phenotypic changes was also found. In addition, a regulatory network involving circadian-rhythms and light signaling pathways was also found, as previously reported in Arabidopsis, which suggest that such a network might also be related with heterosis in hybrid rice. Altogether, the present study provides another view for understanding the molecular mechanism underlying heterosis in rice.

Effects of salt and waterlogging stresses and their combination on leaf photosynthesis, chloroplast ATP synthesis, and antioxidant capacity in wheat

The objective of this study was to investigate the effects of salt (ST) and waterlogging (WL) stresses and their combination (SW) on leaf photosynthesis, chloroplast ATP synthesis, and antioxidant capacity in wheat (Triticum aestivum L.). Two winter wheat cultivars, Huaimai 17 and Yangmai 12, differing in their tolerance to ST and WL stresses were used. The plants were grown in pots and were subjected to ST, WL, and SW from 7 days after anthesis (DAA). The WL and SW treatments lasted for 5 days, while the ST treatment was continuously imposed during the grain filling stage. Significant decrease in net photosynthetic rate (PN) of the flag leaf was observed under the ST and SW treatments from 10 DAA in Yangmai 12 and at 18 DAA in both cultivars, which could be stomatal closure related. At 18 DAA, clear reduction in PN under the ST and SW treatments was observed, which was associated with chlorosis, damages to the photosystem II (PSII), enhanced lipid peroxidation, and depressed ATP synthesis in the chloroplasts of the flag leaf. Whereas, WL treatment alone had slightly negative effect on PN, which was mainly attributed to leaf chlorosis and waste in harvested energy by the PSII reaction center dispersed via non-photochemical approaches.