Genetic basis of grain yield heterosis in an “immortalized F2” maize population

Key message

Genetic basis of grain yield heterosis relies on the cumulative effects of dominance, overdominance, and epistasis in maize hybrid Yuyu22.

Abstract

Heterosis, i.e., when F₁ hybrid phenotypes are superior to those of the parents, continues to play a critical role in boosting global grain yield. Notwithstanding our limited insight into the genetic and molecular basis of heterosis, it has been exploited extensively using different breeding approaches. In this study, we investigated the genetic underpinnings of grain yield and its components using “immortalized F₂” and recombinant inbred line populations derived from the elite hybrid Yuyu22. A high-density linkage map consisting of 3,184 bins was used to assess (1) the additive and additive-by-additive effects determined using recombinant inbred lines; (2) the dominance and dominance-by-dominance effects from a mid-parent heterosis dataset; and (3) the various genetic effects in the “immortalized F₂” population. Compared with a low-density simple sequence repeat map, the bin map identified more quantitative trait loci, with higher LOD scores and better accuracy of detecting quantitative trait loci. The bin map showed that, among all traits, dominance was more important to heterosis than other genetic effects. The importance of overdominance/pseudo-overdominance was proportional to the amount of heterosis. In addition, epistasis contributed to heterosis as well. Phenotypic variances explained by the QTLs detected were close to the broad-sense heritabilities of the observed traits. Comparison of the analyzed results obtained for the “immortalized F₂” population with those for the mid-parent heterosis dataset indicated identical genetic modes of action for mid-parent heterosis and grain yield performance of the hybrid.     

Extent of heterosis in rice (Oryza sativa L.) under cold stress conditions — yield and its components

Summary The nature and extent of heterosis for grain yield and its component characters was estimated in a seven-parent F₁-diallel cross in rice under the cold stress conditions of Palampur, representing sub-temperate climate of the mid hills of Himachal Pradesh. The highest heterotic effects for grain yield were observed in Himalaya 1/Phul Patas 72 and China 988/Himdhan hybrids, which gave 60.36 and 32.48% heterosis, respectively, over mid parent, 38.76 and 26.86% heterosis, respectively, over high parent and 32.30 and 26.86% heterosis, respectively, over the best variety, Himdhan. Heterosis for grain yield in these crosses was due to an increase in tiller number, panicle length, spikelets/panicle and 1,000-grain weight. Large number of crosses exhibited significant heterosis for high spikelet sterility and the majority of them had significant heterosis for late flowering, taking more than 120 days to flower, which resulted in lack of heterosis for grain yield in such crosses due to cold stress at the reproductive stage.Part of the thesis submitted by the senior author to Himachal Pradesh Agricultural University, Palampur (H.P.) India, in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Agriculture     

Natural genetic variation in photosynthesis: an untapped resource to increase crop yield potential?

Raising crop yield potential is a major goal to ensure food security for the growing global population. Photosynthesis is the primary determinant of crop productivity and any gain in photosynthetic CO₂ assimilation per unit of leaf area (A) has the potential to increase yield. Significant intraspecific variation in A is known to exist in various autotrophic organs that represent an unexploited target for crop improvement. However, the large number of factors that influence photosynthetic rates often makes it difficult to measure or estimate A under dynamic field conditions (i.e. fluctuating light intensities or temperatures). This complexity often results in photosynthetic capacity, rather than realized photosynthetic rates being used to assess natural variation in photosynthesis. Here we review the work on natural variation in A, the different factors determining A and their interaction in yield formation. A series of drawbacks and perspectives are presented for the most common analyses generally used to estimate A. The different yield components and their determination based on different photosynthetic organs are discussed with a major focus on potential exploitation of various traits for crop improvement. To conclude, an example of different possibilities to increase yield in wheat through enhancing A is illustrated.     

Comparative mapping in F2∶3 and F6∶7 generations of quantitative trait loci for grain yield and yield components in maize

This study was conducted to compare maize quantitative trait loci (QTL) detection for grain yield and yield components in F₂₃ and F₆₇ recombinant inbred (RI) lines from the same population. One hundred and eighty-six F₆₇ RIs from a Mo17×H99 population were grown in a replicated field experiment and analyzed at 101 loci detected by restriction fragment length polymorphisms (RFLPs). Single-factor analysis of variance was conducted for each locus-trait combination to identify QTL. For grain yield, 6 QTL were detected accounting for 22% of the phenotypic variation. A total of 63 QTL were identified for the seven grain yield components with alleles from both parents contributing to increased trait values. Several genetic regions were associated with more than one trait, indicating possible linked and/or pleiotropic effects. In a comparison with 150 F₂₃ lines from the same population, the same genetic regions and parental effects were detected across generations despite being evaluated under diverse environmental conditions. Some of the QTL detected in the F₂₃ seem to be dissected into multiple, linked QTL in the F₆₇ generation, indicating better genetic resolution for QTL detection with RIs. Also, genetic effects at QTL are smaller in the F₆₇ generation for all traits.Abbreviations RFLPs Restriction fragment length polymorphisms - QTL quantitative trait loci - RIs recombinant inbreds Journal Paper no. J-16261 of the Iowa Agric and Home Economics Exp Stn Project no. 3134     

Transgenic maize phosphoenolpyruvate carboxylase alters leaf–atmosphere CO2 and 13CO2 exchanges in Oryza sativa

Photosynthesis Research - The engineering process of C4 photosynthesis into C3 plants requires an increased activity of phosphoenolpyruvate carboxylase (PEPC) in the cytosol of leaf mesophyll...     

Do increases in agricultural yield spare land for nature?

Feeding a rapidly expanding human population will require a large increase in the supply of agricultural products during the coming decades. This may lead to the transformation of many landscapes from natural vegetation cover to agricultural land use, unless increases in crop yields reduce the need for new farmland. Here, we assess the evidence that past increases in agricultural yield have spared land for wild nature. We investigated the relationship between the change in the combined energy yield of the 23 most energetically important food crops over the period 1979–1999 and the change in per capita cropland area for 124 countries over the same period. Per capita area of the 23 staple crops tended to decrease in developing countries where large yield increases occurred. However, this was counteracted by a tendency for the area used to grow crops other than staples to increase in the countries where staple crop yields increased. There remained a weak tendency in developing countries for the per capita area of all cropland to decline as staple crop yield increased, a pattern that was most evident in developing countries with the highest per capita food supplies. In developed countries, there was no evidence that higher staple crop yields were associated with decreases in per capita cropland area. This may be because high agricultural subsidies in developed countries override any land-sparing pattern that might otherwise occur. Declines in the area of natural forest were smaller in countries where the yield of staple crops increased most, when the negative effects of human population increases on forest area were controlled for. Our results show that land-sparing is a weak process that occurs under a limited set of circumstances, but that it can have positive outcomes for the conservation of wild nature.     

A removable spacer peptide in an α-factor-leader/insulin precursor fusion protein improves processing and concomitant yield of the insulin precursor in Saccharomyces cerevisiae

An α-factor leader/insulin precursor fusion protein was produced in Saccharomyces cerevisiae and metabolically labeled in order to analyse the efficiency of maturation and secretion. A substantial fraction of the secreted material was found in a hyperglycosylated unprocessed form, indicating incomplete Kex2p endopeptidase maturation. Introduction of a spacer peptide (EAEAEAK) after the dibasic Kex2p site, creating a N-terminal extension of the insulin precursor, greatly increased the Kex2p catalytic efficiency and the fermentation yield of insulin precursor. The N-terminal extension features a Lys to allow subsequent proteolytic removal by trypsin or the Achromobacter lyticus Lys-specific protease. Dipeptidyl aminopeptidase A (DPAPA) activity removing Glu-Ala dipeptides from the extension was inhibited by adding a Glu N-terminally to the extension. Unexpectedly, this modified N-terminal extension (EEAEAEAK) was partially cleaved after the Lys during fermentation. This monobasic proteolytic activity was demonstrated to be associated with Yap3p. Yap3p cleavage could be prevented by insertion of a Pro before the Lys (EEAEAEAPK)     

The rise in chlorophyll a fluorescence yield and decay in delayed light emission in Tris-washed chloroplasts in the 6–100 μs time range after an excitation flash

Parallel measurements of the rise in chlorophyll a fluorescence yield and delayed light emission decay, after a 10 ns saturating excitation flash, have been made in tris(hydroxymethyl)aminomethane-washed chloroplasts. Various electron donor systems (Mn²⁺; ascorbate; reduced phenylenediamine and benzidine) were used in conjuction with different preillumination regimes to alter [P⁺-680], the oxidized form of the Photosystem II reaction center chlorophyll a. Conditions giving rise to high [P⁺-680] resulted in only a small rise in fluorescence yield, an inhibition of a 6 μs delayed light component, and an enhancement of a 60 μs component of delayed light emission. These results confirm the hypothesis that P⁺-680 acts as a quencher of fluorescence and that delayed light emission in the microsecond time range is due to the back reaction of P⁺-680 and Q^?. (Q is the first “stable” electron acceptor of Photosystem II.) Two preillumination flashes are required before the full effect of Tris washing is observed in the delayed light emission decay and fluorescence yield rise; this suggests that a capacity to hold two charges exists between the Tris block and P⁺-680. Tris washing has no direct effect on the movement of electrons from Z (the first electron donor to P⁺-680) to P⁺-680. Finally, Mn²⁺ donates electrons to P⁺-680 via Z.     

Casein Kinases I and 2α Phosphorylate Oryza Sativa Pseudo-Response Regulator 37 (OsPRR37) in Photoperiodic Flowering in Rice

Flowering time (or heading date) is controlled by intrinsic genetic programs in response to environmental cues, such as photoperiod and temperature. Rice, a facultative short-day (SD) plant, flowers early in SD and late in long-day (LD) conditions. Casein kinases (CKs) generally act as positive regulators in many signaling pathways in plants. In rice, Heading date 6 (Hd6) and Hd16 encode CK2α and CKI, respectively, and mainly function to delay flowering time. Additionally, the major LD-dependent floral repressors Hd2/Oryza sativa Pseudo-Response Regulator 37 (OsPRR37; hereafter PRR37) and Ghd7 also confer strong photoperiod sensitivity. In floral induction, Hd16 acts upstream of Ghd7 and CKI interacts with and phosphorylates Ghd7. In addition, Hd6 and Hd16 also act upstream of Hd2. However, whether CKI and CK2α directly regulate the function of PRR37 remains unclear. Here, we use in vitro pull-down and in vivo bimolecular fluorescence complementation assays to show that CKI and CK2α interact with PRR37. We further use in vitro kinase assays to show that CKI and CK2α phosphorylate different regions of PRR37. Our results indicate that direct posttranslational modification of PRR37 mediates the genetic interactions between these two protein kinases and PRR37. The significance of CK-mediated phosphorylation for PRR37 and Ghd7 function is discussed.     

D-ribose,an overlooked player in type 2 diabetes mellitus?

<正>Type 2 diabetes mellitus is a metabolic disorder that is characterized by high blood glucose due to either insulin resistance or insulin deficiency[1].A direct correlation between D-glucose and diabetic complications has long been established,and is the focus of most research in this field.In contrast,D-Ribose has been overlooked so far as a potential risk player in the development of diabetes.     

Can genetic engineering of lignin deposition be accomplished without an unacceptable yield penalty?

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Highlights► Manipulation of the phenylpropanoid polymer lignin can improve bioenergy crops but often leads to dwarfing. ► Decreasing lignin leads to vascular collapse, but it is unclear that this is the cause of dwarfing. ► Non-lignin phenylpropanoids may be essential for normal plant growth and development. ► Accumulation of lignin biosynthetic intermediates or byproducts may have toxic effects. ► Dwarfism may be mediated by an active cell wall integrity monitoring pathway.     

Genotypic variability in physiological,biomass and yield response to drought stress?in pigeonpea

Three pigeonpea (Cajanus cajan L. Millsp.) genotypes- GT-1, AKP-1 and PRG-158 with varying crop duration, growth habit and flowering pattern were evaluated for variability in their response for drought stress. Drought stress was imposed at initiation of flowering and the observations on biomass and seed yield parameters were recorded at harvest. The magnitude of response of individual component to drought stress was found to be genotype specific. Drought stress significantly decreased photosynthetic rate (P_N), transpiration rate (Tr) and relative water content (RWC) in all the genotypes, however the magnitude of reduction differed with genotype. With drought stress, the reduction of P_N was highest in GT-1 while reduction in Tr was highest in PRG-158. The genotype AKP-1, accumulated significantly higher concentrations of osmotic solutes especially proline under water deficit stress, this facilitated it to maintain higher relative water content (RWC) and lower malondialdehyde (MDA) content as compared to other genotypes. Drought stress also impacted biomass production and their partitioning to vegetative and reproductive components at harvest. There was significant variability between the genotypes for seed yield under drought stress while it was non-significant under well-watered condition. Drought stress enhanced flower drop and decreased flower to pod conversion resulting in reduced pod number and seed number in PRG-158 and GT-1. The genotype AKP-1 recorded superior performance for seed yield under stress environment due to its ability in maintaining pod and seed number as well as improved test weight (100 seed weight). Under drought stress, significant positive association of seed yield with proline, seed number, pod number and test weight clearly indicating their role in drought tolerance.     

A first interspecific Oryza sativa×Oryza glaberrima microsatellite-based genetic linkage map

Oryza glaberrima is an endemic African cultivated rice species. To provide a tool for evaluation and utilisation of the potential of O. glaberrima in rice breeding, we developed an interspecific O. glaberrima×Oryza sativa genetic linkage map. It was based on PCR markers, essentially microsatellites and STSs. Segregation of markers was examined in a backcross (O. sativa/O. glaberrima//O. sativa) population. Several traits were measured on the BC₁ plants, and major genes and QTLs were mapped for these traits. Several of these genes correspond well to previously identified loci. The overall map length was comparable to those observed in indica×japonica crosses, indicating that recombination between the two species occurs without limitation. However, three chromosomes show discrepancies with the indica×japonica maps. The colinearity with intraspecific maps was very good, confirming previous cytological observations. A strong segregation-distortion hot spot was observed on chromosome 6 near the waxy gene, indicating the presence of s ₁₀ , a sporo-gametophytic sterility gene previously identified by Sano (1990). The main interests of such a PCR-based map for African rice breeding are discussed, including gene and QTL localisation, marker-assisted selection, and the development of interspecific introgression lines. Received: 1 June 1991 / Accepted: 22 June 1999     

Wheat grain yield on saline soils is improved by an ancestral Na⁺ transporter gene

The ability of wheat to maintain a low sodium concentration ([Na(+)]) in leaves correlates with improved growth under saline conditions. This trait, termed Na(+) exclusion, contributes to the greater salt tolerance of bread wheat relative to durum wheat. To improve the salt tolerance of durum wheat, we explored natural diversity in shoot Na(+) exclusion within ancestral wheat germplasm. Previously, we showed that crossing of Nax2, a gene locus in the wheat relative Triticum monococcum into a commercial durum wheat (Triticum turgidum ssp. durum var. Tamaroi) reduced its leaf [Na(+)] (ref. 5). Here we show that a gene in the Nax2 locus, TmHKT1;5-A, encodes a Na(+)-selective transporter located on the plasma membrane of root cells surrounding xylem vessels, which is therefore ideally localized to withdraw Na(+) from the xylem and reduce transport of Na(+) to leaves. Field trials on saline soils demonstrate that the presence of TmHKT1;5-A significantly reduces leaf [Na(+)] and increases durum wheat grain yield by 25% compared to near-isogenic lines without the Nax2 locus.     

Is maximization of molar yield in metabolic networks favoured by evolution?

Stoichiometric analysis of metabolic networks allows the calculation of possible metabolic flux distributions in the absence of kinetic data. In order to predict which of the possible fluxes are present under certain conditions, additional constraints and optimization principles can be applied. One approach of calculating unknown fluxes (frequently called flux balance analysis) is based on the optimality principle of maximizing the molar yield of biotransformations. Here, the relevance and applicability of that approach are examined, and it is compared with the principle of maximizing pathway flux. We discuss diverse experimental evidence showing that, often, those biochemical pathways are operative that allow fast but low-yield synthesis of important products, such as fermentation in Saccharomyces cerevisiae and several other yeast species. Together with arguments based on evolutionary game theory, this leads us to the conclusion that maximization of molar yield is by no means a universal principle.