Weedy red rice (Oryza sativa; WRR), a close relative of cultivated rice, is a highly competitive weed that commonly infests rice fields and can also naturally interbreed with rice. Useful genes for biotic stress have been maintained in WRR and can be explored for breeding. Here we describe genetic and physiological traits of WRR that can be beneficial in preventing major rice diseases. Rice blast, caused by the hemibiotrophic fungal pathogen Magnaporthe oryzae, and sheath blight disease, caused by the necrotrophic pathogen Rhizoctonia solani, are the two most damaging biotic stresses of rice. Many major and minor resistance genes and QTL have been identified in cultivated and wild rice relatives. However, novel QTL were recently found in the two major U.S. biotypes of WRR, blackhull-awned (BH) and strawhull-awnless (SH), suggesting that WRR has evolved novel genetic mechanisms to cope with these biotic stresses. Twenty-eight accessions of WRR (PI 653412-PI 653439) from the southern USA were characterized and placed in the National Small Grains Collection, and are available for identification of novel genetic factors to prevent biotic stress.
Proso millet is an important short-duration crop that adapts well to varied climatic conditions and is grown worldwide for food, feed and fodder purposes. Owing to a lack of genetic improvement, the crop has experienced no yield improvement and provides low income to farmers. In this study, 200 accessions of proso millet originating in 30 countries were evaluated in two rainy seasons to assess phenotypic diversity for morpho-agronomic and grain nutritional traits and to identify high grain-yielding and grain nutrient-rich accessions. Proso millet diversity was structured by geographical region, by country within region, and by racial group. Race patentissimum showed high diversity and ovatum low diversity, and diverged widely from each other. The lowest divergence was observed between races compactum and ovatum. Eighteen high grain-yielding, 10 large-seeded, and 26 two or more grain nutrients-rich accessions were identified, and highly diverse pairs of accessions within and between trait groups were identified. They included IPm 9 and IPm 2661 for high grain yield and large seed size; and IPm 2069, IPm 2076, and IPm 2537 for high Fe, Zn, Ca, and protein contents. IPm 2875 had a seed coat that is readily removed by threshing. This study provides valuable information to proso millet researchers about agronomic and nutritional traits in accessions that could be tested for regional adaption and yield for direct release as cultivars, and could be used in breeding for developing high grain-yielding and nutrient-rich cultivars.
Identification of metabolites responsible for tolerance to low nitrogen availability (low-N) will aid in the genetic improvement of rice yield under nitrogen deficiency. In this study, a backcross introgression line (G9) and its recurrent parent Shuhui 527 (SH527), which show differential responses to low-N stress, were used to identify metabolites associated with low-N tolerance in rice. Differences in metabolite contents in the leaves of G9 and SH527 at three growth stages under low-N stress were assessed by gas chromatography-mass spectrometry. Many metabolites, including amino acids and derivatives, were highly enriched in G9 compared with SH527 under the control condition, suggesting that the two genotypes had basal metabolite differences. Low-N stress induced genotype-specific as well as growth stage-dependent metabolite changes. Metabolites induced specifically in G9 that were involved in glycolysis and tricarboxylic acid metabolism were enriched at the tillering and grain filling stages, and metabolites involved in nitrogen and proline metabolism were enriched at the booting stage. Enrichment of pyroglutamate, glutamate, 2-oxoglutarate, sorbose, glycerate-2-P, and phosphoenolpyruvic acid in G9 suggests that these metabolites could be involved in low-N stress tolerance. The results presented here provide valuable information for further elucidation of the molecular mechanisms of low-N tolerance in crops.
Rice yield is an important and complex agronomic trait controlled by multiple genes. In recent decades, dozens of yield-associated genes in rice have been cloned, many of which can increase production in the form of loss or degeneration of function. However, mutations occurring randomly under natural conditions have provided very limited genetic resources for yield increases. In this study, potentially yield-increasing alleles of two genes closely associated with yield were edited artificially. The recently developed CRISPR/Cas9 system was used to edit two yield genes: Grain number 1a (Gn1a) and DENSE AND ERECT PANICLE1 (DEP1). Several mutants were identified by a target sequence analysis. Phenotypic analysis confirmed one mutant allele of Gn1a and three of DEP1 conferring yield superior to that conferred by other natural high-yield alleles. Our results demonstrate that favorable alleles of the Gnla and DEP1 genes, which are considered key factors in rice yield increases, could be developed by artificial mutagenesis using genome editing technology.
Compared to drought-susceptible rice cultivars (DSRs), drought-resistance rice cultivars (DRRs) could drastically reduce the amount of irrigation water input and simultaneously result in higher grain yield under water-saving irrigation conditions. However, the mechanisms underlying these properties are unclear. We investigated how improved agronomic traits contribute to higher yield and higher water use efficiency (WUE) in DRRs than in DSRs under alternate wetting and drying (AWD). Two DRRs and two DSRs were field-grown in 2015 and 2016 using two different irrigation regimes: continuous flooding (CF) and AWD. Under CF, no statistical differences in grain yield and WUE were observed between DRRs and DSRs. Irrigation water under the AWD regime was 275-349?mm, an amount 49.8%-56.2% of that (552-620?mm) applied under the CF regime. Compared to CF, AWD significantly decreased grain yield in both DRRs and DSRs, with a more significant reduction in DSRs, and WUE was increased in DRRs, but not in DSRs, by 9.9%-23.0% under AWD. Under AWD, DRRs showed a 20.2%-26.2% increase in grain yield and an 18.6%-24.5% increase in WUE compared to DSRs. Compared to DSRs, DRRs showed less redundant vegetative growth, greater sink capacity, higher grain filling efficiency, larger root biomass, and deeper root distribution under AWD. We conclude that these improved agronomic traits exert positive influences on WUE in DRRs under AWD.
Dry direct-seeded rice grown in raised beds is becoming an important practice in the wheat-rice rotation system in China. However, little information has been available on the effect of various irrigation regimes on grain yield, water productivity (WP), nitrogen use efficiency (NUE), and greenhouse gas emission in this practice. This study investigated the question using two rice cultivars in 2015 and 2016 grown in soil with wheat straw incorporated into it. Rice seeds were directly seeded into raised beds, which were maintained under aerobic conditions during the early seedling period. Three irrigation regimes: continuous flooding (CF), alternate wetting and drying (AWD), and furrow irrigation (FI), were applied from 4.5-leaf-stage to maturity. Compared with CF, both AWD and FI significantly increased grain yield, WP, and internal NUE, with greater increases under the FI regime. The two cultivars showed the same tendency in both years. Both AWD and FI markedly increased soil redox potential, root and shoot biomass, root oxidation activity, leaf photosynthetic NUE, and harvest index and markedly decreased global warming potential, owing to substantial reduction in seasonal CH4 emissions. The results demonstrate that adoption of either AWD or FI could increase grain yield and resource-use efficiency and reduce environmental risks in dry direct-seeded rice grown on raised beds with wheat straw incorporation in the wheat-rice rotation system.
Triticum urartu (AA, 2n?=?2x?=?14), a wild grass endemic to the Fertile Crescent (FC), is the progenitor of the A subgenome in common wheat. It belongs to the primary gene pool for wheat improvement. Here, we evaluated the yellow rust (caused by Puccinia striiformis f. sp. tritici, Pst) reactions of 147 T. urartu accessions collected from different parts of the FC. The reactions varied from susceptibility to strong resistance. In general, there were more accessions with stronger resistance to race CYR33 than to CYR 32. In most cases the main form of defense was a moderate resistance characterized by the presence of necrotic/chlorotic lesions with fewer Pst uredinia on the leaves. Forty two accessions displayed resistance to both races. Histological analysis showed that Pst growth was abundant in the compatible interaction but significantly suppressed by the resistant response. Gene silencing mediated by Barley stripe mosaic virus was effective in two T. urartu accessions with different resistance responses, indicating that this method can expedite future functional analysis of resistance genes. Our data suggest that T. urartu is a valuable source of resistance to yellow rust, and represents a model for studying the genetic, genomic and molecular basis underlying interaction between wheat and Pst.
Tomato spotted wilt (TSW) is a serious virus disease of peanut in the United States. Breeding for TSWV resistance would be facilitated by the implementation of marker-assisted selection in breeding programs; however, genes associated with resistance have not been identified. Association mapping is a type of genetic mapping that can exploit relationships between markers and traits in many lineages. The objectives of this study were to examine genetic diversity and population structure in the U.S. peanut mini-core collection using simple sequence repeat (SSR) markers, and to conduct association mapping between SSR markers and TSWV resistance in cultivated peanuts. One hundred and thirty-three SSR markers were used for genotyping 104 accessions. Four subpopulations, generally corresponding to botanical varieties, were classified by population structure analysis. Association mapping analysis indicated that five markers: pPGPseq5D5, GM1135, GM1991, TC23C08, and TC24C06, were consistently associated with TSW resistance by the Q, PCA, Q+K, and PCA+K models. These markers together explained 36.4% of the phenotypic variance. Moreover, pPGPseq5D5 and GM1991 were associated with both visual symptoms of TSWV and ELISA values with a high R2. The potential of these markers for use in a marker-assisted selection program to breed peanut for resistance to TSWV is discussed.
Accurate leaf area simulation is critical for the performance of crop growth models. Area of fully expanded individual leaves of maize hybrids released before 1995 (defined as old hybrids) has been simulated using a bell-shaped function (BSF) and the relationship between its parameters and total leaf number (TLNO). However, modern high-yielding maize hybrids show different canopy architectures. The function parameters calibrated for old hybrids will not accurately represent modern hybrids. In this study, we evaluated these functions using a dataset including old and modern hybrids that have been widely planted in China in recent years. Maximum individual leaf area (Y0) and corresponding leaf position (X0) were not predicted well by TLNO (R2?=?0.56 and R2?=?0.70) for modern hybrids. Using recalibrated shape parameters a and b with values of Y0 and X0 for modern hybrids, the BSF accurately predicted individual leaf area (R2?=?0.95-0.99) and total leaf area of modern hybrids (R2?=?0.98). The results show that the BSF is still a robust way to predict the fully expanded leaf area of maize when parameters a and b are modified and Y0 and X0 are fitted. Breeding programs have led to increases in TLNO of maize but have not altered Y0 and X0, reducing the correlation between Y0, X0, and TLNO. For modern hybrids, the values of Y0 and X0 are hybrid-specific. Modern hybrids tend to have less-negative values of parameter a and more-positive values of parameter b in the leaf profile. Growth conditions, such as plant density and environmental conditions, also affect the fully expanded leaf area but were not considered in the original published equations. Thus, further research is needed to accurately estimate values of Y0 and X0 of individual modern hybrids to improve simulation of maize leaf area in crop growth models.
Premature senescence after anthesis reduces crop yields. Delaying leaf senescence could maintain photosynthetic activity for a longer period and lead to a higher photosynthetic rate. Recent studies have provided some insights into the interaction between cytokinin and nitrogen (N) in the regulation of plant development. In the present study, foliar application of exogenous 6-benzylaminopurine (6-BA) and lovastatin, an inhibitor of cytokinin synthesis, was combined with three N rates [0?kg?ha−1 (low nitrogen, LN), 240?kg?ha−1 (normal nitrogen, NN), and 360?kg?ha−1 (high nitrogen, HN)] in two wheat cultivars, Wennong 6 (with a staygreen phenotype) and Jimai 20 (with a non-staygreen phenotype). Flag leaf senescence was assessed using a Gompertz growth curve. Grain mass, dry matter accumulation and distribution, total N of flag leaf, and concentrations of zeatin riboside (ZR) and abscisic acid (ABA) were also used to evaluate the functional characteristics of flag leaves. Grain mass was negatively correlated with initial senescence rate (r0) and duration of rapid chlorophyll loss (Chlloss), whereas it was positively correlated with maximum senescence rate (rmax), average senescence rate (raver), persistence phase (Chlper), total duration of flag leaf (Chltotal) and inflection point cumulative temperature (M). Compared to Jimai 20, Wennong 6 had larger raver, Chlper, and Chltotal. The concentration of ZR was highest under the 6-BA?×?NN treatment, followed by the 6-BA?×?HN and 6-BA?×?LN treatments. However, the concentration of ABA showed the opposite trend. It was concluded that the staygreen phenotype Wennong 6 was associated with greater grain mass and altered cytokinin metabolism and could be classified as a functional staygreen type. Foliar application of 6-BA interacting with N at the NN level (240?kg?ha−1) may be a beneficial strategy for improving grain yield of wheat by regulating endogenous hormones and the flag leaf senescence process. Increasing endogenous cytokinin promoted the transport of dry matter to grain.
A molecular phylogenetic analysis of the genus Corchorus (Grewioideae, Malvaceae s.l.) is presented, based on sequences of the nuclear ribosomal DNA internal transcribed spacer (ITS) region for 144 accessions representing 47 species. Several other genera from the subfamily Grewioideae, namely Pseudocorchorus, Triumfetta, Sparrmannia, Entelea, and Grewia, were included as outgroups. The monophyly of the genus was well supported by all phylogenetic analyses (maximum likelihood, Bayesian approaches, and parsimony), and Corchorus was divided into four major clades. The majority of African species formed a statistically highly supported and distinct clade separated from the other pantropically distributed species. Several endemic species from Australia, New Caledonia, and tropical America were nested within this distinct clade, indicating dispersal from Africa to the rest of the pantropics. Based on the taxa included in this study, the two cultivated species (C. olitorius and C. capsularis) shared a common ancestry with wild species of C. africanus, C. brevicornatus, C. pseudocapsularis, C. pseudo-olitorius, C. urticifolius, C. pilosus, C. orinocensis, and C. cunninghamii. Pseudocorchorus, previously considered an accepted genus, was nested within the genus Corchorus and shared a common ancestry especially with C. depressus and C. siliquosus. Based on morphological and anatomical similarity as well as the results of the present molecular findings, inclusion of the six Pseudocorchorus species into Corchorus is proposed, with Pseudocorchorus as a synonym of Corchorus. Of the included outgroup taxa, Triumfetta is the closest sister to Corchorus, while the common ancestor of Corchorus/Pseudocorchorus, Triumfetta, Sparrmannia, and Entelea is Grewia. A further phylogenetic study with more taxa mainly from Australia, together with additional molecular markers and morphological investigation, would help to test the hypothesis on the biogeography and growth form evolution of the genus Corchorus.
