Pathogen‐inducible Ta‐Lr34res expression in heterologous barley confers disease resistance without negative pleiotropic effects

Plant diseases are a serious threat to crop production. The informed use of naturally occurring disease resistance in plant breeding can greatly contribute to sustainably reduce yield losses caused by plant pathogens. The Ta‐Lr34res gene encodes an ABC transporter protein and confers partial, durable, and broad spectrum resistance against several fungal pathogens in wheat. Transgenic barley lines expressing Ta‐Lr34res showed enhanced resistance against powdery mildew and leaf rust of barley. While Ta‐Lr34res is only active at adult stage in wheat, Ta‐Lr34res was found to be highly expressed already at the seedling stage in transgenic barley resulting in severe negative effects on growth. Here, we expressed Ta‐Lr34res under the control of the pathogen‐inducible Hv‐Ger4c promoter in barley. Sixteen independent barley transformants showed strong resistance against leaf rust and powdery mildew. Infection assays and growth parameter measurements were performed under standard glasshouse and near‐field conditions using a convertible glasshouse. Two Hv‐Ger4c::Ta‐Lr34res transgenic events were analysed in detail. Plants of one transformation event had similar grain production compared to wild‐type under glasshouse and near‐field conditions. Our results showed that negative effects caused by constitutive high expression of Ta‐Lr34res driven by the endogenous wheat promoter in barley can be eliminated by inducible expression without compromising disease resistance. These data demonstrate that Ta‐Lr34res is agronomically useful in barley. We conclude that the generation of a large number of transformants in different barley cultivars followed by early field testing will allow identifying barley lines suitable for breeding.     

Optimization of TaDREB3 gene expression in transgenic barley using cold‐inducible promoters

Constitutive over‐expression of the TaDREB3 gene in barley improved frost tolerance of transgenic plants at the vegetative stage of plant development, but leads to stunted phenotypes and 3‐ to 6‐week delays in flowering compared to control plants. In this work, two cold‐inducible promoters with contrasting properties, the WRKY71 gene promoter from rice and the Cor39 gene promoter from durum wheat, were applied to optimize expression of TaDREB3. The aim of the work was to increase plant frost tolerance and to decrease or prevent negative developmental phenotypes observed during constitutive expression of TaDREB3. The OsWRKY71 and TdCor39 promoters had low‐to‐moderate basal activity and were activated by cold treatment in leaves, stems and developing spikes of transgenic barley and rice. Expression of the TaDREB3 gene, driven by either of the tested promoters, led to a significant improvement in frost tolerance. The presence of the functional TaDREB3 protein in transgenic plants was confirmed by the detection of strong up‐regulation of cold‐responsive target genes. The OsWRKY71 promoter–driven TaDREB3 provides stronger activation of the same target genes than the TdCor39 promoter. Analysis of the development of transgenic plants in the absence of stress revealed small or no differences in plant characteristics and grain yield compared with wild‐type plants. The WRKY71–TaDREB3 promoter–transgene combination appears to be a promising tool for the enhancement of cold and frost tolerance in crop plants but field evaluation will be needed to confirm that negative development phenotypes have been controlled.     

The TaDREB3 transgene transferred by conventional crossings to different genetic backgrounds of bread wheat improves drought tolerance

Drought tolerance of the wheat cultivar Bobwhite was previously enhanced by transformation with a construct containing the wheat DREB3 gene driven by the stress‐inducible maize Rab17 promoter. Progeny of a single T₂ transgenic line were used as pollinators in crosses with four elite bread wheat cultivars from Western Australia: Bonnie Rock, IGW‐2971, Magenta and Wyalkatchem, with the aim of evaluating transgene performance in different genetic backgrounds. The selected pollinator line, BW8‐9‐10‐3, contained multiple transgene copies, had significantly improved drought tolerance compared with wild‐type plants and showed no growth and development penalties or abnormalities. A single hybrid plant was selected from each cross‐combination for three rounds of backcrossing with the corresponding maternal wheat cultivar. The transgene was detected in all four F₁BC₃ combinations, but stress‐inducible transgene expression was found in only three of the four combinations. Under well‐watered conditions, the phenotypes and grain yield components of the F₂BC₃ transgene‐expressing lines were similar to those of corresponding recurrent parents and null‐segregants. Under severe drought conditions, the backcross lines demonstrated 12–18% higher survival rates than the corresponding control plants. Two from four F₃BC₃ transgenic lines showed significantly higher yield (18.9% and 21.5%) than control plants under limited water conditions. There was no induction of transgene expression under cold stress, and therefore, no improvement of frost tolerance observed in the progenies of drought‐tolerant F₃BC₃ lines.     

Mechanisms of waterlogging tolerance in wheat – a review of root and shoot physiology

We review the detrimental effects of waterlogging on physiology, growth and yield of wheat. We highlight traits contributing to waterlogging tolerance and genetic diversity in wheat. Death of seminal roots and restriction of adventitious root length due to O₂ deficiency result in low root:shoot ratio. Genotypes differ in seminal root anoxia tolerance, but mechanisms remain to be established; ethanol production rates do not explain anoxia tolerance. Root tip survival is short‐term, and thereafter, seminal root re‐growth upon re‐aeration is limited. Genotypes differ in adventitious root numbers and in aerenchyma formation within these roots, resulting in varying waterlogging tolerances. Root extension is restricted by capacity for internal O₂ movement to the apex. Sub‐optimal O₂ restricts root N uptake and translocation to the shoots, with N deficiency causing reduced shoot growth and grain yield. Although photosynthesis declines, sugars typically accumulate in shoots of waterlogged plants. Mn or Fe toxicity might occur in shoots of wheat on strongly acidic soils, but probably not more widely. Future breeding for waterlogging tolerance should focus on root internal aeration and better N‐use efficiency; exploiting the genetic diversity in wheat for these and other traits should enable improvement of waterlogging tolerance.     

Overexpression of an Arabidopsis thaliana galactinol synthase gene improves drought tolerance in transgenic rice and increased grain yield in the field

Drought stress has often caused significant decreases in crop production which could be associated with global warming. Enhancing drought tolerance without a grain yield penalty has been a great challenge in crop improvement. Here, we report the Arabidopsis thaliana galactinol synthase 2 gene (AtGolS2) was able to confer drought tolerance and increase grain yield in two different rice (Oryza sativa) genotypes under dry field conditions. The developed transgenic lines expressing AtGolS2 under the control of the constitutive maize ubiquitin promoter (Ubi:AtGolS2) also had higher levels of galactinol than the non‐transgenic control. The increased grain yield of the transgenic rice under drought conditions was related to a higher number of panicles, grain fertility and biomass. Extensive confined field trials using Ubi:AtGolS2 transgenic lines in Curinga, tropical japonica and NERICA4, interspecific hybrid across two different seasons and environments revealed the verified lines have the proven field drought tolerance of the Ubi:AtGolS2 transgenic rice. The amended drought tolerance was associated with higher relative water content of leaves, higher photosynthesis activity, lesser reduction in plant growth and faster recovering ability. Collectively, our results provide strong evidence that AtGolS2 is a useful biotechnological tool to reduce grain yield losses in rice beyond genetic differences under field drought stress.     

Identification and Analysis of Powdery Mildew‐Responsive miRNAs in Wheat

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is one of the most consistently damaging diseases of common wheat worldwide and greatly affects crop productivity. Recently, several plant microRNAs (miRNAs) have been reported as gene expression regulators related to various adverse environments. However, up to now, less is known on the roles of miRNAs in powdery mildew infection response of wheat. In this study, miRNA expression patterns were investigated for identifying Bgt‐responsive miRNAs in wheat leaves using a plant miRNA microarray platform. A total of 79 miRNAs from 24 families were detected in wheat leaves. Among those, seven miRNAs were further validated to be involved in wheat powdery mildew response and two of them have never been reported. In addition, their target expression profiles showed a negative correlation with that of the seven miRNAs in mock‐ and Bgt‐infected samples furtherly proved, which in turn as the robust evidence, that those seven powdery mildew‐responsive miRNAs are highly reliable. These findings could extend the current view about miRNAs as ubiquitous regulators under stress conditions.     

Identifying the critical period for waterlogging on yield and its components in wheat and barley

Background and aims

Crop tolerance to waterlogging depends on factors such as species sensitivity and the stage of development that waterlogging occurs. The aim of this study was to identify the critical period for waterlogging on grain yield and its components, when applied during different stages of crop development in wheat and barley.

Methods

Two experiments were carried out (E1: early sowing date, under greenhouse; E2: late sowing date, under natural conditions). Waterlogging was imposed during 15–20 days in 5 consecutive periods during the crop cycle (from Leaf 1 emergence to maturity).

Results

The greatest yield penalties occurred when waterlogging was applied from Leaf 7 appearance on the main stem to anthesis (from 34 to 92 % of losses in wheat, and from 40 to 79 % in barley for E1 and E2 respectively). Waterlogging during grain filling reduced yield to a lesser degree. In wheat, reductions in grain number were mostly explained by reduced grain number per spike while in barley, by variations in the number of spikes per plant.

Conclusions

The time around anthesis was identified as the most susceptible period to waterlogging in wheat and barley. Exposing the crop to more stressful conditions, e.g. delaying sowing date, magnified the negative responses to waterlogging, although the most sensitive stage (around anthesis) remained unchanged.