During evolution of land plants, the haploid gametophytic stage has been strongly reduced in size and the diploid sporophytic phase has become the dominant growth form. Both male and female ...gametophytes are parasitic to the sporophyte and reside in separate parts of the flower located either on the same plant or on different plants. For fertilization to occur, bi-cellular or tri-cellular male gametophytes (pollen grains) have to travel to the immobile female gametophyte in the ovary. To survive exposure to a hostile atmosphere, pollen grains are thought to enter a state of complete or partial developmental arrest (DA). DA in pollen is strongly associated with acquisition of desiccation tolerance (DT) to extend pollen viability during air travel, but occurrence of DA in pollen is both species-dependent and at the same time strongly dependent on the reigning environmental conditions at the time of dispersal. Several environmental stresses (heat, drought, cold, humidity) are known to affect pollen production and viability. Climate change is also posing a serious threat to plant reproductive behavior and crop productivity. It is therefore timely to gain a better understanding of how DA and pollen viability are controlled in plants and how pollen viability can be protected to secure crop yields in a changing environment. Here, we provide an overview of how DA and pollen viability are controlled and how the environment affects them. We make emphasis on what is known and areas where a deeper understanding is needed.
Grain number is the only yield component that is directly associated with increased grain yield in important cereal crops like wheat. Historical yield studies show that increases in grain yield are ...always accompanied by an increase in grain number. Adverse weather conditions can cause severe fluctuations in grain yield and substantial yield losses in cereal crops. The problem is global and despite its impact on world food production breeding and selection approaches have only met with limited success. A specific period during early reproductive development, the young microspore stage of pollen development, is extremely vulnerable to abiotic stress in self-fertilising cereals (wheat, rice, barley, sorghum). A better understanding of the physiological and molecular processes that lead to stress-induced pollen abortion may provide us with the key to finding solutions for maintaining grain number under abiotic stress conditions. Due to the complexity of the problem, stress-proofing our main cereal crops will be a challenging task and will require joint input from different research disciplines.
Environmental restraints like cold, drought and heat adversely affect growth and development in different ways and at different plant developmental stages, leading to reduced crop yield ...
•Abiotic stresses affect yield and productivity of crop plants.•Stress tolerance in crops correlates with maintenance of growth and productivity.•Early stress-responsive genes control growth ...responses.•Selection strategies should use stress-induced traits and focus on early general stress response.•Plant hormone interactions control development and abiotic stress tolerance.
Progress in improving abiotic stress tolerance of crop plants using classic breeding and selection approaches has been slow. This has generally been blamed on the lack of reliable traits and phenotyping methods for stress tolerance. In crops, abiotic stress tolerance is most often measured in terms of yield-capacity under adverse weather conditions. “Yield” is a complex trait and is determined by growth and developmental processes which are controlled by environmental signals throughout the life cycle of the plant. The use of model systems has allowed us to gradually unravel how plants grow and develop, but our understanding of the flexibility and opportunistic nature of plant development and its capacity to adapt growth to environmental cues is still evolving. There is genetic variability for the capacity to maintain yield and productivity under abiotic stress conditions in crop plants such as cereals. Technological progress in various domains has made it increasingly possible to mine that genetic variability and develop a better understanding about the basic mechanism of plant growth and abiotic stress tolerance. The aim of this paper is not to give a detailed account of all current research progress, but instead to highlight some of the current research trends that may ultimately lead to strategies for stress-proofing crop species. The focus will be on abiotic stresses that are most often associated with climate change (drought, heat and cold) and those crops that are most important for human nutrition, the cereals.
Main conclusion
Transcriptomic analyses identified anther-expressed genes in wheat likely to contribute to heat tolerance and hence provide useful genetic markers. The genes included those involved ...in hormone biosynthesis, signal transduction, the heat shock response and anther development.
Pollen development is particularly sensitive to high temperature heat stress. In wheat, heat-tolerant and heat-sensitive cultivars have been identified, although the underlying genetic causes for these differences are largely unknown. The effects of heat stress on the developing anthers of two heat-tolerant and two heat-sensitive wheat cultivars were examined in this study. Heat stress (35 °C) was found to disrupt pollen development in the two heat-sensitive wheat cultivars but had no visible effect on pollen or anther development in the two heat-tolerant cultivars. The sensitive anthers exhibited a range of developmental abnormalities including an increase in unfilled and clumped pollen grains, abnormal pollen walls and a decrease in pollen viability. This subsequently led to a greater reduction in grain yield in the sensitive cultivars following heat stress. Transcriptomic analyses of heat-stressed developing wheat anthers of the four cultivars identified a number of key genes which may contribute to heat stress tolerance during pollen development. Orthologs of some of these genes in Arabidopsis and rice are involved in regulation of the heat stress response and the synthesis of auxin, ethylene and gibberellin. These genes constitute candidate molecular markers for the breeding of heat-tolerant wheat lines.
ABSTRACT
Reproductive stage water stress leads to spikelet sterility in wheat. Whereas drought stress at anthesis affects mainly grain size, stress at the young microspore stage of pollen development ...is characterized by abortion of pollen development and reduction in grain number. We identified genetic variability for drought tolerance at the reproductive stage. Drought‐tolerant wheat germplasm is able to maintain carbohydrate accumulation in the reproductive organs throughout the stress treatment. Starch depletion in the ovary of drought‐sensitive wheat is reversible upon re‐watering and cross‐pollination experiments indicate that the ovary is more resilient than the anther. The effect on anthers and pollen fertility is irreversible, suggesting that pollen sterility is the main cause of grain loss during drought conditions in wheat. The difference in storage carbohydrate accumulation in drought‐sensitive and drought‐tolerant wheat is correlated with differences in sugar profiles, cell wall invertase gene expression and expression of fructan biosynthesis genes in anther and ovary (sucrose : sucrose 1‐fructosyl‐transferase, 1‐SST; sucrose : fructan 6‐fructosyl‐transferase, 6‐SFT). Our results indicate that the ability to control and maintain sink strength and carbohydrate supply to anthers may be the key to maintaining pollen fertility and grain number in wheat and this mechanism may also provide protection against other abiotic stresses.
Cold temperatures cause pollen sterility and large reductions in grain yield in temperate rice growing regions of the world. Induction of pollen sterility by cold involves a disruption of sugar ...transport in anthers, caused by the cold-induced repression of the apoplastic sugar transport pathway in the tapetum. Here we demonstrate that the phytohormone ABA is a potential signal for cold-induced pollen sterility (CIPS). Cold treatment of the cold-sensitive cultivar Doongara resulted in increased anther ABA levels. Exogenous ABA treatment at the young microspore stage induced pollen sterility and affected cell wall invertase and monosaccharide transporter gene expression in a way similar to cold treatment. In the cold-tolerant cultivar R31, ABA levels were significantly lower under normal circumstances and remained low after cold treatment. The differences in endogenous ABA levels in Doongara and R31 correlated with differences in expression of the ABA biosynthetic genes encoding zeaxanthin epoxidase (OSZEP1) and 9-cis-epoxycarotenoid dioxygenase (OSNCED2, OSNCED3) in anthers. The expression of three ABA-8-hydroxylase genes (ABA8OX1, 2 and 3) in R31 anthers was higher under control conditions and was regulated differently by cold compared with Doongara. Our results indicate that the cold tolerance phenotype of R31 is correlated with lower endogenous ABA levels and a different regulation of ABA metabolism.
Consistent gains in grain yield in dry environments have been made by empirical breeding although there is disturbing evidence that these gains may have slowed. There are few examples where an ...understanding of the physiology and the genetics of putative important drought-related traits has led to improved yields. Success will first depend on identifying the most important traits in the target regions. It will then depend on accurate and fast phenotyping, which, in turn, will lead to: (1) trait-based selection being immediately transferable into breeding operations and (2) being able to identify the underlying genes or the important genomic regions (quantitative trait loci), perhaps leading to efficient marker-based selection (MBS). Genetic complexity, extent of genotype×environment (G×E) interaction and sampling cost per line will determine value of phenotyping over MBS methods. Here, we review traits of importance in dry environments and review whether molecular or phenotypic selection methods are likely to be the most effective in crop improvement programs and where the main bottlenecks to selection are. We also consider whether selection for these traits should be made in dry environments or environments where there is no soil water limitation. The development of lines/populations for trait validation studies and for varietal development is also described. We firstly conclude that despite the spectacular improvements in molecular technologies, fast and accurate phenotyping remains the major bottleneck to enhancing yield gains in water-limited environments. Secondly, for most traits of importance in dry environments, selection is generally conducted most effectively in favourable moisture environments.
Anther development progresses through 15 distinct developmental stages in wheat, and accurate determination of anther developmental stages is essential in anther and pollen studies. A detailed ...outline of the development of the wheat anther through its entire developmental program, including the 15 distinct morphological stages, is presented. In bread wheat (
), anther developmental stages were correlated with five measurements, namely auricle distance, spike length, spikelet length, anther length and anther width. Spike length and auricle distance were shown to be suitable for rapid anther staging within cultivars. Anther length is an accurate measurement in determining anther stages and may be applicable for use between cultivars. Tapetal Programmed Cell Death (PCD) in wheat begins between late tetrad stage (stage 8) and the early young microspore stage (stage 9) of anther development. Tapetal PCD continues until the vacuolate pollen stage (stage 11), at which point the tapetum fully degrades. The timing of tapetal PCD initiation is slightly delayed compared to that in rice, but is two stages earlier than in the model dicot Arabidopsis. The
gene, which encodes a transcription factor regulating the timing of tapetal PCD, reaches its peak expression at the onset of tapetal PCD in wheat.
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