Spike developmental stages and ABA role in spikelet primordia abortion contribute to the final yield in barley (Hordeum vulgare L.)

Anatomical insights into the vascular layout of the barley rachis: implications for transport and spikelet connection

BACKGROUND AND AIMS

Vascular patterning is intimately related to plant form and function. Here, using barley (Hordeum vulgare) as a model, we studied the vascular anatomy of the spike-type inflorescence. The main aim of the present work was to clarify the relationship between rachis (spike axis) vasculature and spike size, to define vascular dynamics and to discuss the implications for transport capacity and its interaction with the spikelets.

METHODS

We used serial transverse internode sections to determine the internode area, vascular area and number of veins along the rachis of several barley lines.

KEY RESULTS

Internode area and total vascular area show a clear positive correlation with spike size, whereas the number of veins is only weakly correlated. The lateral periphery of the rachis contains large mature veins of constant size, whereas the central part is occupied by small immature veins. Spikelet-derived veins entering the rachis often merge with the immature rachis veins but never merge with the mature veins. An increase in floret fertility through the conversion of a two-rowed barley into an isogenic six-rowed line, in addition to a decrease in floret fertility owing to enhanced pre-anthesis tip degeneration caused by the mutation tip sterile 2.b (tst2.b), significantly affected vein size but had limited to no effects on the number of veins or internode area.

CONCLUSIONS

The rachis vasculature is the result of a two-step process involving an initial layout followed by size adjustment according to floret fertility/spike size. The restriction of large mature vessels to the periphery and that of small immature vessels to the centre of the rachis suggests that long-distance transport and local supply to spikelets are spatially separated processes. The identification of spikelet-derived veins entering the rachis without fusing with its vasculature indicates that a vascular continuity between rachis and spikelets might be non-essential.

Endophyte Inoculation and Elevated Potassium Supply on Productivity, Growth and Physiological Parameters of Spring Barley (Hordeum vulgare L.) Genotypes over Contrasting Seasons

In recent years, recurrent droughts have significantly affected spring barley production, reducing the quantity and quality of grain. This study aims to identify genotype-specific traits and the drought resilience of six different Hordeum vulgare L. (spring barley) genotypes, while also examining the potential of potassium application and fungal endophyte Serendipita indica inoculation to mitigate the negative effects of dry periods during the growing season. Field experiments were conducted over a three-year period from 2020 to 2022, measuring physiological, growth, and yield parameters. To get insight into the physiological state of the plants, we measured the soluble sugars content and the ratio of stable carbon isotopes in the flag leaf tissue, which reflects conditions during its formation. The dominant factors that influenced the measured parameters were the genotypes and seasons, as well as their interaction, rather than other experimental factors. The results showed that the Spitfire and Accordine varieties were the best performing in both the 2020 and 2021 seasons, as indicated by their yield. However, in the drier 2022 season, the yield of these two varieties decreased significantly (to 55% for Spitfire and to 69% for Accordine of their yield in 2021), while for the arid-region genotypes, it remained at the same level as the previous year. This study sheds light on the potential of various genotypes to withstand periods of drought and the effectiveness of using potassium application and S. indica inoculation as mitigation approaches.

A transcriptional atlas identifies key regulators and networks for the development of spike tissues in barley

Grain number and size determine grain yield in crops and are closely associated with spikelet fertility and grain filling in barley (Hordeum vulgare). Abortion of spikelet primordia within individual barley spikes causes a 30%-50% loss in the potential number of grains during development from the awn primordium stage to the tipping stage, after that grain filling is the primary factor regulating grain size. To identify transcriptional signatures associated with spike development, we use a six-rowed barley cultivar (Morex) to develop a spatiotemporal transcriptome atlas containing 255 samples covering 17 stages and 5 positions along the spike. We identify several fundamental regulatory networks, in addition to key regulators of spike development and morphology. Specifically, we show HvGELP96, encoding a GDSL domain-containing protein, as a regulator of spikelet fertility and grain number. Our transcriptional atlas offers a powerful resource to answer fundamental questions in spikelet development and degeneration in barley.

Multilayered regulation of developmentally programmed pre-anthesis tip degeneration of the barley inflorescence

Leaf and floral tissue degeneration is a common feature in plants. In cereal crops such as barley (Hordeum vulgare L.), pre-anthesis tip degeneration (PTD) starts with growth arrest of the inflorescence meristem dome, which is followed basipetally by the degeneration of floral primordia and the central axis. Due to its quantitative nature and environmental sensitivity, inflorescence PTD constitutes a complex, multilayered trait affecting final grain number. This trait appears to be highly predictable and heritable under standardized growth conditions, consistent with a developmentally programmed mechanism. To elucidate the molecular underpinnings of inflorescence PTD, we combined metabolomic, transcriptomic, and genetic approaches to show that barley inflorescence PTD is accompanied by sugar depletion, amino acid degradation, and abscisic acid responses involving transcriptional regulators of senescence, defense, and light signaling. Based on transcriptome analyses, we identified GRASSY TILLERS1 (HvGT1), encoding an HD-ZIP transcription factor, as an important modulator of inflorescence PTD. A gene-edited knockout mutant of HvGT1 delayed PTD and increased differentiated apical spikelets and final spikelet number, suggesting a possible strategy to increase grain number in cereals. We propose a molecular framework that leads to barley PTD, the manipulation of which may increase yield potential in barley and other related cereals.

Delayed development of basal spikelets in wheat explains their increased floret abortion and rudimentary nature

Large differences exist in the number of grains per spikelet across an individual wheat (Triticum aestivum L.) spike. The central spikelets produce the highest number of grains, while apical and basal spikelets are less productive, and the most basal spikelets are commonly only developed in rudimentary form. Basal spikelets are delayed in initiation, yet they continue to develop and produce florets. The precise timing or the cause of their abortion remains largely unknown. Here, we investigated the underlying causes of basal spikelet abortion using shading applications in the field. We found that basal spikelet abortion is likely to be the consequence of complete floret abortion, as both occur concurrently and have the same response to shading treatments. We detected no differences in assimilate availability across the spike. Instead, we show that the reduced developmental age of basal florets pre-anthesis is strongly associated with their increased abortion. Using the developmental age pre-abortion, we were able to predict final grain set per spikelet across the spike, alongside the characteristic gradient in the number of grains from basal to central spikelets. Future efforts to improve spikelet homogeneity across the spike could thus focus on improving basal spikelet establishment and increasing floret development rates pre-abortion.

A framework for improving wheat spike development and yield based on the master regulatory TOR and SnRK gene systems

The low rates of yield gain in wheat breeding programs create an ominous situation for the world. Amongst the reasons for this low rate are issues manifested in spike development that result in too few spikelets, fertile florets, and therefore grains being produced. Phases in spike development are particularly sensitive to stresses of various kinds and origins, and these are partly responsible for the deficiencies in grain production and slow rates of gain in yield. The diversity of developmental processes, stresses, and the large numbers of genes involved make it particularly difficult to prioritize approaches in breeding programs without an overarching, mechanistic framework. Such a framework, introduced here, is provided around the master regulator target of rapamycin and sucrose non-fermenting-1 (SNF1)-related protein kinase complexes and their control by trehalose-6-phosphate and other molecules. Being master regulators of the balance between growth and growth inhibition under stress, these provide genetic targets for creating breakthroughs in yield enhancement. Examples of potential targets and experimental approaches are described.

Abscisic acid and its role in the modulation of plant growth, development, and yield stability

Abscisic acid (ABA) is known to confer stress tolerance; however, at elevated levels it impairs plant growth under prolonged stress. Paradoxically, at its basal level, ABA plays many vital roles in promoting plant growth and development, including modulation of tillering, flowering, and seed development, as well as seed maturation. In this review, we provide insight into novel discoveries of ABA fluxes, ABA signaling responses, and their impact on yield stability. We discuss ABA homeostasis implicated under pre- and postanthesis drought and its impact on productive tillers, grain number determination, and seed development to address yield stability in cereal crops while considering the new knowledge that emerged from the model plant systems.

Spikelet abortion in six-rowed barley is mainly influenced by final spikelet number, with potential spikelet number acting as a suppressor trait

The potential to increase barley grain yield lies in the indeterminate nature of its inflorescence meristem, which produces spikelets, the basic reproductive unit in grasses that are linked to reproductive success. During early reproductive growth, barley spikes pass through the maximum yield potential-a stage after which no new spikelet ridges are produced. Subsequently, spikelet abortion (SA), a phenomenon in which spikelets abort during spike growth, imposes a bottleneck for increasing the grain yield potential. Here, we studied the potential of main culm spikes by counting potential spikelet number (PSN) and final spikelet number (FSN), and computed the corresponding SA (%) in a panel of 417 six-rowed spring barleys. Our phenotypic data analyses showed a significantly large within- and across-years genotypic variation with high broad-sense heritability estimates for all the investigated traits, including SA. Asian accessions displayed the lowest SA, indicating the presence of favourable alleles that may be exploited in breeding programs. A significantly negative Pearson's product-moment correlation was observed between FSN and SA. Our path analysis revealed that PSN and FSN explain 93% of the observed phenotypic variability for SA, with PSN behaving as a suppressor trait that magnifies the effect of FSN. Based on a large set of diverse barley accessions, our results provide a deeper phenotypic understanding of the quantitative genetic nature of SA, its association with traits of high agronomic importance, and a resource for further genetic analyses.

An ABA Functional Analogue B2 Enhanced Salt Tolerance by Inducing the Root Elongation and Reducing Peroxidation Damage in Maize Seedlings

Salt stress negatively affects maize growth and yield. Application of plant growth regulator is an effective way to improve crop salt tolerance, therefore reducing yield loss by salt stress. Here, we used a novel plant growth regulator B2, which is a functional analogue of ABA. With the aim to determine whether B2 alleviates salt stress on maize, we studied its function under hydroponic conditions. When the second leaf was fully developed, it was pretreated with 100 µM ABA, 0.01 µM B2, 0.1 µM B2, and 1 µM B2, independently. After 5 days treatment, NaCl was added into the nutrient solution for salt stress. Our results showed that B2 could enhance salt tolerance in maize, especially when the concentration was 1.0 µMol·L⁻¹. Exogenous application of B2 significantly enhanced root growth, and the root/shoot ratio increased by 7.6% after 6 days treatment under salt stress. Compared with control, the ABA level also decreased by 31% after 6 days, which might have resulted in the root development. What is more, B2 maintained higher photosynthetic capacity in maize leaves under salt stress conditions and increased the activity of antioxidant enzymes and decreased the generation rate of reactive oxygen species by 16.48%. On the other hand, B2 can enhance its water absorption ability by increasing the expression of aquaporin genes ZmPIP1-1 and ZmPIP1-5. In conclusion, the novel plant growth regulator B2 can effectively improve the salt tolerance in maize.

The Adaptation and Tolerance of Major Cereals and Legumes to Important Abiotic Stresses

Abiotic stresses, including drought, extreme temperatures, salinity, and waterlogging, are the major constraints in crop production. These abiotic stresses are likely to be amplified by climate change with varying temporal and spatial dimensions across the globe. The knowledge about the effects of abiotic stressors on major cereal and legume crops is essential for effective management in unfavorable agro-ecologies. These crops are critical components of cropping systems and the daily diets of millions across the globe. Major cereals like rice, wheat, and maize are highly vulnerable to abiotic stresses, while many grain legumes are grown in abiotic stress-prone areas. Despite extensive investigations, abiotic stress tolerance in crop plants is not fully understood. Current insights into the abiotic stress responses of plants have shown the potential to improve crop tolerance to abiotic stresses. Studies aimed at stress tolerance mechanisms have resulted in the elucidation of traits associated with tolerance in plants, in addition to the molecular control of stress-responsive genes. Some of these studies have paved the way for new opportunities to address the molecular basis of stress responses in plants and identify novel traits and associated genes for the genetic improvement of crop plants. The present review examines the responses of crops under abiotic stresses in terms of changes in morphology, physiology, and biochemistry, focusing on major cereals and legume crops. It also explores emerging opportunities to accelerate our efforts to identify desired traits and genes associated with stress tolerance.

Proline-Mediated Drought Tolerance in the Barley (Hordeum vulgare L.) Isogenic Line Is Associated with Lateral Root Growth at the Early Seedling Stage

A vigorous root system in barley promotes water uptake from the soil under water-limited conditions. We investigated three spring barley genotypes with varying water stress responses using rhizoboxes at the seedling stage. The genotypes comprised two elite German cultivars, Barke and Scarlett, and a near-isogenic line, NIL 143. The isogenic line harbors the wild allele pyrroline-5-carboxylate synthase1-P5cs1. Root growth in rhizoboxes under reduced water availability conditions caused a significant reduction in total root length, rooting depth, root maximum width, and root length density. On average, root growth was reduced by more than 20% due to water stress. Differences in organ proline concentrations were observed for all genotypes, with shoots grown under water stress exhibiting at least a 30% higher concentration than the roots. Drought induced higher leaf and root proline concentrations in NIL 143 compared with any of the other genotypes. Under reduced water availability conditions, NIL 143 showed less severe symptoms of drought, higher lateral root length, rooting depth, maximum root width, root length density, and convex hull area compared with Barke and Scarlett. Within the same comparison, under water stress, NIL 143 had a higher proportion of lateral roots (+30%), which were also placed at deeper substrate horizons. NIL 143 had a less negative plant water potential and higher relative leaf water content and stomatal conductance compared with the other genotypes under water stress. Under these conditions, this genotype also maintained an enhanced net photosynthetic rate and exhibited considerable fine root growth (diameter class 0.05–0.35 mm). These results show that water stress induces increased shoot and root proline accumulation in the NIL 143 barley genotype at the seedling stage and that this effect is associated with increased lateral root growth.

A Wild Allele of Pyrroline-5-Carboxylate Synthase1 Leads to Proline Accumulation in Spikes and Leaves of Barley Contributing to Improved Performance Under Reduced Water Availability

Water stress (WS) during spike development strongly affects final grain yield and grain quality in cereals. Proline, an osmoprotectant amino-acid, may contribute to alleviating the effects of cell and tissue dehydration. We studied five spring barley genotypes contrasting in their drought response, including two introgression lines, S42IL-143 and S42IL-141, harboring a Pyrroline-5-carboxylate synthase1- P5cs1 allele originating from the wild barley accession ISR42-8. We tested the hypothesis that barley genotypes harboring a wild allele at P5cs1 locus are comparatively more drought-tolerant at the reproductive stage by inducing proline accumulation in their immature spikes. At the booting stage, we subjected plants to well-watered and WS treatments until physiological maturity. Several morpho-physiological traits had significant genotype by treatment interaction and reduction under WS. Varying levels of genotypic proline accumulation and differences in WS tolerance were observed. Spike proline accumulation was higher than leaf proline accumulation for all genotypes under WS. Also, introgression lines carrying a wild allele at P5cs1 locus had a markedly higher spike and leaf proline content compared with the other genotypes. These introgression lines showed milder drought symptoms compared with elite genotypes, remained photosynthetically active under WS, and maintained their intrinsic water use efficiency. These combined responses contributed to the achievement of higher final seed productivity. Magnetic resonance imaging (MRI) of whole spikes at the soft dough stage showed an increase in seed abortion among the elite genotypes compared with the introgression lines 15 days after WS treatment. Our results suggest that proline accumulation at the reproductive stage contributes to the maintenance of grain formation under water shortage.

Abscisic Acid-Enemy or Savior in the Response of Cereals to Abiotic and Biotic Stresses?

Abscisic acid (ABA) is well-known phytohormone involved in the control of plant natural developmental processes, as well as the stress response. Although in wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) its role in mechanism of the tolerance to most common abiotic stresses, such as drought, salinity, or extreme temperatures seems to be fairly well recognized, not many authors considered that changes in ABA content may also influence the sensitivity of cereals to adverse environmental factors, e.g., by accelerating senescence, lowering pollen fertility, and inducing seed dormancy. Moreover, recently, ABA has also been regarded as an element of the biotic stress response; however, its role is still highly unclear. Many studies connect the susceptibility to various diseases with increased concentration of this phytohormone. Therefore, in contrast to the original assumptions, the role of ABA in response to biotic and abiotic stress does not always have to be associated with survival mechanisms; on the contrary, in some cases, abscisic acid can be one of the factors that increases the susceptibility of plants to adverse biotic and abiotic environmental factors.