Seed Shattering: A Trait of Evolutionary Importance in Plants

Comparative histology of abscission zones reveals the extent of convergence and divergence in seed shattering in weedy and cultivated rice

The modification of seed shattering has been a recurring theme in rice evolution. The wild ancestor of cultivated rice disperses its seeds, but reduced shattering was selected during multiple domestication events to facilitate harvesting. Conversely, selection for increased shattering occurred during the evolution of weedy rice, a weed invading cultivated rice fields that has originated multiple times from domesticated ancestors. Shattering requires formation of a tissue known as the abscission zone (AZ), but how the AZ has been modified throughout rice evolution is unclear. We quantitatively characterized the AZ characteristics of relative length, discontinuity, and intensity in 86 cultivated and weedy rice accessions. We reconstructed AZ evolutionary trajectories and determined the degree of convergence among different cultivated varieties and among independent weedy rice populations. AZ relative length emerged as the best feature to distinguish high and low shattering rice. Cultivated varieties differed in average AZ morphology, revealing lack of convergence in how shattering reduction was achieved during domestication. In contrast, weedy rice populations typically converged on complete AZs, irrespective of origin. By examining AZ population-level morphology, our study reveals its evolutionary plasticity, and suggests that the genetic potential to modify the ecologically and agronomically important trait of shattering is plentiful in rice lineages.

Early silique-shedding wild radish (Raphanus raphanistrum L.) phenotypes persist in a long-term harvest weed seed control managed field in Western Australia

BACKGROUND

This study introduces a wild radish population collected from Yelbeni in the Western Australian grainbelt that evolved an early silique abscission (shedding) trait to persist despite long-term harvest weed seed control (HWSC) use. In 2017, field-collected seed (known herein as Yelbeni) was compared to surrounding ruderal and field-collected populations in a fully randomized common garden study.

RESULTS

The Yelbeni population exhibited a higher rate of silique abscission when compared to the ruderal populations collected from the site before wheat (Triticum aestivum L.) harvest (assessed at soft dough stage, Zadoks 83). A similar common garden study was conducted in the subsequent season (2018) using progeny reproduced on a single site without stress. The HWSC-selected progeny (Yelbeni P) shed 1048 (±288) siliques before wheat maturity at the soft dough stage (Zadoks 83) compared to 25 (±7) siliques from the pooled control populations. The Yelbeni P population only flowered 6 days earlier (FT50 as determined by log-logistic analysis) than pooled control populations, which is unlikely to fully account for the increased rate of silique abscission. The Yelbeni P population also located its lowest siliques below the lowest height for harvest interception (10 cm), which is likely to increase HWSC evasion. The mechanism inducing early silique-shedding is yet to be determined; however, wild radish is known for its significant genetic variability and has demonstrated its capacity to adapt to environmental and management stresses.

CONCLUSION

This study demonstrates that the repeated use of HWSC can lead to the selection of HWSC-avoidance traits including early silique-shedding before harvest and/or locating siliques below the harvest cutting height for interception. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Phenotypic screening of seed retention and histological analysis of the abscission zone in Festuca pratensis and Lolium perenne

BACKGROUND

Seed retention is the basic prerequisite for seed harvest. However, only little breeding progress has been achieved for this trait in the major forage grasses. The aim of this study was to evaluate the potential of plant genetic resources of the important fodder grasses Festuca pratensis Huds. and Lolium perenne L. as source for seed retention in the breeding process. Furthermore, the morphology of the abscission zone, where shattering occurs, was studied on the cell tissue level in different developmental stages of contrasting accessions.

RESULTS

150 and 286 accessions of Festuca pratensis and Lolium perenne were screened for seed retention, respectively. Contrasting accessions were selected to be tested in a second year. We found a great variation in seed retention in Festuca pratensis and Lolium perenne, ranging from 13 to 71% (average: 35%) and 12 to 94% (average: 49%), respectively, in the first year. Seed retention was generally lower in the second year. Cultivars were within the accessions with highest seed retention in Festuca pratensis, but had lower seed retention than ecotypes in Lolium perenne. Field-shattered seeds had a lower thousand grain weight than retained seeds. Cell layers of the abscission zone appeared already in early seed stages and were nested within each other in accessions with high seed retention, while there were two to three superimposed layers in accessions with low seed retention.

CONCLUSIONS

Plant genetic resources of Lolium perenne might be a valuable source for breeding varieties with high seed retention. However, simultaneous selection for high seed weight is necessary for developing successful commercial cultivars.

Volunteer Plants' Occurrence and the Environmental Adaptability of Genetically Modified Fodder Corn upon Unintentional Release into the Environment

The number of corn cultivars that have been improved using genetically modified technology continues to increase. However, concerns about the unintentional release of living-modified organisms (LMOs) into the environment still exist. Specifically, there are cases where LMO crops grown as fodder are released into the environment and form a volunteer plant community, which raises concerns about their safety. In this study, we analyzed the possibility of weediness and volunteer plants' occurrence when GMO fodder corn grains distributed in Korea are unintentionally released into the environment. Volunteer plants' occurrence was investigated by directly sowing grains in an untreated field. The results showed that the germination rate was extremely low, and even if a corn seed germinated, it could not grow into an adult plant and would die due to weed competition. In addition, the germination rate of edible and fodder grains was affected by temperature (it was high at 20 °C and 30 °C but low at 40 °C and extremely low at 10 °C), and it was higher in the former than in the latter. And the germination rate was higher in Daehakchal (edible corn grains) than in Gwangpyeongok (fodder corn grains). The environmental risk assessment data obtained in this study can be used for future evaluations of the weediness potential of crops and the development of volunteer plant suppression technology in response to unintentional GMO release.

Advances in Rice Seed Shattering

Seed shattering is an important trait that wild rice uses to adapt to the natural environment and maintain population reproduction, and weedy rice also uses it to compete with the rice crop. The loss of shattering is a key event in rice domestication. The degree of shattering is not only one of the main reasons for rice yield reduction but also affects its adaptability to modern mechanical harvesting methods. Therefore, it is important to cultivate rice varieties with a moderate shattering degree. In this paper, the research progress on rice seed shattering in recent years is reviewed, including the physiological basis, morphological and anatomical characteristics of rice seed shattering, inheritance and QTL/gene mapping of rice seed shattering, the molecular mechanism regulating rice seed shattering, the application of seed-shattering genes, and the relationship between seed-shattering genes and domestication.

Reducing crop losses by gene-editing control of organ developmental physiology

Some physiological processes in reproductive organs, if not controlled, can lead to crop loss even in the absence of environmental stress. These processes may occur pre- or post- harvest, and in diverse species and include abscission processes in cereal grain, e.g., shattering and in immature fruit, e.g., preharvest drop, preharvest sprouting of cereals, and postharvest senescence in fruit. Some of the molecular mechanisms and genetic determinants underlying these processes are now better detailed, making it possible to refine them by gene editing. Here, we discuss using advanced genomics to identify genetic determinants underlying crop physiological traits. Examples of improved phenotypes developed for preharvest problems are provided, and suggestions for reducing postharvest fruit losses by gene and promoter editing were made.

Can the Wild Perennial, Rhizomatous Rice Species Oryza longistaminata be a Candidate for De Novo Domestication?

As climate change intensifies, the development of resilient rice that can tolerate abiotic stresses is urgently needed. In nature, many wild plants have evolved a variety of mechanisms to protect themselves from environmental stresses. Wild relatives of rice may have abundant and virtually untapped genetic diversity and are an essential source of germplasm for the improvement of abiotic stress tolerance in cultivated rice. Unfortunately, the barriers of traditional breeding approaches, such as backcrossing and transgenesis, make it challenging and complex to transfer the underlying resilience traits between plants. However, de novo domestication via genome editing is a quick approach to produce rice with high yields from orphans or wild relatives. African wild rice, Oryza longistaminata, which is part of the AA-genome Oryza species has two types of propagation strategies viz. vegetative propagation via rhizome and seed propagation. It also shows tolerance to multiple types of abiotic stress, and therefore O. longistaminata is considered a key candidate of wild rice for heat, drought, and salinity tolerance, and it is also resistant to lodging. Importantly, O. longistaminata is perennial and propagates also via rhizomes both of which are traits that are highly valuable for the sustainable production of rice. Therefore, O. longistaminata may be a good candidate for de novo domestication through genome editing to obtain rice that is more climate resilient than modern elite cultivars of O. sativa.

Shattering and yield expression of sesame (Sesamum indicum L) genotypes influenced by paclobutrazol concentration under rainfed conditions of Pothwar

Seed shattering is a critical challenge that significantly reduces sesame production by 50%. These shattering losses can be reduced by selecting shattering resistant genotypes or by incorporating modern agronomic management such as paclobutrazol, which can boost productivity and prevent seed shattering in sesame. Two-years of field trials were conducted to examine the effect of sesame genotypes, environment, and paclobutrazol (PBZ) concentrations. Twelve sesame genotypes were used in a four-way factorial RCBD with three replications and five PBZ concentrations (T₀ = Control; T₁ = 150; T₂ = 300; T₃ = 450; and T₄ = 600 mg L^- 1) under rainfed conditions of Pothwar. The findings revealed significant variations in the major effects of all examined variables (genotypes, locations, years, and PBZ levels). Sesame genotypes PI-154304 and PI-175907 had the highest plant height, number of capsule plant^- 1, seed capsule^- 1, 1000 seed weight, biological yield, and seed yield, while also having the lowest seed losses and shattering percentage. Regarding environments, NARC-Islamabad generated the highest plant height, number of capsule plant^- 1, shattering percentage, and biological yield; however, the URF-Koont produced the highest seed yield with the lowest shattering percentage. Additionally, plant height, capsules plant^- 1, and biological yield were higher in 2021, while seed capsule^- 1, 1000 seed weight, seed losses, shattering percentage, and seed yield were higher in 2020. PBZ concentration affected all measured parameters; plant height and number of seed capsule^- 1 decreased with increasing PBZ concentrations. 450 mg L^- 1 PBZ concentration generated the highest biomass, number of capsules plant^- 1, and seed yield. At the same time, PBZ concentration 600 mg L^- 1 generated the smallest plant, the lowest seed capsules^- 1, the greatest thousand seed weight, and the lowest shattering percentage. The study concluded that paclobutrazol could dramatically reduce shattering percentage and shattering losses while increasing economic returns through better productivity. Based on the findings, the genotypes PI-154304 and PI-175907 with paclobutrazol level 450 mgL^- 1 may be suggested for cultivation in Pothwar farming community under rainfed conditions, as they showed promising shattering resistance as well as enhanced growth and yield.

Agricultural weeds: the contribution of domesticated species to the origin and evolution of feral weeds

Agricultural weeds descended from domesticated ancestors, directly from crops (endoferality) and/or from crop-wild hybridization (exoferality), may have evolutionary advantages by rapidly acquiring traits pre-adapted to agricultural habitats. Understanding the role of crops on the origin and evolution of agricultural weeds is essential to develop more effective weed management programs, minimize crop losses due to weeds, and accurately assess the risks of cultivated genes escaping. In this review, we first describe relevant traits of weediness: shattering, seed dormancy, branching, early flowering and rapid growth, and their role in the feralization process. Furthermore, we discuss how the design of "super-crops" can affect weed evolution. We then searched for literature documenting cases of agricultural weeds descended from well-domesticated crops, and describe six case studies of feral weeds evolved from major crops: maize, radish, rapeseed, rice, sorghum, and sunflower. Further studies on the origin and evolution of feral weeds can improve our understanding of the physiological and genetic mechanisms underpinning the adaptation to agricultural habitats and may help to develop more effective weed-control practices and breeding better crops. © 2022 Society of Chemical Industry.

Contrasting Regulators of the Onset and End of the Seed Release Phenology of a Temperate Desert Shrub Nitraria tangutorum

Seed release is crucial in the reproductive cycle of many desert plant species, but their responses to precipitation changes are still unclear. To clarify the response patterns, we conducted a long-term in situ water addition experiment with five treatments, including natural precipitation (control) plus an extra 25%, 50%, 75%, and 100% of the local mean annual precipitation (145 mm), in a temperate desert in northwestern China. Both the onset and end of the seed release phenophase of the locally dominant shrub, Nitraria tangutorum, were observed from 2012 to 2018. The results showed that both the onset and end time of seed release, especially the end time, were significantly affected by water addition treatment. On average, the end time of seed release was advanced by 3.9 d, 7.3 d, 10.8 d, and 3.8 d under +25%, +50%, +75%, and +100% water addition treatments, respectively, over the seven-year study, compared with the control. The changes in the onset time were relatively small (only several hours), and the duration of seed release was shortened by 4.0 d, 7.5 d, 10.8 d, and 2.0 d under +25%, +50%, +75%, and +100% water addition treatments, respectively. The onset and end time of seed release varied greatly between the years. Preceding fruit ripening and summer temperature jointly regulated the inter-annual variation of the onset time of seed release, while the cumulative summer precipitation played a key role in driving the inter-annual variation of the end time. The annual mean temperature controlled the inter-annual variation of the seed release duration, and these interactions were all non-linear.

Vignette of Vigna domestication: From archives to genomics

The genus Vigna comprises fast-growing, diploid legumes, cultivated in tropical and subtropical parts of the world. It comprises more than 200 species among which Vigna angularis, Vigna radiata, Vigna mungo, Vigna aconitifolia, Vigna umbellata, Vigna unguiculata, and Vigna vexillata are of enormous agronomic importance. Human selection along with natural variability within these species encompasses a vital source for developing new varieties. The present review convokes the early domestication history of Vigna species based on archeological pieces of evidence and domestication-related traits (DRTs) together with genetics of domestication. Traces of early domestication of Vigna have been evidenced to spread across several temperate and tropical regions of Africa, Eastern Asia, and few parts of Europe. Several DRTs of Vigna species, such as pod shattering, pod and seed size, dormancy, seed coat, seed color, maturity, and pod dehiscence, can clearly differentiate wild species from their domesticates. With the advancement in next-generation high-throughput sequencing techniques, exploration of genetic variability using recently released reference genomes along with de novo sequencing of Vigna species have provided a framework to perform genome-wide association and functional studies to figure out different genes related to DRTs. In this review, genes and quantitative trait loci (QTLs) related to DRTs of different Vigna species have also been summarized. Information provided in this review will enhance the in-depth understanding of the selective pressures that causes crop domestication along with nature of evolutionary selection made in unexplored Vigna species. Furthermore, correlated archeological and domestication-related genetic evidence will facilitate Vigna species to be considered as suitable model plants.

miR319-Regulated TCP3 Modulates Silique Development Associated with Seed Shattering in Brassicaceae

Seed shattering is an undesirable trait that leads to crop yield loss. Improving silique resistance to shattering is critical for grain and oil crops. In this study, we found that miR319-targeted TEOSINTE BRANCHED 1, CYCLOIDEA, and PROLIFERATING CELL NUCLEAR ANTIGEN BINDING FACTOR (TCPs) inhibited the process of post-fertilized fruits (silique) elongation and dehiscence via regulation of FRUITFULL (FUL) expression in Arabidopsis thaliana and Brassica napus. AtMIR319a activation resulted in a longer silique with thickened and lignified replum, whereas overexpression of an miR319a-resistant version of AtTCP3 (mTCP3) led to a short silique with narrow and less lignified replum. Further genetic and expressional analysis suggested that FUL acted downstream of TCP3 to negatively regulate silique development. Moreover, hyper-activation of BnTCP3.A8, a B. napus homolog of AtTCP3, in rapeseed resulted in an enhanced silique resistance to shattering due to attenuated replum development. Taken together, our findings advance our knowledge of TCP-regulated silique development and provide a potential target for genetic manipulation to reduce silique shattering in Brassica crops.

Mechanism of pod shattering in the forage legume Medicago ruthenica

Pod shattering is a seed dispersal strategy and an important agronomical trait in domesticated crops. The relationship between pod shattering and pod morphology in the genus Medicago is well known; however, the detailed mechanism underlying pod dehiscence in Medicago ruthenica, a perennial legume used for forage production, is unknown. Here, the pod ventral sutures of shatter-resistant and shatter-susceptible M. ruthenica genotypes were examined at 8, 12, 16, and 20 d after flowering. The mechanism of pod shattering was analyzed through microscopic observations, polygalacturonase (PG) and cellulase (CE) activity analyses, and RNA-sequencing (RNA-Seq), and the results were verified via reverse transcriptase-quantitative polymerase chain reaction. Pod shattering at the ventral suture in M. ruthenica occurs via a combination of two mechanisms: degradation of the middle lamella at the abscission layers (ALs) and detachment of lignified cells on either side of the ALs triggered by physical forces. Increased PG and CE activities in the pod ventral suture are essential for AL cell-autolysis in the shatter-susceptible genotype. RNA-Seq revealed that 11 genes encoding PG and CE were highly expressed in the ventral sutures of the shatter-susceptible genotype. The expression levels of auxin biosynthesis-related genes decreased in the AL cells and they were negatively associated with pod dehiscence. These results enhance our understanding of the pod shattering mechanism not only in M. ruthenica but also in other leguminous plants.

Cannabis Seedlings Inherit Seed-Borne Bioactive and Anti-Fungal Endophytic Bacilli

Throughout the hundreds of millions of years of co-evolution, plants and microorganisms have established intricate symbiotic and pathogenic relationships. Microbial communities associated with plants are in constant flux and can ultimately determine whether a plant will successfully reproduce or be destroyed by their environment. Inheritance of beneficial microorganisms is an adaptation plants can use to protect germinating seeds against biotic and abiotic stresses as seedlings develop. The interest in Cannabis as a modern crop requires research into effective biocontrol of common fungal pathogens, an area that has seen little research. This study examines the seed-borne endophytes present across 15 accessions of Cannabis grown to seed across Western Canada. Both hemp and marijuana seedlings inherited a closely related group of bioactive endophytic Bacilli. All Cannabis accessions possessed seed-inherited Paenibacillus mobilis with the capacity to solubilize mineral phosphate. Additionally, seeds were found to carry genera of fungal isolates known to be Cannabis pathogens and post-harvest molds: Alternaria, Penicillium, Cladosporium, Chaetomium, Aspergillus, Rhizopus, and Fusarium. Thirteen seed-borne endophytes showed antibiotic activity against Alternaria, Aspergillus, Penicillium, and Fusarium. This study suggests both fungal pathogens and bacterial endophytes that antagonize them are vectored across generations in Cannabis as they compete over this shared niche.

Regulatory Modules Involved in the Degradation and Modification of Host Cell Walls During Cuscuta campestris Invasion

Haustoria of parasitic plants have evolved sophisticated traits to successfully infect host plants. The degradation and modification of host cell walls enable the haustorium to effectively invade host tissues. This study focused on two APETALA2/ETHYLENE RESPONSE FACTOR (ERF) genes and a set of the cell wall enzyme genes principally expressed during the haustorial invasion of Cuscuta campestris Yuncker. The orthogroups of the TF and cell wall enzyme genes have been implicated in the cell wall degradation and modification activities in the abscission of tomatoes, which are currently the phylogenetically closest non-parasitic model species of Cuscuta species. Although haustoria are generally thought to originate from root tissues, our results suggest that haustoria have further optimized invasion potential by recruiting regulatory modules from other biological processes.