The anatomy of abscission zones is diverse among grass species

Transcriptome analyses for revealing leaf abscission of Cyclocarya paliurus stem segments in vitro

Leaf abscission of Cyclocarya paliurus stem segments in vitro is very serious, and more than 90% of the leaves gradually fall off with prolonged culture time, which hinders breeding. This study investigated the molecular mechanism of leaf abscission. The emerged leaves of C. paliurus stem segments were cultured for 22 days (T0) in vitro; leaves at 27 days (T1) and leaves that had fallen after ≥ 32 days (T2) were used as materials for analysis of the physiological characteristics and transcriptome data. During the leaf abscission process of C. paliurus, the Indole-3-acetic acid (IAA) content gradually decreased, whereas the carotenoid, 1-aminocyclopropane-1-carboxylic acid (ACC) and lignin contents and pectinase and cellulase activities significantly increased; 1807 and 10,908 DEGs were obtained in T0 vs T1 and T1 vs T2, respectively. The plant hormone signal transduction pathway, phenylpropanoid biosynthesis pathway and flavonoid biosynthesis pathway were significantly enriched in the KEGG metabolic pathway analysis. The differential expression of related genes affected AUX and Ethylene (ETH) biosynthesis and signal transduction, lignin synthesis, ROS metabolism, leaf color changes. Weighted gene coexpression network analysis (WGCNA) identified 10 hub genes (U-box protein, ERF5, ERF109, ERF4, SAUR36, CML19, MYC2-like,SPHK1, TOE3, POD55) that interact to activate abscission signaling, which subsequently influences the genes expression involved in the biosynthesis and signal transduction of auxin and ethylene; this resulted in an imbalance of endogenous hormone levels in the leaves, leading to the upregulation of pectinase, cellulase, and lignin biosynthesis genes and acceleration of the rupture of the abscission zone (AZ) cell and vascular cell wall, which ultimately led to leaf abscission. The present study illustrates a regulatory mechanism of leaf abscission of C. paliurus stem segments in vitro, which provides potential application value for guiding the inhibition of leaf abscission in vitro.

Histological Staining and Hydrogen Peroxide Visualization of the Abscission Zone in Setaria viridis

The abscission zone (AZ) consists of specialized cell layers where cell separation or breakage occurs that result in organ detachment. Microscopic observation of the AZ is crucial for understanding its function. The AZ undergoes cellular and physiological changes prior to abscission, such as cell death, loss of chlorophyll, and the production of reactive oxygen species (ROS). These changes can be visualized using specific dyes and indicators under light or fluorescent microscopes. However, one challenge of using these dyes is their inefficient penetration into the tissue, especially when the epidermal layer has thick secondary cell walls. In this chapter, a detailed protocol to overcome this challenge is described. Using the fruit AZ of Setaria viridis, in which the epidermal cell wall is thick and lignified, we gently fix the dissected tissue, embed it in the Cryomatrix, and trim off the outer cell layers using a cryostat. The tissue with exposed inner cells can then be stained with fluorescent dyes to visualize organelles of interest, or 3,3'-diaminobenzidine (DAB) to visualize hydrogen peroxide accumulated in the tissue.

Transcriptome Analysis of the Seed Shattering Mechanism in Psathyrostachys juncea Using Full-Length Transcriptome Sequencing

Seed shattering (SS) functions are a survival mechanism in plants, enabling them to withstand adverse environmental conditions and ensure reproduction. However, this trait limits seed yield. Psathyrostachys juncea, a perennial forage grass with many favorable traits, is constrained by SS, limiting its broader application. To investigate the mechanisms underlying SS, second-generation Illumina sequencing and third-generation PacBio sequencing were conducted on abscission zone tissues of P. juncea at 7, 14, 21, and 28 days after heading. GO enrichment analysis identified several significant biological processes, including the "cell wall macromolecule catabolic process", "cell wall polysaccharide catabolic process", "hemicellulose catabolic process", and "xylan catabolic process", all involved in cell wall degradation. KEGG enrichment analysis showed that differentially expressed genes were predominantly enriched in pathways related to "starch and sucrose metabolism", "fructose and mannose metabolism", "phenylpropanoid biosynthesis", "pentose and glucuronate interconversions", and "galactose metabolism", each linked to both the synthesis and degradation of the cell wall. Further analysis of the "starch and sucrose metabolism" pathway revealed genes encoding fructokinase, hexokinase, β-glucosidase, sucrose phosphate synthase, sucrose synthase, and endoglucanase, all of which affected cellulose content. Reduced cellulose content can alter cell wall structure, leading to SS. These findings provide new insights into the regulation of SS in P. juncea and offer valuable references for other species within the Poaceae family.

Phytohormonal dynamics in the abscission zone of Korla fragrant pear during calyx abscission: a visual study

Introduction

Phytohormones play a crucial role in regulating the abscission of plant organs and tissues.

Methods

In this study, the ultrastructure of the sepals of Korla fragrant pears was observed using a transmission electron microscope, and high-performance liquid and gas chromatography were used to analyze the dynamic changes of phytohormones in the abscission zone during the calyx abscission process of Korla fragrant pears, and mass spectrometry imaging was applied to ascertain the spatial distribution of phytohormones.

Results

The results revealed that the mitochondria in the abscission zone of the decalyx fruits were regularly distributed around the cell wall, and the chloroplasts were moderately present. In contrast, in the persistent calyx fruit, the corresponding parts of the abscission zone showed a scattered distribution of mitochondria within the cells, and there was a higher number of chloroplasts, which also contained starch granules inside. Mass spectrometry imaging revealed that ABA was enriched in the abscission zone of the decalyx fruit, and their ionic signal intensities were significantly stronger than those of the persistent calyx fruit. However, the ionic signal intensities of Indole-3-acetic acid (IAA) and Gibberellin A3 (GA3) of the persistent calyx fruit were significantly stronger than those in the abscission zone of the decalyx fruit and were concentrated in the persistent calyx fruit. 1-Aminocyclopropanecarboxylic Acid (ACC) did not show distinct regional distribution in both the decalyx and persistent calyx fruits. Furthermore, before the formation of the abscission zone, the levels of IAA, GA3, and zeatin (ZT) in the abscission zone of the decalyx fruits were significantly lower than those in the persistent calyx fruits by 37.9%, 57.7%, and 33.0%, respectively, while the levels of abscisic acid (ABA) and ethylene (ETH) were significantly higher by 21.9% and 25.0%, respectively. During the formation of the abscission zone, the levels of IAA, GA3, and ZT in the abscission zone of the decalyx fruits were significantly lower than those in the persistent calyx fruits by 41.7%, 71.7%, and 24.6%, respectively, while the levels of ABA and ETH were significantly higher by 15.2% and 80.0%, respectively. After the formation of the abscission zone, the levels of IAA and GA3 in the abscission zone of the decalyx fruits were lower than those in the persistent calyx fruits by 20.8% and 47.8%, respectively, while the levels of ABA and ETH were higher by 271.8% and 26.9%, respectively. In summary, during the calyx abscission process of Korla fragrant pears, IAA and GA3 in the abscission zone inhibited abscission, while ABA and ETH promoted calyx abscission. These research findings enrich the understanding of the regulatory mechanism of plant hormones on calyx abscission and provide a theoretical basis for the study of exogenous plant growth regulators for regulating calyx abscission in Korla fragrant pear.

Multifaceted mechanisms controlling grain disarticulation in the Poaceae

Cereal shattering and threshability, both involving disarticulation of grains from the mother plant, are important traits for cereal domestication and improvement. Recent studies highlighted diverse mechanisms influencing shattering and threshability, either through development of the disarticulation zone or floral structures enclosing or supporting the disarticulation unit. Differential lignification in the disarticulation zone is essential for rice shattering but not required for many other grasses. During shattering, the disarticulation zone undergoes either abscission leading to cell separation or cell breakage. Threshability can be affected by the morphology and toughness of the enclosing floral structures, and in some species, by the inherent weakness of the disarticulation zone. Fine-tuning shattering and threshability is essential for breeding wild and less domesticated cereals.

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.

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.

Melica as an emerging model system for comparative studies in temperate Pooideae grasses

BACKGROUND AND AIMS

Pooideae grasses contain some of the world's most important crop and forage species. Although much work has been conducted on understanding the genetic basis of trait diversification within a few annual Pooideae, comparative studies at the subfamily level are limited by a lack of perennial models outside 'core' Pooideae. We argue for development of the perennial non-core genus Melica as an additional model for Pooideae, and provide foundational data regarding the group's biogeography and history of character evolution.

METHODS

Supplementing available ITS and ndhF sequence data, we built a preliminary Bayesian-based Melica phylogeny, and used it to understand how the genus has diversified in relation to geography, climate and trait variation surveyed from various floras. We also determine biomass accumulation under controlled conditions for Melica species collected across different latitudes and compare inflorescence development across two taxa for which whole genome data are forthcoming.

KEY RESULTS

Our phylogenetic analyses reveal three strongly supported geographically structured Melica clades that are distinct from previously hypothesized subtribes. Despite less geographical affinity between clades, the two sister 'Ciliata' and 'Imperfecta' clades segregate from the more phylogenetically distant 'Nutans' clade in thermal climate variables and precipitation seasonality, with the 'Imperfecta' clade showing the highest levels of trait variation. Growth rates across Melica are positively correlated with latitude of origin. Variation in inflorescence morphology appears to be explained largely through differences in secondary branch distance, phyllotaxy and number of spikelets per secondary branch.

CONCLUSIONS

The data presented here and in previous studies suggest that Melica possesses many of the necessary features to be developed as an additional model for Pooideae grasses, including a relatively fast generation time, perenniality, and interesting variation in physiology and morphology. The next step will be to generate a genome-based phylogeny and transformation tools for functional analyses.

Gibberellin signaling regulates lignin biosynthesis to modulate rice seed shattering

The elimination of seed shattering was a key step in rice (Oryza sativa) domestication. In this paper, we show that increasing the gibberellic acid (GA) content or response in the abscission region enhanced seed shattering in rice. We demonstrate that SLENDER RICE1 (SLR1), the key repressor of GA signaling, could physically interact with the rice seed shattering-related transcription factors quantitative trait locus of seed shattering on chromosome 1 (qSH1), O. sativa HOMEOBOX 15 (OSH15), and SUPERNUMERARY BRACT (SNB). Importantly, these physical interactions interfered with the direct binding of these three regulators to the lignin biosynthesis gene 4-COUMARATE: COENZYME A LIGASE 3 (4CL3), thereby derepressing its expression. Derepression of 4CL3 led to increased lignin deposition in the abscission region, causing reduced rice seed shattering. Importantly, we also show that modulating GA content could alter the degree of seed shattering to increase harvest efficiency. Our results reveal that the "Green Revolution" phytohormone GA is important for regulating rice seed shattering, and we provide an applicable breeding strategy for high-efficiency rice harvesting.

The YABBY gene SHATTERING1 controls activation rather than patterning of the abscission zone in Setaria viridis

Abscission is predetermined in specialized cell layers called the abscission zone (AZ) and activated by developmental or environmental signals. In the grass family, most identified AZ genes regulate AZ anatomy, which differs among lineages. A YABBY transcription factor, SHATTERING1 (SH1), is a domestication gene regulating abscission in multiple cereals, including rice and Setaria. In rice, SH1 inhibits lignification specifically in the AZ. However, the AZ of Setaria is nonlignified throughout, raising the question of how SH1 functions in species without lignification. Crispr-Cas9 knockout mutants of SH1 were generated in Setaria viridis and characterized with histology, cell wall and auxin immunofluorescence, transmission electron microscopy, hormonal treatment and RNA-Seq analysis. The sh1 mutant lacks shattering, as expected. No differences in cell anatomy or cell wall components including lignin were observed between sh1 and the wild-type (WT) until abscission occurs. Chloroplasts degenerated in the AZ of WT before abscission, but degeneration was suppressed by auxin treatment. Auxin distribution and expression of auxin-related genes differed between WT and sh1, with the signal of an antibody to auxin detected in the sh1 chloroplast. SH1 in Setaria is required for activation of abscission through auxin signaling, which is not reported in other grass species.

Grain shattering by cell death and fracture in Eragrostis tef

Abscission, known as shattering in crop species, is a highly regulated process by which plants shed parts. Although shattering has been studied extensively in cereals and a number of regulatory genes have been identified, much diversity in the process remains to be discovered. Teff (Eragrostis tef) is a crop native to Ethiopia that is potentially highly valuable worldwide for its nutritious grain and drought tolerance. Previous work has suggested that grain shattering in Eragrostis might have little in common with other cereals. In this study, we characterize the anatomy, cellular structure, and gene regulatory control of the abscission zone (AZ) in E. tef. We show that the AZ of E. tef is a narrow stalk below the caryopsis, which is common in Eragrostis species. X-ray microscopy, scanning electron microscopy, transmission electron microscopy, and immunolocalization of cell wall components showed that the AZ cells are thin walled and break open along with programmed cell death (PCD) at seed maturity, rather than separating between cells as in other studied species. Knockout of YABBY2/SHATTERING1, documented to control abscission in several cereals, had no effect on abscission or AZ structure in E. tef. RNA sequencing analysis showed that genes related to PCD and cell wall modification are enriched in the AZ at the early seed maturity stage. These data show that E. tef drops its seeds using a unique mechanism. Our results provide the groundwork for understanding grain shattering in Eragrostis and further improvement of shattering in E. tef.

Floral organ abscission in Arabidopsis requires the combined activities of three TALE homeodomain transcription factors

Floral organ abscission is a separation process in which sepals, petals, and stamens detach from the plant at abscission zones. Here, we investigated the collective role of three amino-acid-loop-extension (TALE) homeobox genes ARABIDOPSIS THALIANA HOMEOBOX GENE1 (ATH1), KNAT6 (for KNOTTED LIKE from Arabidopsis thaliana) and KNAT2, which form a module that patterns boundaries under the regulation of BLADE-ON-PETIOLE 1 and 2 (BOP1/2) co-activators. These TALE homeodomain transcription factors were shown to maintain boundaries in the flower, functioning as a unit to coordinate the growth, patterning, and activity of abscission zones. Together with BOP1 and BOP2, ATH1 and its partners KNAT6 and KNAT2 collectively contribute to the differentiation of lignified and separation layers of the abscission zone. The genetic interactions of BOP1/2 and ATH1 with INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) were also explored. We showed that BOP1/2 co-activators and ATH1 converge with the IDA signalling pathway to promote KNAT6 and KNAT2 expression in the abscission zone and cell separation. ATH1 acts as a central regulator in floral organ abscission as it controls the expression of other TALE genes in abscission zone cells.

Molecular, cellular, and developmental foundations of grass diversity

Humans have cultivated grasses for food, feed, beverages, and construction materials for millennia. Grasses also dominate the landscape in vast parts of the world, where they have adapted morphologically and physiologically, diversifying to form ~12,000 species. Sequences of hundreds of grass genomes show that they are essentially collinear; nonetheless, not all species have the same complement of genes. Here, we focus on the molecular, cellular, and developmental bases of grain yield and dispersal-traits that are essential for domestication. Distinct genes, networks, and pathways were selected in different crop species, reflecting underlying genomic diversity. With increasing genomic resources becoming available in nondomesticated species, we anticipate advances in coming years that illuminate the ecological and economic success of the grasses.

Morphology and Anatomy of Branch-Branch Junctions in Opuntia ficus-indica and Cylindropuntia bigelovii: A Comparative Study Supported by Mechanical Tissue Quantification

The Opuntioideae include iconic cacti whose lateral branch-branch junctions are intriguing objects from a mechanical viewpoint. We have compared Opuntia ficus-indica, which has stable branch connections, with Cylindropuntia bigelovii, whose side branches abscise under slight mechanical stress. To determine the underlying structures and mechanical characteristics of these stable versus shedding cacti junctions, we conducted magnetic resonance imaging, morphometric and anatomical analyses of the branches and tensile tests of individual tissues. The comparison revealed differences in geometry, shape and material properties as follows: (i) a more pronounced tapering of the cross-sectional area towards the junctions supports the abscission of young branches of C. bigelovii. (ii) Older branches of O. ficus-indica form, initially around the branch-branch junctions, collar-shaped periderm tissue. This secondary coverage mechanically stiffens the dermal tissue, giving a threefold increase in strength and a tenfold increase in the elastic modulus compared with the epidermis. (iii) An approximately 200-fold higher elastic modulus of the vascular bundles of O. ficus-indica is a prerequisite for the stable junction of its young branches. Our results provide, for both biological and engineered materials systems, important insights into the geometric characteristics and mechanical properties of branching joints that are either stable or easily detachable.

The rachis cannot hold, plants fall apart. A commentary on: 'The unique disarticulation layer formed in the rachis of Aegilops longissima likely results from the spatial co-expression of Btr1 and Btr2'

This article comments on: Xiaoxue Zeng, Gang Chen, Lei Wang, Akemi Tagiri, Shinji Kikuchi, Hidenori Sassa and Takao Komatsuda, The unique disarticulation layer formed in the rachis of Aegilops longissima probably results from the spatial co-expression of Btr1 and Btr2, Annals of Botany, Volume 127, Issue 3, 16 February 2021, Pages 297–304, https://doi.org/10.1093/aob/mcaa147

The Ectopic Expression of Btr2 in Aegilops tauschii Switches the Disarticulation Layer From Above to Below the Rachis Node

Seed dispersal among wild species belonging to the tribe Triticeae is typically achieved by the formation of a brittle rachis. The trait relies on the development of a disarticulation layer, most frequently above the rachis node (resulting in wedge type dispersal units), but in some species below the rachis node (resulting in barrel type dispersal units). The genes responsible for the former type are the complementary pair Btr1 and Btr2, while the genetic basis of the latter type has yet to be determined. Aegilops tauschii forms barrel type dispersal units and previous study showed this species lacked an intact copy of Btr1. Here it has been demonstrated that Ae. tauschii carries two of Btr2; and that Btr2 transcript is present in a region below the rachis node where the abscission zone forms. The implication is that in this species, the Btr2 product is involved in the formation of barrel type dispersal units.