Seed dormancy and ABA signaling: the breakthrough goes on

Hormonal regulation and crosstalk during early endosperm and seed coat development

KEY MESSAGE

This review covers the latest developments on the regulation of early seed development by phytohormones. The development of seeds in flowering plants starts with the fertilization of the maternal gametes by two paternal sperm cells. This leads to the formation of two products, embryo and endosperm, which are surrounded by a tissue of maternal sporophytic origin, called the seed coat. The development of each of these structures is under tight genetic control. Moreover, several phytohormones have been shown to modulate the development of all three seed compartments and have been implicated in the communication between them. This is particularly relevant, as embryo, endosperm, and seed coat have to coordinate their development for successful seed formation. Here, we review the latest advances on the hormonal regulation of early seed development in the model plant species Arabidopsis thaliana, with a focus on the endosperm and the seed coat. Moreover, we highlight how phytohormones serve as mechanisms of non-cell autonomous communication between these two compartments and how they are determinant in shaping seed formation.

Unraveling site-specific seed formation abnormalities in Picea neoveitchii Mast. trees via widely metabolomic and transcriptomic analysis

Picea neoveitchii Mast. is a rare and threatened species of evergreen coniferous tree in China, commonly facing issues such as damaged seeds, abnormal seed growth, and empty seed shells. These abnormalities vary by location; unfortunately, the reasons behind these inconsistencies are completely unknown. This study compared seeds from two 150-year-old trees located in Taibai (Shaanxi province, TB150) and Zhouqu (Gansu province, ZQ150). The results showed significant differences in 43 metabolites and hormone levels, with higher levels of indole-3-acetic acid (IAA), methyl jasmonate (MeJA), and brassinosteroid (BR) in ZQ150, which were associated with more viable seeds. In contrast, TB150 exhibited more damaged seeds and empty seed shells due to higher abscisic acid (ABA) levels. Moreover, to further investigate these inconsistencies, we performed de-novo transcriptomic assembly and functional annotation of unigenes using high-throughput sequencing. A total of 2,355 differentially expressed unigenes were identified between TB150 and ZQ150, with 1,280 upregulated and 1,075 downregulated. Hormone signaling and sugar metabolism-related unigenes were further examined for their role in seed development. ZQ150 increased the number of normal seeds by enhancing endogenous IAA levels and upregulating auxin signaling and sugar metabolism-related genes. Conversely, TB150 showed more empty seed shells, correlated with elevated ABA levels and the activation of ABA signaling genes. We hypothesize that enhanced IAA levels and the upregulation of sugar metabolism and auxin signaling genes promote normal seed development.

REPRESSOR OF UV-B PHOTOMORPHOGENESIS proteins target ABSCISIC ACID INSENSITIVE 5 for degradation to promote early plant development

REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and REPRESSOR OF UV-B PHOTOMORPHOGENESIS 2 (RUP2) are WD-40 domain-containing proteins that have been extensively characterized for their role in UV-B signaling. However, the roles of the RUP proteins outside the canonical UV-signaling pathway are less known. Here, we identify that RUP1 and RUP2 play important roles in ABA signaling to regulate seed germination and early seedling development in Arabidopsis thaliana. Our protein interaction studies confirmed that RUP1 and RUP2 physically interact with ABA INSENSITIVE 5 (ABI5). In the presence of abscisic acid, rup1, rup2, and rup1rup2 exhibited reduced germination and seedling establishment compared with the wild type. Germination and seedling establishment in rup1rup2abi5-8 were similar to abi5-8, suggesting that RUP1 and RUP2 suppress ABA-mediated inhibition of germination and early seedling development in an ABI5-dependent manner. The DDB1-binding WD40 protein RUP2 promoted the ubiquitination of ABI5 to regulate its degradation. ABI5, in turn, establishes a negative feedback loop to inhibit the expression of RUP1/RUP2. ABI5 also inhibited the direct binding of ELONGATED HYPOCOTYL 5 (HY5) to the promoters of RUP1 and RUP2 under ABA. This study highlights the coordinated action of RUP1, RUP2, ABI5, and HY5 in regulating early plant development.

Myosin-binding protein 13 mediates primary seed dormancy via abscisic acid biosynthesis and signaling in Arabidopsis

Dormancy is an essential characteristic that enables seeds to survive in unfavorable conditions while germinating when conditions are favorable. Myosin-binding proteins (MyoBs) assist in the movement of organelles along actin microfilaments by attaching to both organelles and myosins. In contrast to studies on yeast and metazoans, research on plant MyoBs is still in its early stages and primarily focuses on tip-growing cells. In this study, we found that Arabidopsis MyoB13 is highly expressed in dry mature seeds. The myob13 mutant, created using CRISPR/Cas9, exhibits a preharvest sprouting phenotype, which can be mitigated by after-ripening treatment, indicating that MyoB13 plays a positive role in primary seed dormancy. Furthermore, we show that MyoB13 negatively regulates ABA biosynthesis and signaling pathways. Notably, the expression of MyoB13 orthologs from maize and soybean can completely restore the phenotype of the Arabidopsis myob13 mutant, suggesting that the function of MyoB13 in ABA-induced seed dormancy is evolutionarily conserved. Therefore, the functional characterization of MyoB13 offers an additional genetic resource to help prevent vivipary in crop species.

From Regulation to Application: The Role of Abscisic Acid in Seed and Fruit Development and Agronomic Production Strategies

Abscisic acid (ABA) is a crucial plant hormone that plays a decisive role in regulating seed and fruit development and is becoming increasingly important in agricultural applications. This article delves into ABA's regulatory functions in plant growth, particularly during the stages of seed and fruit development. In the seed phase, elevated ABA levels help maintain seed dormancy, aiding seed survival under unfavorable conditions. During fruit development, ABA regulates pigment synthesis and sugar accumulation, influencing the nutritional value and market quality of the fruit. This article highlights three main strategies for applying ABA in agricultural production: the use of ABA analogs, the development of ABA signal modulators, and breeding techniques based on ABA signaling. ABA analogs can mimic the natural functions of ABA, while ABA signal modulators, including enhancers and inhibitors, are used to finely tune plant responses to ABA, optimizing crop performance under specific growth conditions. Furthermore, breeding strategies based on ABA signaling aim to select crop varieties that effectively utilize ABA pathways through genetic engineering and other technologies. ABA is not only a key regulator of plant growth and development but also holds great potential for modern agricultural practices.

ABSCISIC ACID INSENSITIVE 3 promotes auxin signalling by regulating SHY2 expression to control primary root growth in response to dehydration stress

Plants combat dehydration stress through different strategies including root architectural changes. Here we show that when exposed to varying levels of dehydration stress, primary root growth in Arabidopsis is modulated by regulating root meristem activity. Abscisic acid (ABA) in concert with auxin signalling adjust primary root growth according to stress levels. ABSCISIC ACID INSENSITIVE 3 (ABI3), an ABA-responsive transcription factor, stands at the intersection of ABA and auxin signalling and fine-tunes primary root growth in response to dehydration stress. Under low ABA or dehydration stress, induction of ABI3 expression promotes auxin signalling by decreasing expression of SHY2, a negative regulator of auxin response. This further enhances the expression of auxin transporter gene PIN1 and cell cycle gene CYCB1;1, resulting in an increase in primary root meristem size and root length. Higher levels of dehydration stress or ABA repress ABI3 expression and promote ABSCISIC ACID INSENSITIVE 5 (ABI5) expression. This elevates SHY2 expression, thereby impairing primary root meristem activity and retarding root growth. Notably, ABI5 can promote SHY2 expression only in the absence of ABI3. Such ABA concentration-dependent expression of ABI3 therefore functions as a regulatory sensor of dehydration stress levels and orchestrates primary root growth by coordinating its downstream regulation.

Long days induce adaptive secondary dormancy in the seeds of the Mediterranean plant Aethionema arabicum

Secondary dormancy is an adaptive trait that increases reproductive success by aligning seed germination with permissive conditions for seedling establishment. Aethionema arabicum is an annual plant and member of the Brassicaceae that grows in environments characterized by hot and dry summers. Aethionema arabicum seeds may germinate in early spring when seedling establishment is permissible. We demonstrate that long-day light regimes induce secondary dormancy in the seeds of Aethionema arabicum (CYP accession), repressing germination in summer when seedling establishment is riskier. Characterization of mutants screened for defective secondary dormancy demonstrated that RGL2 mediates repression of genes involved in gibberellin (GA) signaling. Exposure to high temperature alleviates secondary dormancy, restoring germination potential. These data are consistent with the hypothesis that long-day-induced secondary dormancy and its alleviation by high temperatures may be part of an adaptive response limiting germination to conditions permissive for seedling establishment in spring and autumn.

Two genes encoding caffeoyl coenzyme A O-methyltransferase 1 (CCoAOMT1) are candidate genes for physical seed dormancy in cowpea (Vigna unguiculata (L.) Walp.)

KEY MESSAGE

The major QTL Sdp1.1+ controlling seed dormancy in cowpea was finely mapped, and two CCoAOMT1 genes were identified as candidate genes for the dormancy. Seed dormancy in wild cowpea may be useful in breeding cultivated cowpea with pre-harvest sprouting resistance. A previous study identified a major quantitative trait locus (QTL) for seed dormancy, Sdp1.1+ , using the population of the cross between cultivated cowpea 'JP81610' and wild cowpea 'JP89083.' However, the molecular basis of seed dormancy in cowpea is not yet known. In this study, we aimed to finely map the locus Sdp1.1+ and identify candidate gene(s) for it. Germination tests demonstrated that the seed coat is the major factor controlling seed dormancy in the wild cowpea JP89083. Microscopic observations revealed that wild cowpea seeds, unlike cultivated cowpea seeds, possessed a palisade cuticle layer. Fine mapping using a large F2 population of the cross JP81610 × JP89083 grown in Thailand revealed a single QTL, Sdp1.1+ , controlling seed dormancy. The Sdp1.1+ was confirmed using a small F2 population of the same cross grown in Japan. The Sdp1.1+ was mapped to a 37.34-Kb region containing three genes. Two closely linked genes, Vigun03g278900 (VuCCoAOMT1a) and Vigun03g290000 (VuCCoAOMT1b), located 4.844 Kb apart were considered as candidate genes for seed dormancy. The two genes encoded caffeoyl coenzyme A O-methyltransferase 1 (CCoAOMT1). DNA sequencing and alignment of VuCCoAOMT1a and VuCCoAOMT1b between JP89083 and JP81610 revealed a single nucleotide polymorphism (SNP) causing an amino acid change in VuCCoAOMT1a and several SNPs leading to six amino acid changes in VuCCoAOMT1b. Altogether, these results indicate that VuCCoAOMT1a and VuCCoAOMT1b are candidate genes controlling physical seed dormancy in the wild cowpea JP89083.

KAR1-induced dormancy release in Avena fatua caryopses involves reduction of caryopsis sensitivity to ABA and ABA/GAs ratio in coleorhiza and radicle

MAIN CONCLUSION

The dormancy release by KAR1 is associated with a reduction of coleorhiza and radicle sensitivity to ABA as well as with reduction the ABA/GAs ratio in the coleorhiza, by a decrease content of ABA, and in the radicle, by a decrease the ABA and an increase of the GAs contents. Both, karrikin 1 (KAR1) and gibberellin A3 (GA3), release dormancy in Avena fatua caryopses, resulting in the emergence of coleorhiza (CE) and radicle (RE). Moreover, KAR1 and GA3 stimulate CE and RE in the presence of abscisic acid (ABA), the stimulation being more effective in CE. The stimulatory effects of KAR1 and GA3 involve also the CE and RE rates. A similar effect was observed at KAR1 concentrations much lower than those of GA3. KAR1 increased the levels of bioactive GA5 and GA6 in embryos and the levels of GA1, GA5, GA3, GA6 and GA4 in radicles. The stimulatory effect of KAR1 on germination, associated with increased levels of gibberellins (GAs) and reduced levels of ABA in embryos, was counteracted by paclobutrazol (PAC), commonly regarded as a GAs biosynthesis inhibitor. Consequently, KAR1 decreased the ABA/GAs ratio, whereas PAC, used alone or in combination with KAR1, increased it. The ABA/GAs ratio was reduced by KAR1 in both coleorhiza and radicle, the effect being stronger in the latter. We present the first evidence that KAR1-induced dormancy release requires a decreased ABA/GAs ratio in coleorhiza and radicle. It is concluded that the dormancy-releasing effect of KAR1 in A. fatua caryopses includes (i) a reduction of the coleorhiza and radicle sensitivity to ABA, and (2) a reduction of the ABA/GAs ratio (i) in the coleorhiza, by decreasing the ABA content, and (ii) in the radicle, by decreasing the ABA and increasing the content GAs, particularly GA1. The results may suggest different mechanisms of dormancy release by KAR1 in monocot and dicot seeds.

Detection of locally adapted genomic regions in wild rice (Oryza rufipogon) using environmental association analysis

Oryza rufipogon is the wild progenitor of cultivated rice Oryza sativa and exhibits high levels of genetic diversity across its distribution, making it a useful resource for the identification of abiotic stress-tolerant varieties and genes that could limit future climate-changed-induced yield losses. To investigate local adaptation in O. rufipogon, we analyzed single nucleotide polymorphism (SNP) data from a panel of 286 samples located across a diverse range of climates. Environmental association analysis (EAA), a genome-wide association study (GWAS)-based method, was used and revealed 15 regions of the genome significantly associated with various climate factors. Genes within these environmentally associated regions have putative functions in abiotic stress response, phytohormone signaling, and the control of flowering time. This provides an insight into potential local adaptation in O. rufipogon and reveals possible locally adaptive genes that may provide opportunities for breeding novel rice varieties with climate change-resilient phenotypes.

A logical way to reprogram plants

Living cells constantly monitor their external and internal environments for changing conditions, stresses or developmental cues. Networks of genetically encoded components sense and process these signals following pre-defined rules in such a way that specific combinations of the presence or absence of certain signals activate suitable responses. Many biological signal integration mechanisms approximate Boolean logic operations, whereby presence or absence of signals are computed as variables with values described as either true or false, respectively. Boolean logic gates are commonly used in algebra and in computer sciences, and have long been recognized as useful information processing devices in electronic circuits. In these circuits, logic gates integrate multiple input values and produce an output signal according to pre-defined Boolean logic operations. Recent implementation of these logic operations using genetic components to process information in living cells has allowed genetic circuits to enable novel traits with decision-making capabilities. Although several literature reports describe the design and use of these logic gates to introduce new functions in bacterial, yeast and mammalian cells, similar approaches in plants remain scarce, likely due to challenges posed by the complexity of plants and the lack of some technological advances, e.g., species-independent genetic transformation. In this mini review, we have surveyed recent reports describing synthetic genetic Boolean logic operators in plants and the different gate architectures used. We also briefly discuss the potential of deploying these genetic devices in plants to bring to fruition a new generation of resilient crops and improved biomanufacturing platforms.

NO-mediated dormancy release of Avena fatua caryopses is associated with decrease in abscisic acid sensitivity, content and ABA/GAs ratios

MAIN CONCLUSION

NO releases caryopsis dormancy in Avena fatua, the effect being dependent on the level of dormancy. The NO effect involves also the reduction of caryopsis sensitivity to ABA and to a decrease in the ABA to GA_s ratio due to a decrease in ABA levels and the lack of effect on GA_s levels before germination is completed. Nitric oxide (NO) from various donors (i.e. SNP, GSNO and acidified KNO₂), applied to dry caryopses or during initial germination, released primary dormancy in caryopses. Dormancy in caryopses was gradually lost during dry storage (after-ripening) at 25 °C, enabling germination at 20 °C in the dark. The after-ripening effect is associated with a decrease in NO required for germination. In addition, NO decreased the sensitivity of dormant caryopses to exogenous abscisic acid (ABA) and decreased the embryos' ABA content before germination was completed. However, NO did not affect the content of bioactive gibberellins (GA_s) from non-13-hydroxylation (GA₄, GA₇) and 13-hydroxylation (GA₁, GA₃, GA_6.) pathways. Paclobutrazol (PAC), commonly regarded as a GA_s biosynthesis inhibitor, counteracted the dormancy-releasing effect of NO and did not affect the GA_s level; however, it increased the ABA content in embryos before germination was completed. Ascorbic acid, sodium benzoate and tiron, scavengers of reactive oxygen species (ROS), reduced the stimulatory effect of NO on caryopsis germination. This work provides new insight on the participation of NO in releasing A. fatua caryopses dormancy and on the relationship of NO with endogenous ABA and GA_s.

Transcriptomic insights into the effects of abscisic acid on the germination of Magnolia sieboldii K. Koch seed

Magnolia sieboldii K. Koch is a deciduous tree species. However, the wild resource of M. sieboldii has been declining due to excessive utilization and seed dormancy. In our previous research, M. sieboldii seeds have morphophysiological dormancy and low germination rates under natural conditions. The aim of the present study was to identify the genes involved in dormancy maintenance. In this study, the germination percentage of M. sieboldii seeds negatively correlated with the content of endogenous abscisic acid (ABA). The hydration of seeds for germination showed three distinct phases. Five key time points were identified: 0 h imbibition (dry seed, GZ), 0 day after imbibition (DAI), 16 DAI, 40 DAI, and 56 DAI. The comprehensive transcript profiles of M. sieboldii seeds treated with ABA and water at the five key germinating stages were obtained. A total of 9641 differentially expressed genes (DEGs) were identified, and 208 and 197 common DEGs were found throughout the ABA and water treatments, respectively. Compared with that in the GZ, 518, 696, 2133, and 1535 DEGs were identified in the SH group at 0, 16, 40 and 56 DAI, respectively. 666, 1725, 1560 and 1415 DEGs were identified in the ABA group at 0, 16, 40, and 56 DAI, respectively. Among the identified DEGs, 12 722 were annotated with GO terms, the top three enriched GO terms were different among the DEGs at 56 DAI in the ABA vs. SH treatments. KEGG pathway enrichment analysis for DEGs indicated that oxidative phosphorylation, protein processing in endoplasmic reticulum, starch and sucrose metabolism play an important role in seed response to ABA. 1926 TFs are obtained and classified into 72 families from the M. sieboldii transcriptome. Results of differential gene expression analysis together with qRT-PCR indicated that phase II is crucial for rapid and successful seed germination. This study is the first to present the global expression patterns of ABA-regulated transcripts in M. sieboldii seeds at different germinating phases.

Metabolites That Confirm Induction and Release of Dormancy Phases in Sweet Cherry Buds

Here we report on metabolites found in a targeted profiling of 'Summit' flower buds for nine years, which could be indicators for the timing of endodormancy release (t₁) and beginning of ontogenetic development (t₁*). Investigated metabolites included chrysin, arabonic acid, pentose acid, sucrose, abscisic acid (ABA), and abscisic acid glucose ester (ABA-GE). Chrysin and water content showed an almost parallel course between leaf fall and t₁*. After 'swollen bud', water content raised from ~60 to ~80% at open cluster, while chrysin content decreased and lost its function as an acetylcholinesterase inhibitor. Both parameters can be suitable indicators for t₁*. Arabonic acid showed a clear increase after t₁*. Pentose acid would be a suitable metabolite to identify t₁ and t₁*, but would not allow describing the ecodormancy phase, because of its continuously low value during this time. Sucrose reached a maximum during ecodormancy and showed a significant correlation with air temperature, which confirms its cryoprotective role in this phase. The ABA content showed maximum values during endodormancy and decreased during ecodormancy, reaching 50% of its content t₁ at t₁*. It appears to be the key metabolite to define the ecodormancy phase. The ABA-GE was present at all stages and phases and was much higher than the ABA content and is a readily available storage pool in cherry buds.

The ethylene receptor mutation etr2b reveals crosstalk between ethylene and ABA in the control of Cucurbita pepo germination

The enhanced salt tolerance of squash ethylene-insensitive mutants during germination and early stages of seedling development suggested that abscisic acid (ABA) could mediate this tolerance. To gain insight into the crosstalk between ethylene and ABA in seed germination, the germination rate and early seedling growth of wild type (WT) and ethylene-insensitive etr2b mutant were compared in seeds germinated under water and exogenous ABA treatment. The etr2b seeds germinated earlier than WT under both water and ABA, and the effect of ABA on radicle length and seedling growth of etr2b was lower than in WT, indicating that etr2b is also insensitive to ABA. The comparison of ABA and ethylene contents and ABA and ethylene gene expression profiles in WT and etr2b dry and imbibed seeds in either water, NaCl or ABA demonstrated a clear crosstalk between ethylene and ABA in germination. The expression profiles of ethylene genes in WT and etr2b indicated that the role of ethylene in seed germination does not appear to follow the canonical ethylene signaling pathway. Instead, etr2b reduces ABA content during formation of the seeds (dry seeds) and in response to seed imbibition and germination, which means diminished dormancy in the ethylene mutant. The etr2b mutation downregulated the expression of ABA biosynthesis and signaling genes during germination, demonstrating the positive role of ethylene receptor gene CpETR2B on seed germination and early seedling growth in squash is mediated by ABA. The reduced effect of exogenous ABA on ethylene production and ethylene gene expression in etr2b seeds suggests that this regulation is also dependent on ethylene.

The Re-Localization of Proteins to or Away from Membranes as an Effective Strategy for Regulating Stress Tolerance in Plants

The membranes of plant cells are dynamic structures composed of phospholipids and proteins. Proteins harboring phospholipid-binding domains or lipid ligands can localize to membranes. Stress perception can alter the subcellular localization of these proteins dynamically, causing them to either associate with or detach from membranes. The mechanisms behind the re-localization involve changes in the lipidation state of the proteins and interactions with membrane-associated biomolecules. The functional significance of such re-localization includes the regulation of molecular transport, cell integrity, protein folding, signaling, and gene expression. In this review, proteins that re-localize to or away from membranes upon abiotic and biotic stresses will be discussed in terms of the mechanisms involved and the functional significance of their re-localization. Knowledge of the re-localization mechanisms will facilitate research on increasing plant stress adaptability, while the study on re-localization of proteins upon stresses will further our understanding of stress adaptation strategies in plants.

Metabolic changes induced by Cuscuta campestris Yunck in the host species Artemisia campestris subsp. variabilis (Ten.) Greuter as a strategy for successful parasitisation

C. campestris parasitisation increases internal host defences at the expense of environmentally directed ones in the host species A. campestris, thus limiting plant defence against progressive parasitisation. Cuscuta campestris Yunck is a holoparasitic species that parasitises wild species and crops. Among their hosts, Artemisia campestris subsp. variabilis (Ten.) Greuter is significantly affected in natural ecosystems. Limited information is available on the host recognition mechanism and there are no data on the interactions between these species and the effects on the primary and specialised metabolism in response to parasitisation. The research aims at evaluating the effect of host-parasite interactions, through a GC-MS untargeted metabolomic analysis, chlorophyll a fluorescence, ionomic and δ¹³C measurements, as well as volatile organic compound (VOC) fingerprint in A. campestris leaves collected in natural environment. C. campestris parasitisation altered plant water status, forcing stomatal opening, stimulating plant transpiration, and inducing physical damages to the host antenna complex, thus reducing the efficiency of its photosynthetic machinery. Untargeted-metabolomics analysis highlighted that the parasitisation significantly perturbed the amino acids and sugar metabolism, inducing an increase in the production of osmoprotectants, which generally accumulate in plants as a protective strategy against oxidative stress. Notably, VOCs analysis highlighted a reduction in sesquiterpenoids and an increase in monoterpenoids levels; involved in plant defence and host recognition, respectively. Moreover, C. campestris induced in the host a reduction in 3-hexenyl-acetate, a metabolite with known repellent activity against Cuscuta spp. We offer evidences that C. campestris parasitisation increases internal host defences via primary metabolites at the expense of more effective defensive compounds (secondary metabolites), thus limiting A. campestris defence against progressive parasitisation.

Pre-Sowing Treatment of Tilia cordata Mill. by Seed Material Fermentation

The aim of the work was to verify whether the fermentation of seed material is a suitable pre-sowing treatment of small-leaved lime. Seed material of two origins was treated for a period of 7, 14 and 21 days using anaerobic fermentation and for 10, 20 and 30 days using aerobic fermentation, the control treatment being the seed after warm-cold stratification. The water content, germination capacity and abscisic acid level of the seed were evaluated, together with the emergence and morphological parameters of annual lime seedlings that had emerged from the tested seeds. It was found that fermented seed had a low abscisic acid level (up to 700 pmol/g), a higher germination capacity (approx. 90%) than mature seed without fermentation and the morphological parameters of the seedlings that had emerged from seed after fermentation reached higher values, compared to those of the seedlings that had emerged from seed after warm-cold stratification. The seed material had to be collected green (e.g., immature seeds) and moistened. A suitable anaerobic fermentation time is 7 days; the aerobic phase can last 10, 20 or 30 days. The fermentation of seed material can shorten the preparation of seed for sowing by 1 year.

Nano-priming as emerging seed priming technology for sustainable agriculture-recent developments and future perspectives

Nano-priming is an innovative seed priming technology that helps to improve seed germination, seed growth, and yield by providing resistance to various stresses in plants. Nano-priming is a considerably more effective method compared to all other seed priming methods. The salient features of nanoparticles (NPs) in seed priming are to develop electron exchange and enhanced surface reaction capabilities associated with various components of plant cells and tissues. Nano-priming induces the formation of nanopores in shoot and helps in the uptake of water absorption, activates reactive oxygen species (ROS)/antioxidant mechanisms in seeds, and forms hydroxyl radicals to loosen the walls of the cells and acts as an inducer for rapid hydrolysis of starch. It also induces the expression of aquaporin genes that are involved in the intake of water and also mediates H2O2, or ROS, dispersed over biological membranes. Nano-priming induces starch degradation via the stimulation of amylase, which results in the stimulation of seed germination. Nano-priming induces a mild ROS that acts as a primary signaling cue for various signaling cascade events that participate in secondary metabolite production and stress tolerance. This review provides details on the possible mechanisms by which nano-priming induces breaking seed dormancy, promotion of seed germination, and their impact on primary and secondary metabolite production. In addition, the use of nano-based fertilizer and pesticides as effective materials in nano-priming and plant growth development were also discussed, considering their recent status and future perspectives.

Multi-Approach Analysis Reveals Pathways of Cold Tolerance Divergence in Camellia japonica

Understanding the molecular mechanism of the cold response is critical to improve horticultural plant cold tolerance. Here, we documented the physiological, transcriptome, proteome, and hormonal dynamics to cold stress in temperate genotype (Tg) and subtropical genotype (Sg) populations of Camellia japonica. Tg C. japonica suffered minimal osmotic and oxidative damage compared to Sg C. japonica under the same cold treatment. Transcriptional and translational differences increased under the cold treatment, indicating that Tg C. japonica was affected by the environment and displayed both conserved and divergent mechanisms. About 60% of the genes responding to cold had similar dynamics in the two populations, but 1,896 transcripts and 455 proteins differentially accumulated in response to the cold between Tg and Sg C. japonica. Co-expression analysis showed that the ribosomal protein and genes related to photosynthesis were upregulated in Tg C. japonica, and tryptophan, phenylpropanoid, and flavonoid metabolism were regulated differently between the two populations under cold stress. The divergence of these genes reflected a difference in cold responsiveness. In addition, the decrease in the abscisic acid (ABA)/gibberellic acid (GA) ratio regulated by biosynthetic signal transduction pathway enhanced cold resistance in Tg C. japonica, suggesting that hormones may regulate the difference in cold responsiveness. These results provide a new understanding of the molecular mechanism of cold stress and will improve cold tolerance in horticultural plants.

Updated role of ABA in seed maturation, dormancy, and germination

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Functional ABA biosynthesis genes show specific roles for ABA accumulation at different stages of seed development and seedling establishment.

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De novo ABA biosynthesis during embryogenesis is required for late seed development, maturation, and induction of primary dormancy.

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ABA plays multiple roles with the key LAFL hub to regulate various downstream signaling genes in seed and seedling development.

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Key ABA signaling genes ABI3, ABI4, and ABI5 play important multiple functions with various cofactors during seed development such as de-greening, desiccation tolerance, maturation, dormancy, and seed vigor.

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The crosstalk between ABA and other phytohormones are complicated and important for seed development and seedling establishment.

Background

Seed is vital for plant survival and dispersion, however, its development and germination are influenced by various internal and external factors. Abscisic acid (ABA) is one of the most important phytohormones that influence seed development and germination. Until now, impressive progresses in ABA metabolism and signaling pathways during seed development and germination have been achieved. At the molecular level, ABA biosynthesis, degradation, and signaling genes were identified to play important roles in seed development and germination. Additionally, the crosstalk between ABA and other hormones such as gibberellins (GA), ethylene (ET), Brassinolide (BR), and auxin also play critical roles. Although these studies explored some actions and mechanisms by which ABA-related factors regulate seed morphogenesis, dormancy, and germination, the complete network of ABA in seed traits is still unclear.

Aim of review

Presently, seed faces challenges in survival and viability. Due to the vital positive roles in dormancy induction and maintenance, as well as a vibrant negative role in the seed germination of ABA, there is a need to understand the mechanisms of various ABA regulators that are involved in seed dormancy and germination with the updated knowledge and draw a better network for the underlying mechanisms of the ABA, which would advance the understanding and artificial modification of the seed vigor and longevity regulation.

Key scientific concept of review

Here, we review functions and mechanisms of ABA in different seed development stages and seed germination, discuss the current progresses especially on the crosstalk between ABA and other hormones and signaling molecules, address novel points and key challenges (e.g., exploring more regulators, more cofactors involved in the crosstalk between ABA and other phytohormones, and visualization of active ABA in the plant), and outline future perspectives for ABA regulating seed associated traits.

Temporal Control of Seed Development in Dicots: Molecular Bases, Ecological Impact and Possible Evolutionary Ramifications

In flowering plants, seeds serve as organs of both propagation and dispersal. The developing seed passes through several consecutive stages, following a conserved general outline. The overall time needed for a seed to develop, however, may vary both within and between plant species, and these temporal developmental properties remain poorly understood. In the present paper, we summarize the existing data for seed development alterations in dicot plants. For genetic mutations, the reported cases were grouped in respect of the key processes distorted in the mutant specimens. Similar phenotypes arising from the environmental influence, either biotic or abiotic, were also considered. Based on these data, we suggest several general trends of timing alterations and how respective mechanisms might add to the ecological plasticity of the families considered. We also propose that the developmental timing alterations may be perceived as an evolutionary substrate for heterochronic events. Given the current lack of plausible models describing timing control in plant seeds, the presented suggestions might provide certain insights for future studies in this field.

The Plastid-Localized AtFtsHi3 Pseudo-Protease of Arabidopsis thaliana Has an Impact on Plant Growth and Drought Tolerance

While drought severely affects plant growth and crop production, the molecular mechanisms of the drought response of plants remain unclear. In this study, we demonstrated for the first time the effect of the pseudo-protease AtFtsHi3 of Arabidopsis thaliana on overall plant growth and in drought tolerance. An AtFTSHi3 knock-down mutant [ftshi3-1(kd)] displayed a pale-green phenotype with lower photosynthetic efficiency and Darwinian fitness compared to wild type (Wt). An observed delay in seed germination of ftshi3-1(kd) was attributed to overaccumulation of abscisic acid (ABA); ftshi3-1(kd) seedlings showed partial sensitivity to exogenous ABA. Being exposed to similar severity of soil drying, ftshi3-1(kd) was drought-tolerant up to 20 days after the last irrigation, while wild type plants wilted after 12 days. Leaves of ftshi3-1(kd) contained reduced stomata size, density, and a smaller stomatic aperture. During drought stress, ftshi3-1(kd) showed lowered stomatal conductance, increased intrinsic water-use efficiency (WUEi), and slower stress acclimation. Expression levels of ABA-responsive genes were higher in leaves of ftshi3-1(kd) than Wt; DREB1A, but not DREB2A, was significantly upregulated during drought. However, although ftshi3-1(kd) displayed a drought-tolerant phenotype in aboveground tissue, the root-associated bacterial community responded to drought.

Comparative Multi-Omics of Tender Shoots from a Novel Evergrowing Tea Cultivar Provide Insight into the Winter Adaptation Mechanism

Tea (Camellia sinensis [L.] O. Kuntze) tree is a perennial plant in which winter dormancy is an important biological adaptation to environmental changes. We discovered and reported a novel tea tree cultivar that can generate tender shoots in winter several years ago, but the molecular mechanism for this unique phenotype remains unknown . Here, we conducted comparative transcriptomics, proteomics and metabolomics along with phytohormone quantitation between the winter and spring tender shoots to investigate the physiological basis and putative regulatory mechanisms of its evergrowing character during winter. Our multi-omics study has led to the following findings. Gibberellin (GA) levels and key enzymes for GA biosynthesis and the signal transduction pathway were increased in the winter shoots, causing the ABA/GA content ratio to decrease, which might play a key regulatory role in maintaining normal growth during winter. The abundance of proteins, genes and metabolites involved in energy metabolism was all increased in winter shoots, indicating that energy is critical for continuous growth under the relatively weak-light and low-temperature environment. Abiotic resistance-related proteins and free amino acids were also increased in abundance in the winter shoots, which possibly represents an adaptation response to winter conditions. These results allowed us to hypothesize a novel molecular mechanism of adaptation for this unique tender shoot evergrowing in winter.

Arabidopsis CCoAOMT1 Plays a Role in Drought Stress Response via ROS- and ABA-Dependent Manners

Plants possess adaptive reprogramed modules to prolonged environmental stresses, including adjustment of metabolism and gene expression for physiological and morphological adaptation. CCoAOMT1 encodes a caffeoyl CoA O-methyltransferase and is known to play an important role in adaptation of Arabidopsis plants to prolonged saline stress. In this study, we showed that the CCoAOMT1 gene plays a role in drought stress response. Transcript of CCoAOMT1 was induced by salt, dehydration (drought), and methyl viologen (MV), and loss of function mutants of CCoAOMT1, ccoaomt1-1, and ccoaomt1-2 exhibit hypersensitive phenotypes to drought and MV stresses. The ccoaomt1 mutants accumulated higher level of H₂O₂ in the leaves and expressed lower levels of drought-responsive genes including RD29B, RD20, RD29A, and ERD1, as well as ABA3 3 and NCED3 encoding ABA biosynthesis enzymes during drought stress compared to wild-type plants. A seed germination assay of ccoaomt1 mutants in the presence of ABA also revealed that CCoAOMT1 functions in ABA response. Our data suggests that CCoAOMT1 plays a positive role in response to drought stress response by regulating H₂O₂ accumulation and ABA signaling.

Fine tuning of hormonal signaling is linked to dormancy status in sweet cherry flower buds

In temperate trees, optimal timing and quality of flowering directly depend on adequate winter dormancy progression, regulated by a combination of chilling and warm temperatures. Physiological, genetic and functional genomic studies have shown that hormones play a key role in bud dormancy establishment, maintenance and release. We combined physiological and transcriptional analyses, quantification of abscisic acid (ABA) and gibberellins (GAs), and modeling to further investigate how these signaling pathways are associated with dormancy progression in the flower buds of two sweet cherry cultivars. Our results demonstrated that GA-associated pathways have distinct functions and may be differentially related with dormancy. In addition, ABA levels rise at the onset of dormancy, associated with enhanced expression of ABA biosynthesis PavNCED genes, and decreased prior to dormancy release. Following the observations that ABA levels are correlated with dormancy depth, we identified PavUG71B6, a sweet cherry UDP-GLYCOSYLTRANSFERASE gene that up-regulates active catabolism of ABA to ABA glucosyl ester (ABA-GE) and may be associated with low ABA content in the early cultivar. Subsequently, we modeled ABA content and dormancy behavior in three cultivars based on the expression of a small set of genes regulating ABA levels. These results strongly suggest the central role of ABA pathway in the control of dormancy progression and open up new perspectives for the development of molecular-based phenological modeling.

Aethionema arabicum genome annotation using PacBio full-length transcripts provides a valuable resource for seed dormancy and Brassicaceae evolution research

Aethionema arabicum is an important model plant for Brassicaceae trait evolution, particularly of seed (development, regulation, germination, dormancy) and fruit (development, dehiscence mechanisms) characters. Its genome assembly was recently improved but the gene annotation was not updated. Here, we improved the Ae. arabicum gene annotation using 294 RNA-seq libraries and 136 307 full-length PacBio Iso-seq transcripts, increasing BUSCO completeness by 11.6% and featuring 5606 additional genes. Analysis of orthologs showed a lower number of genes in Ae. arabicum than in other Brassicaceae, which could be partially explained by loss of homeologs derived from the At-α polyploidization event and by a lower occurrence of tandem duplications after divergence of Aethionema from the other Brassicaceae. Benchmarking of MADS-box genes identified orthologs of FUL and AGL79 not found in previous versions. Analysis of full-length transcripts related to ABA-mediated seed dormancy discovered a conserved isoform of PIF6-β and antisense transcripts in ABI3, ABI4 and DOG1, among other cases found of different alternative splicing between Turkey and Cyprus ecotypes. The presented data allow alternative splicing mining and proposition of numerous hypotheses to research evolution and functional genomics. Annotation data and sequences are available at the Ae. arabicum DB (https://plantcode.online.uni-marburg.de/aetar_db).

Wake-up alarm: virtual time-lapse gene expression landscape illuminates mechanisms underlying dormancy breaking of germinating spores

Dormancy breaking is a common physiological phenomenon that is shared by eukaryotes. Germination of spores in fungi is one of the most representative cases of dormancy breaking. Understanding the mechanisms of spore germination is therefore fundamental to basic studies on the control of cell proliferation and differentiation, as well as agricultural applications and medical investigation of fungal pathogenesis. In fission yeast, spores are generated as a consequence of sexual differentiation under nutrient starvation, remaining dormant until further nourishment, but little is known about how dormant spores germinate in response to environmental change. In a breakthrough, methods for single-cell-based gene expression profiling have recently been introduced. Several mRNA expression profiles were assembled from single spore cells during dormancy or germination. Single-cell RNA-seq profiles were aligned sequentially according to their similarities. The alignment of transcriptomes visualised how gene expression varies over time upon dormancy breaking. In this review, we revisit knowledge from previous studies on germination, select candidate genes that may be involved in germination, and query their expression from the temporal transcriptomic dataset so that studies on S. pombe germination can be extended further.

Effects of Maternal Environment on Seed Germination and Seedling Vigor of Petunia × hybrida under Different Abiotic Stresses

Seed germination and seedling vigor can be affected by environmental cues experienced by the mother plant. However, information about how the maternal environment affects seed quality is scarce in ornamental plants. This study aimed to investigate the effects of two different maternal environments on the seed germination and seedling vigor of Petunia × hybrida under a variety of abiotic stresses. Petunia mother plants were grown in either a greenhouse during the summer months or an indoor controlled-temperature-and-light environment. Collected seeds were subjected to external stressors, including polyethylene glycol (PEG), sodium chloride (NaCl), high temperature, and abscisic acid (ABA), to determine seed germination percentage and seedling vigor. Results indicated that seeds harvested from the mother plants grown in a controlled environment germinated better than seeds harvested from the mother plants grown in the greenhouse when suboptimal germination conditions were applied. Additionally, the seedlings from the controlled maternal environment performed better in both ABA and salinity stress tests than the greenhouse seedlings. Interestingly, the greenhouse seedlings displayed less reactive oxygen species (ROS) damage and lower electrolyte leakage than the controlled environment seedlings under dehydration stress. The difference in germination and seedling vigor of seeds from the two different maternal environments might be due to the epigenetic memory inherited from the mother plants. This study highlighted the strong impact of the maternal environment on seed germination and seedling vigor in Petunia and may assist in high-quality seed production in ornamental plants.

Genome sequence analysis of the beneficial Bacillus subtilis PTA-271 isolated from a Vitis vinifera (cv. Chardonnay) rhizospheric soil: assets for sustainable biocontrol

BACKGROUND

Bacillus subtilis strains have been widely studied for their numerous benefits in agriculture, including viticulture. Providing several assets, B. subtilis spp. are described as promising plant-protectors against many pathogens and as influencers to adaptations in a changing environment. This study reports the draft genome sequence of the beneficial Bacillus subtilis PTA-271, isolated from the rhizospheric soil of healthy Vitis vinifera cv. Chardonnay at Champagne Region in France, attempting to draw outlines of its full biocontrol capacity.

RESULTS

The PTA-271 genome has a size of 4,001,755 bp, with 43.78% of G + C content and 3945 protein coding genes. The draft genome of PTA-271 putatively highlights a functional swarming motility system hypothesizing a colonizing capacity and a strong interacting capacity, strong survival capacities and a set of genes encoding for bioactive substances. Predicted bioactive compounds are known to: stimulate plant growth or defenses such as hormones and elicitors, influence beneficial microbiota, and counteract pathogen aggressiveness such as effectors and many kinds of detoxifying enzymes.

CONCLUSIONS

Plurality of the putatively encoded biomolecules by Bacillus subtilis PTA-271 genome suggests environmentally robust biocontrol potential of PTA-271, protecting plants against a broad spectrum of pathogens.

Avena fatua caryopsis dormancy release is associated with changes in KAR1 and ABA sensitivity as well as with ABA reduction in coleorhiza and radicle

The dormancy release in Avena fatua caryopses was associated with a reduction in the ABA content in embryos, coleorhiza and radicle. The coleorhiza proved more sensitive to KAR₁ and less sensitive to ABA than the radicle. The inability of dormant caryopses and ABA-treated non-dormant caryopses to complete germination is related to inhibition and delayed of cell-cycle activation, respectively. As freshly harvested Avena fatua caryopses are dormant at 20 °C, they cannot complete germination; the radicle is not able to emerge. Both karrikin 1 (KAR₁) and dry after-ripening release dormancy, enabling the emergence of, first, the coleorhiza and later the radicle. The after-ripening removes caryopse sensitivity to KAR₁ and decreases the sensitivity to abscisic acid (ABA). The coleorhiza was found to be more sensitive to KAR₁, and less sensitive to ABA, than radicles. Effects of KAR₁ and after-ripening were associated with a reduction of the embryo's ABA content during caryopsis germination. KAR₁ was found to decrease the ABA content in the coleorhiza and radicles. Germination of after-ripened caryopses was associated with the progress of cell-cycle activation before coleorhiza emergence. Inhibition of the germination completion due to dormancy or treating the non-dormant caryopses with ABA was associated with a total and partial inhibition of cell-cycle activation, respectively.

Avena fatua caryopsis dormancy release is associated with changes in KAR1 and ABA sensitivity as well as with ABA reduction in coleorhiza and radicle

The dormancy release in Avena fatua caryopses was associated with a reduction in the ABA content in embryos, coleorhiza and radicle. The coleorhiza proved more sensitive to KAR₁ and less sensitive to ABA than the radicle. The inability of dormant caryopses and ABA-treated non-dormant caryopses to complete germination is related to inhibition and delayed of cell-cycle activation, respectively. As freshly harvested Avena fatua caryopses are dormant at 20 °C, they cannot complete germination; the radicle is not able to emerge. Both karrikin 1 (KAR₁) and dry after-ripening release dormancy, enabling the emergence of, first, the coleorhiza and later the radicle. The after-ripening removes caryopse sensitivity to KAR₁ and decreases the sensitivity to abscisic acid (ABA). The coleorhiza was found to be more sensitive to KAR₁, and less sensitive to ABA, than radicles. Effects of KAR₁ and after-ripening were associated with a reduction of the embryo's ABA content during caryopsis germination. KAR₁ was found to decrease the ABA content in the coleorhiza and radicles. Germination of after-ripened caryopses was associated with the progress of cell-cycle activation before coleorhiza emergence. Inhibition of the germination completion due to dormancy or treating the non-dormant caryopses with ABA was associated with a total and partial inhibition of cell-cycle activation, respectively.

Identification, Gene Structure, and Expression of BnMicEmUP: A Gene Upregulated in Embryogenic Brassica napus Microspores

Microspores of Brassica napus can be diverted from normal pollen development into embryogenesis by treating them with a mild heat shock. As microspore embryogenesis closely resembles zygotic embryogenesis, it is used as model for studying the molecular mechanisms controlling embryo formation. A previous study comparing the transcriptomes of three-day-old sorted embryogenic and pollen-like (non-embryogenic) microspores identified a gene homologous to AT1G74730 of unknown function that was upregulated 8-fold in the embryogenic cells. In the current study, the gene was isolated and sequenced from B. napus and named BnMicEmUP (B. napus microspore embryogenesis upregulated gene). Four forms of BnMicEmUP mRNA and three forms of genomic DNA were identified. BnMicEmUP2,3 was upregulated more than 7-fold by day 3 in embryogenic microspore cultures compared to non-induced cultures. BnMicEmUP1,4 was highly expressed in leaves. Transient expression studies of BnMicEmUP3::GFP fusion protein in Nicotiana benthamiana and in stable Arabidopsis transgenics showed that it accumulates in chloroplasts. The features of the BnMicEmUP protein, which include a chloroplast targeting region, a basic region, and a large region containing 11 complete leucine-rich repeats, suggest that it is similar to a bZIP PEND (plastid envelope DNA-binding protein) protein, a DNA binding protein found in the inner envelope membrane of developing chloroplasts. Here, we report that the BnMicEmUP3 overexpression in Arabidopsis increases the sensitivity of seedlings to exogenous abscisic acid (ABA). The BnMicEmUP proteins appear to be transcription factors that are localized in plastids and are involved in plant responses to biotic and abiotic environmental stresses; as well as the results obtained from this study can be used to improve crop yield.

Signal Integration in Plant Abiotic Stress Responses via Multistep Phosphorelay Signaling

Plants growing in any particular geographical location are exposed to variable and diverse environmental conditions throughout their lifespan. The multifactorial environmental pressure resulted into evolution of plant adaptation and survival strategies requiring ability to integrate multiple signals that combine to yield specific responses. These adaptive responses enable plants to maintain their growth and development while acquiring tolerance to a variety of environmental conditions. An essential signaling cascade that incorporates a wide range of exogenous as well as endogenous stimuli is multistep phosphorelay (MSP). MSP mediates the signaling of essential plant hormones that balance growth, development, and environmental adaptation. Nevertheless, the mechanisms by which specific signals are recognized by a commonly-occurring pathway are not yet clearly understood. Here we summarize our knowledge on the latest model of multistep phosphorelay signaling in plants and the molecular mechanisms underlying the integration of multiple inputs including both hormonal (cytokinins, ethylene and abscisic acid) and environmental (light and temperature) signals into a common pathway. We provide an overview of abiotic stress responses mediated via MSP signaling that are both hormone-dependent and independent. We highlight the mutual interactions of key players such as sensor kinases of various substrate specificities including their downstream targets. These constitute a tightly interconnected signaling network, enabling timely adaptation by the plant to an ever-changing environment. Finally, we propose possible future directions in stress-oriented research on MSP signaling and highlight its potential importance for targeted crop breeding.

Plant behaviour: an evolutionary response to the environment?

Plants are not just passive living beings that exist in nature. They are complex and highly adaptable species that react sensitively to environmental forces/stimuli with movement, morphological changes and through the communication via volatile molecules. In a way, plants mimic some traits of animal and human behaviour; they compete for limited resources by gaining more area for more sunlight and spread their roots underground. Furthermore, in order to survive and thrive, they evolve and 'learn' to control various environmental stress factors in order to increase the yield of flowering, fertilization and germination processes. The concept of associating complex behaviour, such as intelligence, with plants is still a highly debatable topic among researchers worldwide. Recent studies have shown that plants are able to discriminate between positive and negative experiences and 'learn' from them. Some botanists have interpreted these behavioural data as a form of primitive cognitive processes. Others have evaluated these responses as biological automatisms of plants determined by adaptation to the environment and absence of intelligence. This review aims to explore adaptive behavioural aspects of various plant species distributed in different ecosystems by emphasizing their biological complexity and survival instincts.

Dead but Not Dead End: Multifunctional Role of Dead Organs Enclosing Embryos in Seed Biology

Dry fruits consist of two types, dehiscent and indehiscent, whereby the fruit is splitting open or remains closed at maturity, respectively. The seed, the dispersal unit (DU) of dehiscent fruits, is composed of three major parts, the embryo and the food reserve, encapsulated by the maternally-derived organ, the seed coat. Indehiscent fruit constitutes the DU in which the embryo is covered by two protective layers (PLs), the seed coat and the fruit coat. In grasses, the caryopsis, a one-seeded fruit, can be further enclosed by the floral bracts to generate two types of DUs, florets and spikelets. All protective layers enclosing the embryo undergo programmed cell death (PCD) at maturation and are thought to provide mainly a physical shield for embryo protection and a means for dispersal. In this review article, I wish to highlight the elaborate function of these dead organs enclosing the embryo as unique storage structures for beneficial substances and discuss their potential role in seed biology and ecology.

Extreme drought alters progeny dispersal unit properties of winter wild oat (Avena sterilis L.)

The dead husk is a vital component of the dispersal unit whose biochemical properties can be modified following exposure to drought. This might affect seed performance and fate, soil properties and consequently plant biodiversity. We investigated the effects of extreme drought on the dispersal unit (DU) properties of winter wild oat (Avena sterilis L.) in the Mediterranean ecosystems focusing on a commonly ignored component of the DU, namely the dead floral bracts (husk). DUs were collected from a climate change experimental research station in the Judean Hills, Israel, simulating extreme drought and from two additional sites differing in the rainfall amounts. Our results showed that drought conditions significantly affected A. sterilis reproductive traits displaying reduced DUs and caryopses weights. The husk contributes profoundly to seed performance showing that germination from the intact DUs or the intact florets 1 was higher, faster and more homogenous compared to naked caryopses; no effect of drought on germination properties was observed. The husk stored hundreds of proteins that retain enzymatic activity and multiple metabolites including phytohormones. Changes in rainfall amounts affected the composition and levels of proteins and other metabolites accumulated in the husk, with a notable effect on abscisic acid (ABA). The husk of both control and drought plants released upon hydration substances that selectively inhibited other species seed germination as well as substances that promoted microbial growth. Our data showed that the dead husk represents a functional component of the DU that have been evolved to nurture the embryo and to ensure its success in its unique habitat. Furthermore, drought conditions can modify husk biochemical properties, which in turn might affect seed performance and fate, soil microbiota and soil fertility and consequently plant species diversity.

Chemical Manipulation of Abscisic Acid Signaling: A New Approach to Abiotic and Biotic Stress Management in Agriculture

The phytohormone abscisic acid (ABA) is the best-known stress signaling molecule in plants. ABA protects sessile land plants from biotic and abiotic stresses. The conserved pyrabactin resistance/pyrabactin resistance-like/regulatory component of ABA receptors (PYR/PYL/RCAR) perceives ABA and triggers a cascade of signaling events. A thorough knowledge of the sequential steps of ABA signaling will be necessary for the development of chemicals that control plant stress responses. The core components of the ABA signaling pathway have been identified with adequate characterization. The information available concerning ABA biosynthesis, transport, perception, and metabolism has enabled detailed functional studies on how the protective ability of ABA in plants might be modified to increase plant resistance to stress. Some of the significant contributions to chemical manipulation include ABA biosynthesis inhibitors, and ABA receptor agonists and antagonists. Chemical manipulation of key control points in ABA signaling is important for abiotic and biotic stress management in agriculture. However, a comprehensive review of the current knowledge of chemical manipulation of ABA signaling is lacking. Here, a thorough analysis of recent reports on small-molecule modulation of ABA signaling is provided. The challenges and prospects in the chemical manipulation of ABA signaling for the development of ABA-based agrochemicals are also discussed.

Maternal environment alters dead pericarp biochemical properties of the desert annual plant Anastatica hierochuntica L

The dead organs enclosing embryos (DOEEs) emerge as central components of the dispersal unit (DU) capable for long-term storage of active proteins and other substances that affect seed performance and fate. We studied the effect of maternal environment (salt and salt+heat) on progeny DU (dry indehiscent fruit) focusing on pericarp properties of Anastatica hierochuntica. Stressed plants displayed increased seed abortion and low level and rate of germination. Hydrated pericarps released antimicrobial factors and allelopathic substances that inhibit germination of heterologous species. Proteome analysis of dead pericarps revealed hundreds of proteins, among them nucleases, chitinases and proteins involved in reactive oxygen species detoxification and cell wall modification. Salt treatment altered the composition and level of proteins stored in the pericarp. We observed changes in protein profile released from seeds of salt-treated plants with a notable increase in a small anti-fungal protein, defensin. The levels of phytohormones including IAA, ABA and salicylic acid were reduced in dead pericarps of stressed plants. The data presented here highlighted the predominant effects of maternal environment on progeny DUs of the desert plant A. hierochuntica, particularly on pericarp properties, which in turn might affect seed performance and fate, soil fertility and consequently plant biodiversity.

Auxin: Hormonal Signal Required for Seed Development and Dormancy

The production of viable seeds is a key event in the life cycle of higher plants. Historically, abscisic acid (ABA) and gibberellin (GAs) were considered the main hormones that regulate seed formation. However, auxin has recently emerged as an essential player that modulates, in conjunction with ABA, different cellular processes involved in seed development as well as the induction, regulation and maintenance of primary dormancy (PD). This review examines and discusses the key role of auxin as a signaling molecule that coordinates seed life. The cellular machinery involved in the synthesis and transport of auxin, as well as their cellular and tissue compartmentalization, is crucial for the development of the endosperm and seed-coat. Thus, auxin is an essential compound involved in integuments development, and its transport from endosperm is regulated by AGAMOUS-LIKE62 (AGL62) whose transcript is specifically expressed in the endosperm. In addition, recent biochemical and genetic evidence supports the involvement of auxins in PD. In this process, the participation of the transcriptional regulator ABA INSENSITIVE3 (ABI3) is critical, revealing a cross-talk between auxin and ABA signaling. Future experimental aimed at advancing knowledge of the role of auxins in seed development and PD are also discussed.

Whole-genome resequencing-based QTL-seq identified candidate genes and molecular markers for fresh seed dormancy in groundnut

The subspecies fastigiata of cultivated groundnut lost fresh seed dormancy (FSD) during domestication and human-made selection. Groundnut varieties lacking FSD experience precocious seed germination during harvest imposing severe losses. Development of easy-to-use genetic markers enables early-generation selection in different molecular breeding approaches. In this context, one recombinant inbred lines (RIL) population (ICGV 00350 × ICGV 97045) segregating for FSD was used for deploying QTL-seq approach for identification of key genomic regions and candidate genes. Whole-genome sequencing (WGS) data (87.93 Gbp) were generated and analysed for the dormant parent (ICGV 97045) and two DNA pools (dormant and nondormant). After analysis of resequenced data from the pooled samples with dormant parent (reference genome), we calculated delta-SNP index and identified a total of 10,759 genomewide high-confidence SNPs. Two candidate genomic regions spanning 2.4 Mb and 0.74 Mb on the B05 and A09 pseudomolecules, respectively, were identified controlling FSD. Two candidate genes-RING-H2 finger protein and zeaxanthin epoxidase-were identified in these two regions, which significantly express during seed development and control abscisic acid (ABA) accumulation. QTL-seq study presented here laid out development of a marker, GMFSD1, which was validated on a diverse panel and could be used in molecular breeding to improve dormancy in groundnut.

Agonist, antagonist and signaling modulators of ABA receptor for agronomic and post-harvest management

Abscisic acid (ABA) is a ubiquitous phytohormone, plays important roles in several physiological processes, including stress adaptation, flowering, seed germination, fruit ripening, and leaf senescence etc. ABA binds with START domain proteins called Pyrabactin Resistance1 (PYR1)/PYR1-like (PYL)/Regulatory Components of ABA Receptors (RCARs) and controls the activity of PP2C phosphatase proteins and in turn the ABA-dependent signaling pathway. Fourteen ABA receptors have been identified in the model plant Arabidopsis thaliana and have shown to be involved in various biological functions. Under field conditions, exogenous application of ABA produces inadequate physiological response due to its rapid conversion into the biologically inactive metabolites. ABA shows selective binding preferences to PYL receptor subtypes and hence produces pleiotropic physiological and phenotypic effects which limit the usage of ABA in agriculture. An agrochemical meant for ameliorating the undesirable physiological effect of the plant should ideally have positive biological attributes without affecting the normal growth, development, and yield. Therefore, to overcome the limitations of ABA for its usage in various agricultural applications, several types of ABA-mimicking agents have been developed. Many compounds have been identified as having significant ABA-agonist/antagonist activity and can be employed to reverse the excessive/moderate ABA action. The present review highlights the potential usage of ABA signaling modulators for managing agronomic and postharvest traits. Besides, designing, development and versatile usage of ABA-mimicking compounds displaying ABA agonists and antagonist activities are discussed in detail.

Golgi-localized cyclophilin 21 proteins negatively regulate ABA signalling via the peptidyl prolyl isomerase activity during early seedling development

Plant possesses particular Golgi-resident cyclophilin 21 proteins (CYP21s) and the catalytic isomerase activities have a negative effect on ABA signalling gene expression during early seedling development. Cyclophilins (CYPs) are essential for diverse cellular process, as these catalyse a rate-limiting step in protein folding. Although Golgi proteomics in Arabidopsis thaliana suggests the existence of several CYPs in the Golgi apparatus, only one putative Golgi-resident CYP protein has been reported in rice (Oryza sativa L.; OsCYP21-4). Here, we identified the Golgi-resident CYP21 family genes and analysed their molecular characteristics in Arabidopsis and rice. The CYP family genes (CYP21-1, CYP21-2, CYP21-3, and CYP21-4) are plant-specific, and their appearance and copy numbers differ among plant species. CYP21-1 and CYP21-4 are common to all angiosperms, whereas CYP21-2 and CYP21-3 evolved in the Malvidae subclass. Furthermore, all CYP21 proteins localize to cis-Golgi, trans-Golgi or both cis- and trans-Golgi membranes in plant cells. Additionally, based on the structure, enzymatic function, and topological orientation in Golgi membranes, CYP21 proteins are divided into two groups. Genetic analysis revealed that Group I proteins (CYP21-1 and CYP21-2) exhibit peptidyl prolyl cis-trans isomerase (PPIase) activity and regulate seed germination and seedling growth and development by affecting the expression levels of abscisic acid signalling genes. Thus, we identified the Golgi-resident CYPs and demonstrated that their PPIase activities are required for early seedling growth and development in higher plants.

An Arabidopsis WD40 repeat-containing protein XIW1 promotes salt inhibition of seed germination

WD40 repeat-containing protein (WD40 protein) acts as versatile scaffolds for protein interactions and participates in a variety of biological processes, such as plant stress and hormone responses. In a recent study, we show that XPO1-Interacting WD40 protein 1 (XIW1) is nuclear accumulated by abscisic acid (ABA), which interacts with ABA insensitive 5 (ABI5) in the nucleus and maintains its stability, thereby affecting seed germination and seedling growth. Here, we show that XIW1 promotes salt inhibition of seed germination (SSG). Mutation of XIW1 reduced the induction of ABI5 and ABA-responsive genes under salt treatment, causing mutant plants with high seed germination and suppression of SSG. In addition, we found that salt treatment facilities XIW1 accumulation in the nucleus. These findings reveal the important role of XIW1 in the regulation of SSG and provide a possible mechanism of XIW1 affecting SSG.

Alleviating dormancy in Brassica oleracea seeds using NO and KAR1 with ethylene biosynthetic pathway, ROS and antioxidant enzymes modifications

BACKGROUND

Seed dormancy is a prevailing condition in which seeds are unable to germinate, even under favorable environmental conditions. Harvested Brassica oleracea (Chinese cabbage) seeds are dormant and normally germinate (poorly) at 21 °C. This study investigated the connections between ethylene, nitric oxide (NO), and karrikin 1 (KAR1) in the dormancy release of secondary dormant Brassica oleracea seeds.

RESULTS

NO and KAR1 were found to induce seed germination, and stimulated the production of ethylene and 1-aminocyclopropane-1-carboxylic acid (ACC), and both ethylene biosynthesis enzyme ACC oxidase (ACO) [1] and ACC synthase (ACS) [2]. In the presence of NO and KAR1, ACS and ACO activity reached maximum levels after 36 and 48 h, respectively. The inhibitor of ethylene 2,5-norbornadiene (NBD) had an adverse effect on Brassica oleracea seed germination (inhibiting nearly 50% of germination) in the presence of NO and KAR1. The benefits from NO and KAR1 in the germination of secondary dormant Brassica oleracea seeds were also associated with a marked increase in reactive oxygen species (ROS) (H₂O₂ and O₂˙-) and antioxidant enzyme activity at early germination stages. Catalase (CAT) and glutathione reductase (GR) activity increased 2 d and 4 d, respectively, after treatment, while no significant changes were observed in superoxide dismutase (SOD) activity under NO and KAR1 applications. An increase in H₂O₂ and O₂˙- levels were observed during the entire incubation period, which increasing ethylene production in the presence of NO and KAR1. Abscisic acid (ABA) contents decreased and glutathione reductase (GA) contents increased in the presence of NO and KAR1. Gene expression studies were carried out with seven ethylene biosynthesis ACC synthases (ACS) genes, two ethylene receptors (ETR) genes and one ACO gene. Our results provide more evidence for the involvement of ethylene in inducing seed germination in the presence of NO and KAR1. Three out of seven ethylene biosynthesis genes (BOACS7, BOACS9 and BOACS11), two ethylene receptors (BOETR1 and BOETR2) and one ACO gene (BOACO1) were up-regulated in the presence of NO and KAR1.

CONCLUSION

Consequently, ACS activity, ACO activity and the expression of different ethylene related genes increased, modified the ROS level, antioxidant enzyme activity, and ethylene biosynthesis pathway and successfully removed (nearly 98%) of the seed dormancy of secondary dormant Brassica olereace seeds after 7 days of NO and KAR1 application.

Expression Patterns of Key Hormones Related to Pea (Pisum sativum L.) Embryo Physiological Maturity Shift in Response to Accelerated Growth Conditions

Protocols have been proposed for rapid generation turnover of temperate legumes under conditions optimized for day-length, temperature, and light spectra. These conditions act to compress time to flowering and seed development across genotypes. In pea, we have previously demonstrated that embryos do not efficiently germinate without exogenous hormones until physiological maturity is reached at 18 days after pollination (DAP). Sugar metabolism and moisture content have been implicated in the modulation of embryo maturity. However, the role of hormones in regulating seed development is poorly described in legumes. To address this gap, we characterized hormonal profiles (IAA, chlorinated auxin [4-Cl-IAA], GA20, GA1, and abscisic acid [ABA]) of developing seeds (10-22 DAP) from diverse pea genotypes grown under intensive conditions optimized for rapid generation turnover and compared them to profiles of equivalent samples from glasshouse conditions. Growing plants under intensive conditions altered the seed hormone content by advancing the auxin, gibberellins (GAs) and ABA profiles by 4 to 8 days, compared with the glasshouse control. Additionally, we observed a synchronization of the auxin profiles across genotypes. Under intensive conditions, auxin peaks were observed at 10 to 12 DAP and GA20 peaks at 10 to 16 DAP, indicative of the end of embryo morphogenesis and initiation of seed desiccation. GA1 was detected only in seeds harvested in the glasshouse. These results were associated with an acceleration of embryo physiological maturity by up to 4 days in the intensive environment. We propose auxin and GA profiles as reliable indicators of seed maturation. The biological relevance of these hormonal fluctuations to the attainment of physiological maturity, in particular the role of ABA and GA, was investigated through the study of precocious in vitro germination of seeds 12 to 22 DAP, with and without exogenous hormones. The extent of sensitivity of developing seeds to exogenous ABA was strongly genotype-dependent. Concentrations between 5 and 10 µM inhibited germination of seeds 18 DAP. Germination of seeds 12 DAP was enhanced 2.5- to 3-fold with the addition of 125 µM GA3. This study provides further insights into the hormonal regulation of seed development and in vitro precocious germination in legumes and contributes to the design of efficient and reproducible biotechnological tools for rapid genetic gain.