Seeds of tomato (Lycopersicon esculentum Mill.) which develop in a fully hydrated environment in the fruit switch from a developmental to a germinative mode without a requirement for desiccation

MaABI5 and MaABF1 transcription factors regulate the expression of MaJOINTLESS during fruit abscission in mulberry (Morus alba L.)

Mulberry holds significant economic value. However, during the ripening stage of its fruit, the phenomenon of abscission, resulting in heavy fruit drop, can severely impact the yield. The formation of off-zone structures is a critical factor in the fruit abscission process, and this process is regulated by multiple transcription factors. One such key gene that plays a significant role in the development of the off-zone in the model plant tomato is JOINTLESS, which promotes the expression of abscission-related genes and regulates the differentiation of abscission zone tissue cells. However, there is a lack of information about fruit abscission mechanism in mulberry. Here, we analyzed the MaJOINTLESS promoter and identified the upstream regulators MaABF1 and MaABI5. These two regulators showed binding with MaJOINTLESS promoter MaABF1 (the ABA Binding Factor/ABA-Responsive Element Binding Proteins) activated the expression of MaJOINTLESS, while MaABI5 (ABSCISIC ACID-INSENSITIVE 5) inhibited the expression of MaJOINTLESS. Finally, the differentially expressed genes (DEGs) were analyzed by transcriptome sequencing to investigate the expression and synergistic relationship of endogenous genes in mulberry during abscission. GO classification and KEGG pathway enrichment analysis showed that most of the DEGs were concentrated in MAPK signaling pathway, flavonoid biosynthesis, citric acid cycle, phytohormone signaling, amino acid biosynthesis, and glycolysis. These results provide a theoretical basis for subsequent in-depth study of physiological fruit abscission in mulberry.

Gene co-expression analysis of tomato seed maturation reveals tissue-specific regulatory networks and hubs associated with the acquisition of desiccation tolerance and seed vigour

BACKGROUND

During maturation seeds acquire several physiological traits to enable them to survive drying and disseminate the species. Few studies have addressed the regulatory networks controlling acquisition of these traits at the tissue level particularly in endospermic seeds such as tomato, which matures in a fully hydrated environment and does not undergo maturation drying. Using temporal RNA-seq analyses of the different seed tissues during maturation, gene network and trait-based correlations were used to explore the transcriptome signatures associated with desiccation tolerance, longevity, germination under water stress and dormancy.

RESULTS

During maturation, 15,173 differentially expressed genes were detected, forming a gene network representing 21 expression modules, with 3 being specific to seed coat and embryo and 5 to the endosperm. A gene-trait significance measure identified a common gene module between endosperm and embryo associated with desiccation tolerance and conserved with non-endospermic seeds. In addition to genes involved in protection such LEA and HSP and ABA response, the module included antioxidant and repair genes. Dormancy was released concomitantly with the increase in longevity throughout fruit ripening until 14 days after the red fruit stage. This was paralleled by an increase in SlDOG1-2 and PROCERA transcripts. The progressive increase in seed vigour was captured by three gene modules, one in common between embryo and endosperm and two tissue-specific. The common module was enriched with genes associated with mRNA processing in chloroplast and mitochondria (including penta- and tetratricopeptide repeat-containing proteins) and post-transcriptional regulation, as well several flowering genes. The embryo-specific module contained homologues of ABI4 and CHOTTO1 as hub genes associated with seed vigour, whereas the endosperm-specific module revealed a diverse set of processes that were related to genome stability, defence against pathogens and ABA/GA response genes.

CONCLUSION

The spatio-temporal co-expression atlas of tomato seed maturation will serve as a valuable resource for the in-depth understanding of the dynamics of gene expression associated with the acquisition of seed vigour at the tissue level.

Seed development and viviparous germination in one accession of a tomato rin mutant

In an experimental field, seed vivipary occurred in one accession of tomato rin mutant fruit at approximately 45-50 days after pollination (DAP). In this study, the possible contributory factors to this viviparous germination were investigated. Firstly, developing seeds were freshly excised from the fruit tissue every 5 days from 25-60 DAP. Germination occurred when isolated seeds were incubated on water, but was inhibited when they remained ex situ in fruit mucilage gel. The effect of abscisic acid (ABA) and osmoticum, separate and together, on germination of developing seeds was investigated. Additionally, ABA content in the seed and mucilage gel, as well as fruit osmolality were measured. The results showed that ABA concentrations in seeds were low during early development and increased later, peaking at about 50 DAP. ABA concentrations in rin accession were similar to those of the control cultivar and thus are not directly associated with the occurrence of vivipary. Developing seeds of rin accession are more sensitive than control seeds to all inhibitory compounds. However, osmolality in rin fruit at later developmental stages becomes less negative that is required to permit germination of developing seeds. Hence, hypo-osmolality in rin fruit may be an important factor in permitting limited viviparous germination.

Seed development and maturation in early spring-flowering Galanthus nivalis and Narcissus pseudonarcissus continues post-shedding with little evidence of maturation in planta

BACKGROUND AND AIMS

Seeds of the moist temperate woodland species Galanthus nivalis and Narcissus pseudonarcissus, dispersed during spring or early summer, germinated poorly in laboratory tests. Seed development and maturation were studied to better understand the progression from developmental to germinable mode in order to improve seed collection and germination practices in these and similar species.

METHODS

Phenology, seed mass, moisture content and ability to germinate and tolerate desiccation were monitored during seed development until shedding. Embryo elongation within seeds was investigated during seed development and under several temperature regimes after shedding.

KEY RESULTS

Seeds were shed at high moisture content (>59 %) with little evidence that dry mass accumulation or embryo elongation were complete. Ability to germinate developed prior to the ability of some seeds to tolerate enforced desiccation. Germination was sporadic and slow. Embryo elongation occurred post-shedding in moist environments, most rapidly at 20 °C in G. nivalis and 15 °C in N. pseudonarcissus. The greatest germination also occurred in these regimes, 78 and 48 %, respectively, after 700 d.

CONCLUSIONS

Seeds of G. nivalis and N. pseudonarcissus were comparatively immature at shedding and substantial embryo elongation occurred post-shedding. Seeds showed limited desiccation tolerance at dispersal.

Gasotransmitters are emerging as new guard cell signaling molecules and regulators of leaf gas exchange

Specialized guard cells modulate plant gas exchange through the regulation of stomatal aperture. The size of the stomatal pore is a direct function of the volume of the guard cells. The transport of solutes across channels in plasma membrane is a crucial process in the maintenance of guard cell water status. The fine tuned regulation of that transport requires an integrated convergence of multiple endogenous and exogenous signals perceived at both the cellular and the whole plant level. Gasotransmitters are novel signaling molecules with key functions in guard cell physiology. Three gasotransmitters, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H(2)S) are involved in guard cell regulatory processes. These molecules are endogenously produced by plant cells and are part of the guard cells responses to drought stress conditions through ABA-dependent pathways. In this review, we summarize the current knowledge of gasotransmitters as versatile molecules interacting with different components of guard cell signaling network and propose them as players in new paradigms to study ABA-independent guard cell responses to water deficit.

Evaluating auxin distribution in tomato (Solanum lycopersicum) through an analysis of the PIN and AUX/LAX gene families

The temporal and spatial control of auxin distribution has a key role in the regulation of plant growth and development, and much has been learnt about the mechanisms that influence auxin pools and gradients in vegetative tissues, particularly in Arabidopsis. For example polar auxin transport, mediated by PIN and AUX/LAX proteins, is central to the control of auxin distribution. In contrast, very little information is known about the dynamics of auxin distribution and the molecular basis of its transport within and between fruit tissues, despite the fact that auxin regulates many aspects of fruit development, which include fruit formation, expansion, ripening and abscission. In addition, functional information regarding the key regulators of auxin fluxes during both vegetative and reproductive development in species other than Arabidopsis is scarce. To address these issues, we have investigated the spatiotemporal distribution of auxin during tomato (Solanum lycopersicum) fruit development and the function of the PIN and AUX/LAX gene families. Differential concentrations of auxin become apparent during early fruit growth, with auxin levels being higher in internal tissues than in the fruit pericarp and the pattern of auxin accumulation depended on polar transport. Ten tomato PIN (SlPIN1 to 10) and five AUX/LAX (SlLAX1 to 5) genes were identified and found to display heterogeneous expression patterns, with tissue and developmental-stage specificity. RNAi-mediated co-silencing of SlPIN4 and SlPIN3 did not affect fruit development, which suggested functional redundancy of PIN proteins, but did lead to a vegetative phenotype, and revealed a role for these genes in the regulation of tomato shoot architecture.

A comparative analysis of embryo and endosperm proteome from seeds of Jatropha curcas

Jatropha curcas is an important economic plant for biodiesel, which is extracted mainly from the endosperm of its mature seeds. Despite the morphological and functional differences between the embryo and endosperm, proteomic characteristics of the two tissues are not yet known. Similar proteomic profiles were observed in the two-dimensional gel electrophoresis maps from the two tissues. There were 380 and 533 major protein spots in the embryo and endosperm, respectively. Fourteen identical spots, showing a notable change, were selected and identified by tandem mass spectrometry. Among these proteins, dihydrolipoamide acetyltransferase (spot 27) participates in tricarboxylic acid cycle, which is an amphibolic pathway. The two parts both included proteins related to stress (spots 8, 115, 118, 125, 130) and signal transduction (spots 7, 100, 108). According to the volume percentage of proteins in embryo and endosperm, the proteins in endosperm (spots 54, 61, 73) were catabolism-related enzymes and reserves to provide the nutrition for seed germination; the proteins in embryo (spots 27, 62, 122) were inclined to anabolism and utilized the nutrition from the endosperm to generate a new life.

Coffee seed physiology

Considerable advances in our understanding of coffee seed physiology have been made in recent years. However, despite intense research efforts, there are many aspects that remain unclear. This paper gives an overview of the current understanding of the more important features concerning coffee seed physiology, and provides information on recent findings on seed development, germination, storage and longevity.

Proteome analysis of embryo and endosperm from germinating tomato seeds

Proteome analysis of embryo and endosperm tissues from germinating tomato seed was conducted using 1-DE, 2-DE, and MS. Mobilization of the most abundant proteins, which showed similar profiles in the two tissues, occurred first in the endosperm. CBB R-250 staining of 2-DE gels revealed 352 and 369 major protein spots in the embryo and endosperm, respectively, at 0 h. Of these, 75 major spots were selected, excised, in-gel digested with trypsin, and analyzed by MALDI-TOF-MS and/or LC-ESI-Q/TOF-MS/MS. Peptide MS and MS/MS data were searched against publicly available protein and EST databases, and 47 proteins identified. Embryo-specific proteins included a BAC19.13 homologue, whereas four proteins specific to the endosperm were tomato mosaic virus coat proteins related to defense mechanisms. The most abundant proteins both in the embryo and endosperm were seed storage proteins, i.e., legumins (11 spots), vicilins (11 spots), albumin (2 spots). Housekeeping enzymes, actin-binding profilin, defense-related protein kinases, nonspecific lipid transfer protein, and proteins involved in general metabolism were also identified. The roles of some of the proteins identified in the embryo and endosperm are discussed in relation to seed germination in tomato.

Communication between the maternal testa and the embryo and/or endosperm affect testa attributes in tomato

Two tomato (Lycopersicon esculentum) mutants with dark testae displaying poor germination rate and percentage on both water and 100 microM gibberellin(4 + 7) were recovered. The mutants were allelic (black seed1-1; bks1-1 and bks1-2), inherited in Mendelian fashion as a recessive gene residing on chromosome 11. They are not allelic to bs (brown seed) -1, -2, or -4, which impair seed germination and possess dark testae. The bks/bs mutants accumulated dark pigment in the cell layers of the testa above the endothelium, which itself accumulated proanthocyanidins similar to wild type. The poor germination performance of bks mutant seeds was because of impediment of the mutant testae to radicle egress. Imbibition on gibberellin(4 + 7) did not ameliorate germination percentage or rate. The toughening of the bks testa and associated poor germination were partially overcome when seeds were not dried before germination or were dried under N(2). The seeds of the bks mutant have elevated activity of at least one enzyme responsible for the detoxification of reactive oxygen species. The bks mutant is epistatic to 12 anthocyaninless mutants of tomato. Bio- and physicochemical analysis of the bks testa determined that it accumulated a melanic substance. Inheritance of bks/bs mutations contrasts with that of the anthocyaninless mutants, which are inherited according to the genotype of the maternally derived testa. This suggests that the testa manufactures components before its demise that can maximize testa strength, whereas the endosperm/embryo produces factors that are conveyed to the testa, mitigating this process.

Abscisic acid and gibberellin differentially regulate expression of genes of the SNF1-related kinase complex in tomato seeds

The SNF1/AMP-activated protein kinase subfamily plays central roles in metabolic and transcriptional responses to nutritional or environmental stresses. In yeast (Saccharomyces cerevisiae) and mammals, activating and anchoring subunits associate with and regulate the activity, substrate specificity, and cellular localization of the kinase subunit in response to changing nutrient sources or energy demands, and homologous SNF1-related kinase (SnRK1) proteins are present in plants. We isolated cDNAs corresponding to the kinase (LeSNF1), regulatory (LeSNF4), and localization (LeSIP1 and LeGAL83) subunits of the SnRK1 complex from tomato (Lycopersicon esculentum Mill.). LeSNF1 and LeSNF4 complemented yeast snf1 and snf4 mutants and physically interacted with each other and with LeSIP1 in a glucose-dependent manner in yeast two-hybrid assays. LeSNF4 mRNA became abundant at maximum dry weight accumulation during seed development and remained high when radicle protrusion was blocked by abscisic acid (ABA), water stress, far-red light, or dormancy, but was low or undetected in seeds that had completed germination or in gibberellin (GA)-deficient seeds stimulated to germinate by GA. In leaves, LeSNF4 was induced in response to ABA or dehydration. In contrast, LeSNF1 and LeGAL83 genes were essentially constitutively expressed in both seeds and leaves regardless of the developmental, hormonal, or environmental conditions. Regulation of LeSNF4 expression by ABA and GA provides a potential link between hormonal and sugar-sensing pathways controlling seed development, dormancy, and germination.

Expression of a GALACTINOL SYNTHASE gene in tomato seeds is up-regulated before maturation desiccation and again after imbibition whenever radicle protrusion is prevented

Raffinose family oligosaccharides (RFOs) have been implicated in mitigating the effects of environmental stresses on plants. In seeds, proposed roles for RFOs include protecting cellular integrity during desiccation and/or imbibition, extending longevity in the dehydrated state, and providing substrates for energy generation during germination. A gene encoding galactinol synthase (GOLS), the first committed enzyme in the biosynthesis of RFOs, was cloned from tomato (Lycopersicon esculentum Mill. cv Moneymaker) seeds, and its expression was characterized in tomato seeds and seedlings. GOLS (LeGOLS-1) mRNA accumulated in developing tomato seeds concomitant with maximum dry weight deposition and the acquisition of desiccation tolerance. LeGOLS-1 mRNA was present in mature, desiccated seeds but declined within 8 h of imbibition in wild-type seeds. However, LeGOLS-1 mRNA accumulated again in imbibed seeds prevented from completing germination by dormancy or water deficit. Gibberellin-deficient (gib-1) seeds maintained LeGOLS-1 mRNA amounts after imbibition unless supplied with gibberellin, whereas abscisic acid (ABA) did not prevent the loss of LeGOLS-1 mRNA from wild-type seeds. The presence of LeGOLS-1 mRNA in ABA-deficient (sitiens) tomato seeds indicated that wild-type amounts of ABA are not necessary for its accumulation during seed development. In all cases, LeGOLS-1 mRNA was most prevalent in the radicle tip. LeGOLS-1 mRNA accumulation was induced by dehydration but not by cold in germinating seeds, whereas both stresses induced LeGOLS-1 mRNA accumulation in seedling leaves. The physiological implications of LeGOLS-1 expression patterns in seeds and leaves are discussed in light of the hypothesized role of RFOs in plant stress tolerance.

Distinct physiological roles of fructokinase isozymes revealed by gene-specific suppression of Frk1 and Frk2 expression in tomato

There are two divergent fructokinase isozymes, Frk1 and Frk2 in tomato (Lycopersicon esculentum Mill.) plants. To investigate the physiological functions of each isozyme, the expression of each fructokinase mRNA was independently suppressed in transgenic tomato plants, and the respective phenotypes were evaluated. Suppression of Frk1 expression resulted in delayed flowering at the first inflorescence. Suppression of Frk2 did not effect flowering time but resulted in growth inhibition of stems and roots, reduction of flower and fruit number, and reduction of seed number per fruit. Localization of Frk1 and Frk2 mRNA accumulation by in situ hybridization in wild-type tomato fruit tissue indicated that Frk2 is expressed specifically in early tomato seed development. Fruit hexose and starch content were not effected by the suppression of either Frk gene alone. The results collectively indicate that flowering time is specifically promoted by Frk1 and that Frk2 plays specific roles in contributing to stem and root growth and to seed development. Because Frk1 and Frk2 gene expression was suppressed individually in transgenic plants, other significant metabolic roles of fructokinases may not have been observed if Frk1 and Frk2 play, at least partially, redundant metabolic roles.

α-Galactosidase is synthesized in tomato seeds during development and is localized in the protein storage vacuoles

The localization of the enzyme α-galactosidase (EC 3.2.1.22) was investigated during its synthesis in developing tomato (Lycopersicon esculentum Mill.) cv. Trust seeds. This enzyme is also present in germinating seeds, where it is involved in the mobilization of carbohydrate reserves during and following seed germination. Subcellular fractionation of developing tomato seeds revealed that there is a cosedimentation between α-galactosidase activity and protein storage vacuoles in a density gradient, which is dependent upon the presence of membranes. A second approach to localizing this enzyme involved the transient transformation of protoplasts from developing tomato seeds. A reporter construct, coding for tomato α-galactosidase, fused N-terminally to the bacterial enzyme chloramphenicol acetyltransferase was used for transient expression. Immunofluorescence microscopy revealed a colocalization between the α-galactosidase - chloramphenicol acetyltransferase fusion protein and the α-tonoplast intrinsic protein, and a partial colocalization with the dark intrinsic protein (both vacuolar proteins). These data indicate that the protein storage vacuole is the intracellular location for α-galactosidase in developing tomato seeds.Key words: α-galactosidase, protein storage vacuole, seed development, seed protoplasts, tomato, tonoplast intrinsic protein.

Two tomato expansin genes show divergent expression and localization in embryos during seed development and germination

Expansins are plant proteins that can induce extension of isolated cell walls and are proposed to mediate cell expansion. Three expansin genes were expressed in germinating tomato (Lycopersicon esculentum Mill.) seeds, one of which (LeEXP4) was expressed specifically in the endosperm cap tissue enclosing the radicle tip. The other two genes (LeEXP8 and LeEXP10) were expressed in the embryo and are further characterized here. LeEXP8 mRNA was not detected in developing or mature seeds but accumulated specifically in the radicle cortex during and after germination. In contrast, LeEXP10 mRNA was abundant at an early stage of seed development corresponding to the period of rapid embryo expansion; it then decreased during seed maturation and increased again during germination. When gibberellin-deficient (gib-1) mutant seeds were imbibed in water, LeEXP8 mRNA was not detected, but a low level of LeEXP10 mRNA was present. Expression of both genes increased when gib-1 seeds were imbibed in gibberellin. Abscisic acid did not prevent the initial expression of LeEXP8 and LeEXP10, but mRNA abundance of both genes subsequently decreased during extended incubation. The initial increase in LeEXP8, but not LeEXP10, mRNA accumulation was blocked by low water potential, but LeEXP10 mRNA amounts fell after longer incubation. When seeds were transferred from abscisic acid or low water potential solutions to water, abundance of both LeEXP8 and LeEXP10 mRNAs increased in association with germination. The tissue localization and expression patterns of both LeEXP8 and LeEXP10 suggest developmentally specific roles during embryo and seedling growth.

Vacuolar H(+)-ATPase is expressed in response to gibberellin during tomato seed germination

Completion of germination (radicle emergence) by gibberellin (GA)-deficient (gib-1) mutant tomato (Lycopersicon esculentum Mill.) seeds is dependent upon exogenous GA, because weakening of the endosperm tissue enclosing the radicle tip requires GA. To investigate genes that may be involved in endosperm weakening or embryo growth, differential cDNA display was used to identify mRNAs differentially expressed in gib-1 seeds imbibed in the presence or absence of GA(4+7). Among these was a GA-responsive mRNA encoding the 16-kD hydrophobic subunit c of the V(0) membrane sector of vacuolar H(+)-translocating ATPases (V-ATPase), which we termed LVA-P1. LVA-P1 mRNA expression in gib-1 seeds was dependent on GA and was particularly abundant in the micropylar region prior to radicle emergence. Both GA dependence and tissue localization of LVA-P1 mRNA expression were confirmed directly in individual gib-1 seeds using tissue printing. LVA-P1 mRNA was also expressed in wild-type seeds during development and germination, independent of exogenous GA. Specific antisera detected protein subunits A and B of the cytoplasmic V(1) sector of the V-ATPase holoenzyme complex in gib-1 seeds only in the presence of GA, and expression was localized to the micropylar region. The results suggest that V-ATPase plays a role in GA-regulated germination of tomato seeds.