Arabidopsis AGAMOUS Regulates Sepal Senescence by Driving Jasmonate Production

Multi-omics reveal differentiation and maintenance of dimorphic flowers in an alpine plant on the Qinghai-Tibet Plateau

Dimorphic flowers growing on a single individual plant play a critical role in extreme adaption and reproductive assurance in plants and have high ecological and evolutionary significance. However, the omics bases underlying such a differentiation and maintenance remain largely unknown. We aimed to investigate this through genomic, transcriptome and metabolomic analyses of dimorphic flowers in an alpine biennial, Sinoswertia tetraptera (Gentianaceae). A high-quality chromosome-level genome sequence (903 Mb) was first assembled for S. tetraptera with 31,359 protein-coding genes annotated. Two rounds of recent independent whole-genome duplication (WGD) were revealed. Numerous genes from the recent species-specific WGD were found to be differentially expressed in the two types of flowers, and this may have helped contribute to the origin of this innovative trait. The genes with contrasting expressions between flowers were related to biosynthesis of hormones, floral pigments (carotenoids and flavonoids) and iridoid compounds, which are involved in both flower development and colour. Metabolomic analyses similarly suggested differential concentrations of these chemicals in the two types of flowers. The expression interactions between multiple genes may together lead to contrasting morphology and chemical concentration and open versus closed pollination of the dimorphic flowers in this species for reproductive assurance.

Hormones and Flower Development in Arabidopsis

Sexual reproduction requires the participation of two gametes, female and male. In angiosperms, gametes develop in specialized organs, pollen (containing the male gametes) develops in the stamens, and the ovule (containing the female gamete) develops in the gynoecium. In Arabidopsis thaliana, the female and male sexual organs are found within the same structure called flower, surrounded by the perianth, which is composed of petals and sepals. During flower development, different organs emerge in an established order and throughout their development distinct tissues within each organ are differentiated. All this requires the coordination and synchronization of several biological processes. To achieve this, hormones and genes work together. These components can interact at different levels generating hormonal interplay and both positive and negative feedback loops, which in turn, gives robustness, stability, and flexibility to flower development. Here, we summarize the progress made on elucidating the role of different hormonal pathways during flower development in Arabidopsis thaliana.

Utilizing Proteomic Approach to Analyze Potential Antioxidant Proteins in Plant against Irradiation

Gamma-ray irradiation is an effective and clean method of sterilization by inactivating microorganisms. It can also be applied to induce anti-oxidants for future application. In this study, the mung bean (Vigna radiata) was exposed to gamma-ray irradiation under the dose of 0, 5 or 10 kGy. With increasing irradiation doses, the concentrations of malondiadehyde decreased while the levels of total flavonoids and DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging activity increased. It has been shown that consuming flavonoids can provide protective effects. In addition, proteomic analysis identified several proteins having anti-oxidant activities in the 5 kGy irradiated group. These proteins are Apocytochrome f, Systemin receptor SR 160, DELLA protein DWARF8, DEAD-box ATP-dependent RNA helicase 9, ζ-carotene desaturase (ZDS), and Floral homeotic protein AGAMOUS. Our findings indicate that plants contain a variety of phytochemicals and antioxidant proteins which may effectively prevent oxidative stress caused by irradiated peroxidation.

Overexpression of the AGL42 gene in cotton delayed leaf senescence through downregulation of NAC transcription factors

Premature leaf senescence negatively influences the physiology and yield of cotton plants. The conserved IDLNL sequence in the C-terminal region of AGL42 MADS-box determines its repressor potential for the down regulation of senescence-related genes. To determine the delay in premature leaf senescence, Arabidopsis AGL42 gene was overexpressed in cotton plants. The absolute quantification of transgenic cotton plants revealed higher mRNA expression of AGL42 compared to that of the non-transgenic control. The spatial expression of GUS fused with AGL42 and the mRNA level was highest in the petals, abscission zone (flower and bud), 8 days post anthesis (DPA) fiber, fresh mature leaves, and senescenced leaves. The mRNA levels of different NAC senescence-promoting genes were significantly downregulated in AGL42 transgenic cotton lines than those in the non-transgenic control. The photosynthetic rate and chlorophyll content were higher in AGL42 transgenic cotton lines than those in the non-transgenic control. Fluorescence in situ hybridization of the AG3 transgenic cotton line revealed a fluorescent signal on chromosome 1 in the hemizygous form. Moreover, the average number of bolls in the transgenic cotton lines was significantly higher than that in the non-transgenic control because of the higher retention of floral buds and squares, which has the potential to improve cotton fiber yield.

Genome-wide identification, phylogenetic and expression pattern analysis of MADS-box family genes in foxtail millet (Setaria italica)

Foxtail millet (Setaria italica) is rich in nutrients and extremely beneficial to human health. We identified and comprehensively analyzed 89 MADS-box genes in the foxtail millet genome. According to the classification of MADS-box genes in Arabidopsis thaliana and rice, the SiMADS-box genes were divided into M-type (37) and MIKC-type (52). During evolution, the differentiation of MIKC-type MADS-box genes occurred before that of monocotyledons and dicotyledons. The SiMADS-box gene structure has undergone much differentiation, and the number of introns in the MIKC-type subfamily is much greater than that in the M-type subfamily. Analysis of gene duplication events revealed that MIKC-type MADS-box gene segmental duplication accounted for the vast majority of gene duplication events, and MIKC-type MADS-box genes played a major role in the amplification of SiMADS-box genes. Collinearity analysis showed highest collinearity between foxtail millet and maize MADS-box genes. Analysis of tissue-specific expression showed that SiMADS-box genes are highly expressed throughout the grain-filling process. Expression analysis of SiMADS-box genes under eight different abiotic stresses revealed many stress-tolerant genes, with induced expression of SiMADS33 and SiMADS78 under various stresses warranting further attention. Further, some SiMADS-box proteins may interact under external stress. This study provides insights for MADS-box gene mining and molecular breeding of foxtail millet in the future.

Comparative Analysis of the MADS-Box Genes Revealed Their Potential Functions for Flower and Fruit Development in Longan (Dimocarpus longan)

Longan (Dimocarpus longan Lour.) is an important economic crop widely planted in tropical and subtropical regions, and flower and fruit development play decisive effects on the longan yield and fruit quality formation. MCM1, AGAMOUS, DEFICIENS, Serum Response Factor (MADS)-box transcription factor family plays important roles for the flowering time, floral organ identity, and fruit development in plants. However, there is no systematic information of MADS-box family in longan. In this study, 114 MADS-box genes were identified from the longan genome, phylogenetic analysis divided them into type I (Mα, Mβ, Mγ) and type II (MIKC*, MIKC ^C ) groups, and MIKC ^C genes were further clustered into 12 subfamilies. Comparative genomic analysis of 12 representative plant species revealed the conservation of type II in Sapindaceae and analysis of cis-elements revealed that Dof transcription factors might directly regulate the MIKC ^C genes. An ABCDE model was proposed for longan based on the phylogenetic analysis and expression patterns of MADS-box genes. Transcriptome analysis revealed that MIKC ^C genes showed wide expression spectrums, particularly in reproductive organs. From 35 days after KClO₃ treatment, 11 MIKC genes were up-regulated, suggesting a crucial role in off-season flower induction, while DlFLC, DlSOC1, DlSVP, and DlSVP-LIKE may act as the inhibitors. The gene expression patterns of longan fruit development indicated that DlSTK, DlSEP1/2, and DlMADS53 could be involved in fruit growth and ripening. This paper carried out the whole genome identification and analysis of the longan MADS-box family for the first time, which provides new insights for further understanding its function in flowers and fruit.

AGAMOUS Gene as a New Sex-Identification Marker in Fig (Ficus carica L.) Is More Efficient Than RAN1

Fig is an ancient gynodioecious fruit tree with females for commercial fruit production and hermaphrodites (males) sometimes used as pollen providers. An early sex-identification method would improve breeding efficiency. Three AGAMOUS (AG) genes were recruited from the Ficus carica genome using AG sequences from Ficus microcarpa and Ficus hispida. FcAG was 5230 bp in length, with 7 exons and 6 introns, and a 744-bp coding sequence. The gene was present in both female and male fig genomes, with a 15-bp deletion in the 7th exon. The other two AG genes (FcAG2-Gall_Stamen and FcAG3-Gall_Stamen) were male-specific, without the 15-bp deletion (759-bp coding sequence), and were only expressed in the gall and stamen of the male fig fruit. Using the deletion as the forward primer (AG-Marker), male plants were very efficiently identified by the presence of a 146-bp PCR product. The previously reported fig male and female polymorphism gene RESPONSIVE-TO-ANTAGONIST1 (RAN1) was also cloned and compared between male and female plants. Fifteen SNPs were found in the 3015-bp protein-coding sequence. Among them, 12 SNPs were identified as having sex-differentiating capacity by checking the sequences of 27 known male and 24 known female cultivars. A RAN1-Marker of 608 bp, including 6 SNPs, was designed, and a PCR and sequencing-based method was verified with 352 fig seedlings from two hybrid populations. Our results confirmed that the newly established AG-Marker is as accurate as the RAN1-Marker, and provide new clues to understanding Ficus sex determination.

Characteristics of the Diploid, Triploid, and Tetraploid Versions of a Cannabigerol-Dominant F1 Hybrid Industrial Hemp Cultivar, Cannabis sativa 'Stem Cell CBG'

Hemp (Cannabis sativa L.) has recently become an important crop due to the growing market demands for products containing cannabinoids. Unintended cross-pollination of C. sativa crops is one of the most important threats to cannabinoid production and has been shown to reduce cannabinoid yield. Ploidy manipulation has been used in other crops to improve agronomic traits and reduce fertility; however, little is known about the performance of C. sativa polyploids. In this study, colchicine was applied to two proprietary, inbred diploid C. sativa inbred lines, ‘TS1-3’ and ‘P163’, to produce the tetraploids ‘TS1-3 (4x)’ and ‘P163 (4x)’. The diploid, triploid, and tetraploid F₁ hybrids from ‘TS1-3’ × ‘P163’, ‘TS1-3 (4x)’ × ‘P163’, and ‘TS1-3 (4x)’ × ‘P163 (4x)’ were produced to test their fertilities, crossing compatibilities, and yields. The results indicated a reduction in fertility in the triploids and the tetraploids, relative to their diploid counterparts. When triploids were used as females, seed yields were less than 2% compared to when diploids were used as females; thus, triploids were determined to be female infertile. The triploids resulting from the crosses made herein displayed increases in biomass and inflorescence weight compared to the diploids created from the same parents in a field setting. Statistical increases in cannabinoid concentrations were not observed. Lastly, asymmetric crossing compatibility was observed between the diploids and the tetraploids of the genotypes tested. The results demonstrate the potential benefits of triploid C. sativa cultivars in commercial agriculture.

Petal abscission in fragrant roses is associated with large scale differential regulation of the abscission zone transcriptome

Flowers of fragrant roses such as Rosa bourboniana are ethylene-sensitive and undergo rapid petal abscission while hybrid roses show reduced ethylene sensitivity and delayed abscission. To understand the molecular mechanism underlying these differences, a comparative transcriptome of petal abscission zones (AZ) of 0 h and 8 h ethylene-treated flowers from R. bourboniana was performed. Differential regulation of 3700 genes (1518 up, 2182 down) representing 8.5% of the AZ transcriptome was observed between 0 and 8 h ethylene-treated R. bourboniana petal AZ. Abscission was associated with large scale up-regulation of the ethylene pathway but prominent suppression of the JA, auxin and light-regulated pathways. Regulatory genes encoding kinases/phosphatases/F-box proteins and transcription factors formed the major group undergoing differential regulation besides genes for transporters, wall modification, defense and phenylpropanoid pathways. Further comparisons with ethylene-treated petals of R. bourboniana and 8 h ethylene-treated AZ (R. hybrida) identified a core set of 255 genes uniquely regulated by ethylene in R. bourboniana AZ. Almost 23% of these encoded regulatory proteins largely conserved with Arabidopsis AZ components. Most of these were up-regulated while an entire set of photosystem genes was prominently down-regulated. The studies provide important information on regulation of petal abscission in roses.

Ectopic expression of an AGAMOUS homologue gene in Jatropha curcas causes early flowering and heterostylous phenotypes

Jatropha curcasseeds are abundant in biodiesel, and low seed yields are linked to poor quality female flowers, which creates a bottleneck for Jatropha seed utilization. Therefore, identifying the genes associated with flowering is crucial for the genetic enrichment of seed yields. Here, we identified an AGAMOUS homologue gene (JcAG) from J. curcas. We found that reproductive organs had higher JcAG expression than vegetative organs, particularly the carpel. Rosette leaves were small and misshapen in 35S:JcAG transgenic lines in comparison with those in wild-type plants. JcAG overexpression caused an extremely early flowering, delayed perianth and stamen filament development, small flowers, and significantly shorter Arabidopsis plants with little fruit. In the JcAG-overexpressing line, the homeotic transformation of sepals into pistillate organs was observed, and floral meristem and organ identity genes were regulated. This study provides insights into the JcAG's function and benefits to our knowledge of the underlying the genetic mechanisms related to floral sex differentiation in Jatropha.

A GARP transcription factor anther dehiscence defected 1 (OsADD1) regulates rice anther dehiscence

Anther dehiscence, one of the essential steps in pollination and double fertilization, is regulated by a complex signaling pathway encompassing hormones and environmental factors. However, key components underlying the signaling pathway that regulate anther dehiscence remain largely elusive. Here, we isolated a rice mutant anther dehiscence defected 1 (Osadd1) that exhibited defects in anther dehiscence and glume open. Map-based cloning revealed that OsADD1 encoded a GARP (Golden2, ARR-B and Psr1) transcription factor. Sequence analysis showed that a single base deletion in Osadd1 mutant resulted in pre-termination of the GARP domain. OsADD1 was constitutively expressed in various tissues, with more abundance in the panicles. The major genes associated with anther dehiscence were affected in the Osadd1 mutant, and the expression level of the cellulose synthase-like D sub-family 4 (OsCSLD4) was significantly decreased. We demonstrate that OsADD1 regulated the expression of OsCSLD4 by binding to its promoter, and affects rice anther dehiscence.

RNA interference suppression of AGAMOUS and SEEDSTICK alters floral organ identity and impairs floral organ determinacy, ovule differentiation, and seed-hair development in Populus

The role of the floral homeotic gene AGAMOUS (AG) and its close homologues in development of anemophilous, unisexual catkins has not previously been studied. We transformed two RNA interference (RNAi) constructs, PTG and its matrix-attachment-region flanked version MPG, into the early-flowering female poplar clone 6K10 (Populus alba) to suppress the expression of its two duplicate AG orthologues. By early 2018, six out of 22 flowering PTG events and 11 out of 12 flowering MPG events showed modified floral phenotypes in a field trial in Oregon, USA. Flowers in catkins from modified events had 'carpel-inside-carpel' phenotypes. Complete disruption of seed production was observed in seven events, and sterile anther-like organs in 10 events. Events with strong co-suppression of both the two AG and two SEEDSTICK (STK) paralogues lacked both seeds and associated seed hairs. Alterations in all of the modified floral phenotypes were stable over 4 yr of study. Trees from floral-modified events did not differ significantly (P < 0.05) from nonmodified transgenic or nontransgenic controls in biomass growth or leaf morphology. AG and STK genes show strong conservation of gene function during poplar catkin development and are promising targets for genetic containment of exotic or genetically engineered trees.

Micromanagement of Developmental and Stress-Induced Senescence: The Emerging Role of MicroRNAs

MicroRNAs are short (19⁻24-nucleotide-long), non-coding RNA molecules. They downregulate gene expression by triggering the cleavage or translational inhibition of complementary mRNAs. Senescence is a stage of development following growth completion and is dependent on the expression of specific genes. MicroRNAs control the gene expression responsible for plant competence to answer senescence signals. Therefore, they coordinate the juvenile-to-adult phase transition of the whole plant, the growth and senescence phase of each leaf, age-related cellular structure changes during vessel formation, and remobilization of resources occurring during senescence. MicroRNAs are also engaged in the ripening and postharvest senescence of agronomically important fruits. Moreover, the hormonal regulation of senescence requires microRNA contribution. Environmental cues, such as darkness or drought, induce senescence-like processes in which microRNAs also play regulatory roles. In this review, we discuss recent findings concerning the role of microRNAs in the senescence of various plant species.