Promoter insertion leads to polyembryony in mango - a case of convergent evolution with citrus.

Sexual reproduction in plants is the main pathway for creating new genetic combinations in modern agriculture. In heterozygous plants, after the identification of a plant with desired traits, vegetative propagation (cloning) is the primary path to create genetically uniform plants. Another natural plant mechanism that creates genetically uniform plants (clones) is apomixis. In fruit crops like citrus and mango, sporophytic apomixis results in polyembryony, where seeds contain multiple embryos, one of which is sexually originated and the others are vegetative clones of the parent mother tree. Utilizing the mango genome and genetic analysis of a diverse germplasm collection, we identified MiRWP as the gene that causes polyembryony in mango. There is a strong correlation between a specific insertion in the gene's promoter region and altered expression in flowers and developing fruitlets, inducing multiple embryos. The MiRWP gene is an ortholog of CitRWP that causes polyembryony in citrus. Based on the data, we speculate that promoter insertion events, which occurred independently in citrus and mango, induced nucellar embryogenesis. The results suggest convergent evolution of polyembryony in the two species. Further work is required to demonstrate the utility of these genes (mango and citrus) in other biological systems as a tool for the clonal production of other crops.

Distinct organ-specific and temporal expression profiles of auxin-related genes during mango fruitlet drop.

In mango, fruitlet abscission initiates with a decrease in polar auxin transport through the abscission zone (AZ), triggered by ethylene. To explore the molecular components affecting this process, we initially conducted experiments with developing fruitlet explants in which fruitlet drop was induced by ethephon, and monitored the expression patterns of distinct indole-3-acetic acid (IAA)-related genes, comparing control vs. ethephon-treated pericarp and AZ profiles. Over the examined time period (48 h), the accumulation of MiPIN1 and MiLAX2 IAA-efflux and influx genes decreased in both control and treated tissues. Nevertheless, ethephon-treated tissues displayed significantly lower levels of these transcripts within 18-24 h. An opposite pattern was observed for MiLAX3, which overall exhibited up-regulation in treated fruitlet tissues. Ethephon treatment also induced an early and pronounced down-regulation of five out of six IAA-responsive genes, and a substantial reduction in the accumulation of two IAA-synthesis related transcripts, contrasting with significant up-regulation of Gretchen Hagen3 transcript (MiGH3.1) encoding an IAA-amino synthetase. Furthermore, for both control and treated AZ, the decrease in IAA-carrier transcripts was associated with a decrease in IAA content and an increase in IAA-Asp:IAA ratio, suggesting that fruitlet drop is accompanied by formation of this non-hydrolyzed IAA-amino acid conjugate. Despite these similarities, ethephon-treated AZ displayed a sharper decrease in IAA content and higher IAA-Asp:IAA ratio within 18 h. Lastly, the response of IAA-related genes to exogenous IAA treatment was also examined. Our results are discussed, highlighting the roles that distinct IAA-related genes might assume during mango fruitlet drop.

Mango (Mangifera indica L.) germplasm diversity based on single nucleotide polymorphisms derived from the transcriptome.

BACKGROUND: Germplasm collections are an important source for plant breeding, especially in fruit trees which have a long duration of juvenile period. Thus, efforts have been made to study the diversity of fruit tree collections. Even though mango is an economically important crop, most of the studies on diversity in mango collections have been conducted with a small number of genetic markers. RESULTS: We describe a de novo transcriptome assembly from mango cultivar 'Keitt'. Variation discovery was performed using Illumina resequencing of 'Keitt' and 'Tommy Atkins' cultivars identified 332,016 single-nucleotide polymorphisms (SNPs) and 1903 simple-sequence repeats (SSRs). Most of the SSRs (70.1%) were of trinucleotide with the preponderance of motif (GGA/AAG)n and only 23.5% were di-nucleotide SSRs with the mostly of (AT/AT)n motif. Further investigation of the diversity in the Israeli mango collection was performed based on a subset of 293 SNPs. Those markers have divided the Israeli mango collection into two major groups: one group included mostly mango accessions from Southeast Asia (Malaysia, Thailand, Indonesia) and India and the other with mainly of Floridian and Israeli mango cultivars. The latter group was more polymorphic (FS=-0.1 on the average) and was more of an admixture than the former group. A slight population differentiation was detected (FST=0.03), suggesting that if the mango accessions of the western world apparently was originated from Southeast Asia, as has been previously suggested, the duration of cultivation was not long enough to develop a distinct genetic background. CONCLUSIONS: Whole-transcriptome reconstruction was used to significantly broaden the mango's genetic variation resources, i.e., SNPs and SSRs. The set of SNP markers described in this study is novel. A subset of SNPs was sampled to explore the Israeli mango collection and most of them were polymorphic in many mango accessions. Therefore, we believe that these SNPs will be valuable as they recapitulate and strengthen the history of mango diversity.

Cloning and molecular characterization of an ethylene receptor gene, MiERS1, expressed during mango fruitlet abscission and fruit ripening.

We isolated and characterized a mango (Mangifera indica L.) cDNA homolog of the ethylene receptor gene ERS1, designated MiERS1. Genomic Southern blot analysis suggested the existence of a second gene with homology to MiERS1. Spatial and temporal expression patterns of MiERS1 were first studied during fruitlet drop and compared with those of a previously identified MiETR1 gene that encodes an ETR1-type ethylene receptor. Experiments were conducted on developing fruitlet explants in which fruitlet abscission was induced by ethephon treatment. Northern analysis revealed a notable increase in MiERS1 mRNA levels in the fruitlet's activated abscission zone within 24 h of ethephon application, followed by a decreasing pattern 48 h post-treatment. A transient, albeit lesser, increase in MiERS1 mRNA levels was also observed in treated fruitlet seed and mesocarp tissues. In contrast, in the abscission zone, accumulation of MiETR1 transcript remained unchanged; a temporal increase in MiETR1 transcript level was observed in the fruitlet mesocarp, whereas in the seed, MiETR1 expression had already dropped by 24 h. Expression profiles of MiERS1 and MiETR1 were then studied during fruit ripening. In agreement with a previous study and coinciding with the climacteric rise in ethylene production, RNA blot analysis revealed that during fruit ripening, MiETR1 mRNA level increases in both mesocarp and seed tissues. Unexpectedly, however, in those same tissues, MiERS1 transcript accumulation was barely detected. Collectively, our data highlight MiERS1's possible specific function in regulating fruitlet abscission rather than fruit ripening.