The avocado, Persea americana, is a fruit crop of immense importance to Mexican agriculture with an increasing demand worldwide. Avocado lies in the anciently diverged magnoliid clade of angiosperms, which has a controversial phylogenetic position relative to eudicots and monocots. We sequenced the nuclear genomes of the Mexican avocado race, P. americana var. drymifolia, and the most commercially popular hybrid cultivar, Hass, and anchored the latter to chromosomes using a genetic map. Resequencing of Guatemalan and West Indian varieties revealed that ∼39% of the Hass genome represents Guatemalan source regions introgressed into a Mexican race background. Some introgressed blocks are extremely large, consistent with the recent origin of the cultivar. The avocado lineage experienced 2 lineage-specific polyploidy events during its evolutionary history. Although gene-tree/species-tree phylogenomic results are inconclusive, syntenic ortholog distances to other species place avocado as sister to the enormous monocot and eudicot lineages combined. Duplicate genes descending from polyploidy augmented the transcription factor diversity of avocado, while tandem duplicates enhanced the secondary metabolism of the species. Phenylpropanoid biosynthesis, known to be elicited by Colletotrichum (anthracnose) pathogen infection in avocado, is one enriched function among tandems. Furthermore, transcriptome data show that tandem duplicates are significantly up- and down-regulated in response to anthracnose infection, whereas polyploid duplicates are not, supporting the general view that collections of tandem duplicates contribute evolutionarily recent “tuning knobs” in the genome adaptive landscapes of given species.

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Serotonin (5-hydroxytryptamine) is a well-known neurotransmitter in mammals and is widely distributed in plants. This compound is synthesized from tryptophan and shares structural similarity with IAA. To date, little is known about the morphological, physiological and molecular responses of plants to serotonin. In this study, we characterized the effects of serotonin on growth and development in Arabidopsis thaliana seedlings. Gas chromatography-mass spectrometry (GC-MS) analysis showed that plants are able to take up serotonin from the growth medium, which coincided with greatly stimulated lateral root development at concentrations from 10 to 160 μM. In contrast, higher doses of serotonin repressed lateral root growth, primary root growth and root hair development, but stimulated adventitious root formation. To investigate the role of serotonin in modulating auxin responses, we performed experiments using transgenic Arabidopsis lines expressing the auxin-responsive marker constructs DR5:uidA, BA3:uidA and HS::AXR3NT-GUS, as well as a variety of Arabidopsis mutants defective at the AUX1, AXR1, AXR2 and AXR4 auxin-related loci. We found that serotonin strongly inhibited both DR5:uidA and BA3:uidA gene expression in primary and adventitious roots and in lateral root primordia. This compound also abolished the effects of IAA or naphthaleneacetic acid on auxin-regulated developmental and genetic responses, indicating an anti-auxin activity in the plant. Mutant analysis further showed that lateral root induction elicited by serotonin was independent of the AUX1 and AXR4 loci but required AXR1 and AXR2. Our results show that serotonin regulates root development probably by acting as a natural auxin inhibitor.

BackgroundAvocado (Persea americana) is an economically important tropical fruit considered to be a good source of fatty acids. Despite its importance, the molecular and cellular characterization of biochemical and developmental processes in avocado is limited due to the lack of transcriptome and genomic information.ResultsThe transcriptomes of seeds, roots, stems, leaves, aerial buds and flowers were determined using different sequencing platforms. Additionally, the transcriptomes of three different stages of fruit ripening (pre-climacteric, climacteric and post-climacteric) were also analyzed. The analysis of the RNAseqatlas presented here reveals strong differences in gene expression patterns between different organs, especially between root and flower, but also reveals similarities among the gene expression patterns in other organs, such as stem, leaves and aerial buds (vegetative organs) or seed and fruit (storage organs). Important regulators, functional categories, and differentially expressed genes involved in avocado fruit ripening were identified. Additionally, to demonstrate the utility of the avocado gene expression atlas, we investigated the expression patterns of genes implicated in fatty acid metabolism and fruit ripening.ConclusionsA description of transcriptomic changes occurring during fruit ripening was obtained in Mexican avocado, contributing to a dynamic view of the expression patterns of genes involved in fatty acid biosynthesis and the fruit ripening process.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1775-y) contains supplementary material, which is available to authorized users.