Molecular and biochemical analyses of avocado (Persea americana) reveal differences in the oil accumulation pattern between the mesocarp and seed during the fruit developmental period

Abstract Background: The avocado ( Persea americana ) mesocarp and seed contain high-value oil with broad industrial applications. The oil contents in these two tissues vary considerably at maturity. Additionally, the molecular mechanism underlying the tissue-specific oil accumulation in the developing avocado mesocarp and seed remains unclear, which has hampered the exploration of the utility of avocado for oil production. Results: To clarify the mechanisms mediating the differences in oil contents and fatty acid compositions, the transcriptomes and oil bodies were compared between the oil-storing tissues during the fruit developmental period. The results revealed the increasing and fluctuating trends in the oil accumulation in the developing avocado mesocarp and seed, respectively. Additionally, striking differences in the lipid droplets between the mature mesocarp and seed were revealed in confocal microscopy images. Subsequently, the gene transcription profiles of the developing mesocarp and seed were characterized via a comprehensive transcriptome analysis involving second-generation sequencing and single-molecule real-time sequencing techniques. The tissue-specific transcription of lipid-related genes contributing to fatty acid synthesis, triacylglycerol assembly, and triacylglycerol storage was examined, with most of the lipid-related genes expressed at higher levels in the developing mesocarp than in the developing seed. A weighted gene co-expression network analysis uncovered 291 transcription factors that were commonly or uniquely correlated with the oil contents in the avocado mesocarp and seed. Moreover, 11 trans-acting and 79 cis-acting long non-coding RNAs were identified as common or unique to the developing avocado mesocarp and seed. These long non-coding RNAs may regulate the expression of 43 lipid-related genes. Finally, a network of genes associated with oil accumulation in the developing avocado mesocarp and seed was established. Conclusions: The results of this study further elucidate the tissue-specific oil biosynthesis and related regulatory network in the avocado mesocarp and seed. Furthermore, tissue-specific lipid-related genes, putative transcription factors, and putativelong non-coding RNAs affecting oil accumulation were identified. Our data may also be useful for characterizing tissue-specific oil accumulation at the transcriptomic level, thereby identifying candidate genes for improving the oil production of related plant species.