Abstract
Massive young fruit abscission usually causes low and unstable yield in litchi (Litchi chinensis Sonn.), an important fruit crop cultivated in tropical and subtropical areas. However, the molecular mechanism of fruit drop has not been fully characterized. This study aimed at identification of molecular components involved in fruitlet abscission in litchi, for which reference genome is not available at present. An improved de novo transcriptome assembly was firstly achieved by using an optimized assembly software, Trinity. Using improved transcriptome assembly as reference, digital transcript abundance (DTA) profiling was performed to screen and identify candidate genes involved in fruit abscission induced by girdling plus defoliation (GPD), a treatment significantly decreased the soluble sugar contents causing carbohydrate stress to fruit. Our results showed that the increasing fruit abscission rate after GPD treatment was associated with higher ethylene production and lower glucose levels in fruit. A total of 2,771 differentially expressed genes were identified as GPD-responsive genes, 857 of which were defined by GO and KEGG enrichment analyses as the candidate genes involved in fruit abscission process. These genes were involved in diverse metabolic processes and pathways, including carbohydrate metabolism, plant hormone synthesis, and signaling, transcription factor activity and cell wall modification that were rapidly induced in the early stages (within 2 days after treatment). qRT-PCR was used to explore the expression pattern of 15 selected candidate genes in the abscission zone, pericarp, and seed, which confirmed the accuracy of our DTA data. More detailed information for different functional categories was also analyzed. This study profiled the gene expression related to fruit abscission induced by carbohydrate stress at whole transcriptome level and thus provided a better understanding of the regulatory mechanism of young fruit abscission in litchi.
Highlights
Litchi (Litchi chinensis Sonn.) is one of the important economic fruit crops widely grown in south China and Southeast Asian areas
Compared to other three assembly software, Trinity generated the best assembly with the largest average (776 bp) and N50 (1,198 bp) gene lengths and utilized 88.70% of the total reads, SOAPdenovo-Trans (k = 23) came in the second, and SOAPdenovo used in our previous study was the worst in the category due to the lowest utilized reads (Table 1)
The Changes in Fruit Abscission Rate, Ethylene Production, and Sugars Contents in Response to girdling plus defoliation (GPD) Treatment Cumulative fruit abscission rate and ethylene production in fruit were compared between the GPD treatment and the CK (Figure 1)
Summary
Litchi (Litchi chinensis Sonn.) is one of the important economic fruit crops widely grown in south China and Southeast Asian areas. Yuan and Huang (1988) reported three to four waves of fruit drop throughout fruit development in different cultivars based on relative abscission rate. Treatment of girdling (a ring of bark and cambium was removed from the branch base) plus defoliation (100% leaf removal) in litchi reduced endogenous IAA concentration and increased the transcript level of two IAA-responsive genes (LcAUX/IAA1 and LcSAUR1), one cell wall degrading enzyme gene (LcPG1) and one ethylene biosynthetic gene (Lc-ACO1), in contrast to the decreasing accumulation of auxin response factor (LcARF1) mRNA, with the concomitant increase in fruit drop (Kuang et al, 2012; Peng et al, 2013; Wu et al, 2013). A deep knowledge of the molecular events involved in fruit abscission induced by carbohydrate stress is still missing
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