Changes of Fruit Abscission and Carbohydrates, Hormones, Related Gene Expression in the Fruit and Pedicel of Macadamia under Starvation Stress

Abstract

In order toexplore the regulation mechanism of macadamia fruitlet abscission induced by ‘starvation stress’, a treatment of girdling and defoliation was applied to the bearing shoots of macadamia cultivar ‘H2’ at the early stage of fruit development, simulating the starvation stress induced by interrupting carbon supply to fruit. The levels of carbohydrates, hormones, and related gene expression in the different tissues (husk, seed, and pedicel) were investigated after treatment. The results showed that a severe fruit drop occurred 3~5 d after starvation stress treatment. The contents of glucose, fructose, and sucrose in both the husk and the seed were significantly decreased, as well as the fructose and sucrose in the pedicel; this large reduction occurred prior to the massive fruit shedding. Starvation stress significantly reduced the GA3 and ZR contents and enhanced the ABA level in the pedicel and the seed, whereas it did not obviously change these hormones in the husk. After treatment, IAA content decreased considerably in both the husk and seed but increased remarkably in the pedicel. In the husk, the expression of genes related to sugar metabolism and signaling (NI, HXK2, TPS, and TPP), as well as the biosynthesis of ethylene (ACO2 and ACS) and ABA (NCED1.1 and AAO3), was significantly upregulated by starvation stress, as well as the stress-responsive transcription factors (AP2/ERF, HD-ZIP12, bZIP124, and ABI5), whereas the BG gene associated with ABA accumulation and the early auxin-responsive genes (Aux/IAA22 and GH3.9) were considerably suppressed during the period of massive fruit abscission. Similar changes in the expression of all genes occurred in the pedicel, except for NI and AP2/ERF, the expression of which was significantly upregulated during the early stage of fruit shedding and downregulated during the period of severe fruit drop. These results suggest that complicated crosstalk among the sugar, IAA, and ABA signaling may be related to macadamia fruitlet abscission induced by carbohydrate starvation.