Abstract
Auxin, a vital plant hormone, regulates a variety of physiological and developmental processes. It is involved in fruit abscission through transcriptional regulation of many auxin-related genes, including early auxin responsive genes (i.e., auxin/indole-3-acetic acid (AUX/IAA), Gretchen Hagen3 (GH3) and small auxin upregulated (SAUR)) and auxin response factors (ARF), which have been well characterized in many plants. In this study, totally five auxin-related genes, including one AUX/IAA (LcAUX/IAA1), one GH3 (LcGH3.1), one SAUR (LcSAUR1) and two ARFs (LcARF1 and LcARF2), were isolated and characterized from litchi fruit. LcAUX/IAA1, LcGH3.1, LcSAUR1, LcARF1 and LcARF2 contain open reading frames (ORFs) encoding polypeptides of 203, 613, 142, 792 and 832 amino acids, respectively, with their corresponding molecular weights of 22.67, 69.20, 11.40, 88.20 and 93.16 kDa. Expression of these genes was investigated under the treatment of girdling plus defoliation which aggravated litchi fruitlet abscission due to the blockage of carbohydrates transport and the reduction of endogenous IAA content. Results showed that transcript levels of LcAUX/IAA1, LcGH3.1 and LcSAUR1 mRNAs were increased after the treatment in abscission zone (AZ) and other tissues, in contrast to the decreasing accumulation of LcARF1 mRNA, suggesting that LcAUX/IAA1, LcSAUR1 and LcARF1 may play more important roles in abscission. Our results provide new insight into the process of fruitlet abscission induced by carbohydrate stress and broaden our understanding of the auxin signal transduction pathway in this process at the molecular level.
Highlights
Fruit abscission, occurring in advance of harvest, is characterized by the activation of a specific abscission zone (AZ) located between the pedicel and fruitlet, resulting in severe economic losses in many fruits [1,2]
Multiple sequence alignment showed that LcAUX/IAA1 contained four conserved domains (I, II, III and IV), which are typical motifs for the Aux/indoleacetic acid (IAA) proteins (Figure 1) [7]
Simian Virus 40 (SV40) and a Mat a2-like nuclear localization signal (NLS), which is present at the end of domain IV (Figure 1) [34]
Summary
Fruit abscission, occurring in advance of harvest, is characterized by the activation of a specific abscission zone (AZ) located between the pedicel and fruitlet, resulting in severe economic losses in many fruits [1,2]. It is well documented that auxin stimulates the rapid transcription of a set of genes called primary or early auxin responsive genes including Aux/IAA, GH3, SAUR [6,7,8]. The AUX/IAA proteins are short-lived nuclear proteins that function as repressors of auxin-induced transcription, and auxin promotes the degradation of this large family of transcriptional regulators, leading to diverse downstream responses [9,10]. GH3 genes encode IAA-conjugating enzymes, which could act as feedback regulators by reducing free auxin levels [11], while SAUR genes code for highly conserved short-lived small proteins and likely function in cell expansion, hypocotyls and stamen filament elongation [12,13]. The promoter regions of these primary/early auxin-responsive genes contain one or more auxin-responsive elements (AuxRE)
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