Effect of plant growth regulators on calyx abscission, fruit quality, and auxin-repressed protein (ARP) gene expression in fruitlets of ‘Dangshansuli’ pear (Pyrus bretschneideri Rehd.)

Abstract

SummarySepals of a pome fruit flower are collectively called the calyx. Calyxes may be shed to produce calyx-shed fruit (CSF), or remain to produce calyx-persistent fruit (CPF). To investigate the relationship between calyx shedding and phytohormone concentrations in fruitlets and fruit quality in ‘Dangshansuli’ pear (Pyrus bretschneideri Rehd.), endogenous levels of gibberellins, indole-3-acetic acid, abscisic acid, and zeatin riboside were measured in CSF and CPF at different stages of development. Manual excision of the calyx at an early stage of fruitlet development was also performed. The auxin-repressed protein gene (ARP) was cloned, and relative levels of ARP gene expression were determined in fruitlets on trees treated by spraying separately with water (control) or each of the plant growth regulators (PGRs) gibberellic acid (GA3), paclobutrazol (Pac), or 1-naphthaleneacetic acid (NAA) using real-time quantitative reverse transcription-PCR. The results indicated that manual calyx excision at an early stage of fruit development altered only the shape and appearance, but not the inner quality of the fruit. Higher endogenous levels of all gibberellins in fruitlets at 10 - 20 d after full bloom (DAFB) played an important role in calyx persistency on fruitlets. The ARP gene was cloned from fruitlets of ‘Dangshansuli’ pear and named PbARP. The full-length cDNA was 707 bp with a 5'-untranslated region (UTR) of 40 bp, a 3'-UTR of 316 bp, and a coding region of 351 bp, encoding a deduced polypeptide containing 116 amino acids.GA3 [100 mg l-1 of 90% (w/w)] inhibited expression of the PbARP gene and increased calyx persistency, whereas 100 mg l-1 of 15% (w/w) Pac enhanced PbARP gene expression and accelerated calyx shedding. Higher levels of PbARP gene expression in fruitlets of ‘Dangshansuli’ pear were associated with calyx shedding.The percentage of CSF increased by > 20% following treatment with 100 mg l-1 of 15% (w/w) Pac, 100 ml l-1 of an emulsifiable concentrate of 40% (w/v) flusilazole (Fls), a mixture of 100 mg l-1 of 15% (w/w) Pac plus 50 mg l-1 of 99% (w/w) benzylaminopurine (BA), or a mixture of 100 ml l-1 of 40% (w/v) Fls plus 50 mg l-1 of 99% (w/w) BA, with no significant difference in fruit set percentage or fruit quality. Persistence or shedding of the calyx in pear fruit is therefore an epigenetic trait that can be regulated by the application of PGRs.