Differential Regulation of Genes Encoding Ethylene Biosynthesis Enzymes and Ethylene Response Sensor Ortholog during Ripening and in Response to Wounding in Avocados

Abstract

<jats:p>We investigated the differential regulation of two 1-aminocyclopropane-1-carboxylate synthase (ACS) genes, one 1-aminocyclopropane-1-carboxylate oxidase (ACO) gene and one ethylene response sensor (<jats:italic>ERS1</jats:italic>) ortholog during ripening and in response to wounding in avocados (<jats:italic>Persea americana</jats:italic> Mill. `Bacon'). The 1-aminocyclopropane-1-carboxylate (ACC) content, ACS activity and detectable expression of <jats:italic>PA-ACS1</jats:italic> mRNA increased and reached a maximum prior to the climacteric peak, whereas ACO activity and the <jats:italic>PA-ACO</jats:italic> mRNA levels increased markedly only at the upsurge of ripening ethylene. A basal level of <jats:italic>PA-ERS1</jats:italic> transcript was detected as from harvest, however, <jats:italic>PA-ERS</jats:italic>1 transcript was hyper-induced at the climacteric peak of ethylene production. 1-Methylcyclopropene (1-MCP) application at thepreclimacteric and the onset of climacteric stages inhibited the ACS and ACO activities, the transcription of <jats:italic>PA-ACS1</jats:italic> and suppressed <jats:italic>PA-ACO</jats:italic> and <jats:italic>PA-ERS1</jats:italic> mRNAs to trace levels. Discontinuation of 1-MCP treatment led to super-induction of <jats:italic>PA-ACS1</jats:italic>, <jats:italic>PA-ACO</jats:italic>, and <jats:italic>PA-ERS1</jats:italic> transcripts. Wound induced ethylene biosynthesis and wound-induced <jats:italic>PA-ACS2</jats:italic> mRNA accumulation were enhanced by 1-MCP, whereas wound-induced <jats:italic>PA-ACO</jats:italic> mRNA accumulation was unaffected by 1-MCP. These results indicate positive feedback regulation of the <jats:italic>PA-ACS1</jats:italic> gene and negative feedback regulation of the <jats:italic>PA-ACS2</jats:italic> gene by ethylene, while <jats:italic>PA-ACO</jats:italic> exhibits positive feedback regulation by ethylene and is also induced by wounding. The hyper-induction of <jats:italic>PA-ERS1</jats:italic> mRNA at relatively high concentrations of ethylene may be a mechanism of avocados to regulate the ethylene responsiveness of the tissues by dissipation of the gas.</jats:p>

Induction of ethylene in avocado fruit in response to chilling stress on tree

“…In contrast, PaACS1, PaACS2, and PaACO transcript levels in control non-chilled avocado were very low on the day of harvest and increased markedly during ripening, reaching high levels after 4 d at 20 1C, similar to those of CS fruits at time 0 ( Figure 3A-C). Thus, the current study verifies previous observations, which have shown detectable expression of avocado ACS and ACO genes and very low activity of ACS and ACO at harvest that increased in correlation with the beginning of the climacteric rise during ripening (Owino et al, 2002). The inability of most avocado varieties to produce ethylene as long as they are attached to the tree results mainly from repression of ACS activity (Sitrit et al, 1986).…”

“…In control nonchilled avocado fruit, PaETR and PaERS1 transcripts were low at time 0, up-regulated at the onset of normal ripening, and correlated with the level of climacteric ethylene production ( Figure 1A vs. 4A and B). A similar expression pattern of ethylene receptors during ripening was shown in tomato (Klee, 2002), avocado (Owino et al, 2002) pear (El-Sharkawy et al, 2003) and apple (Tatsuki and Endo, 2006). PaERS1 gene expression was affected by chilling stress more than by ripening ( Figure 4B), while PaETR gene expression was equally affected by chilling stress and normal ripening ( Figure 4A).…”

“…The first strand cDNAs were synthesized by reverse transcriptase (ABgene, Epson, UK) from 1 mg of total RNA, pretreated with 1.5 units of RQ1 (Promega, Madison, WI, USA). The ethylene receptor PaERS1 cDNA fragment was amplified using the specific primers designed against PaERS1 (Owino et al, 2002) and the PaCTR1 cDNA fragments of the genes were amplified using the degenerate primers designed on the basis of conserved sequence motifs of the CTR genes. The primers were as follows:…”

“…This was followed by a marked increase during the climacteric rise, reaching a peak shortly before the ethylene peak (Sitrit et al, 1986). ACO activity, on the other hand, increased markedly only at the upsurge of ripening ethylene (Owino et al, 2002). It was shown that cold storage stimulated ACO and ACS gene expression in apples (Tian et al, 2002) and in pears (El-Sharkawy et al, 2004;Fonseca et al, 2005).…”

“…The study of ethylene response-pathway elements and their expression patterns has become an important aspect of fruit ripening studies in various fruit species (Owino et al, 2002;Wiersma et al, 2007).…”