Mature fruit abscission (MFA) is a genetically controlled process, through poorly characterized mechanisms in fleshy fruit that include extensive transcriptional changes. While global transcriptome analyses have been used to investigate immature fruit abscission in fleshy fruit, no global gene expression changes specific to MFA have been described. Here we use pyrosequencing to characterize the transcriptomes of the olive abscission zone (AZ) during cell separation in order to understand MFA control at the stage of AZ activation. Analysis of gene expression from these AZs reveals that membrane microdomains involving sterols/sphingolipids and remorins together with signaling proteins are potentially involved in MFA. This is accompanied by gene activity related to sphingolipid turnover, suggesting potentially the involvement of long-chain base metabolism in regulating MFA. Activation of vesicle trafficking involving small GTPases is probably required for cell wall modifications during abscission. Analysis of transcription factors indicates that most members of the MYB and bZIP families are abundantly represented in the fruit AZ, and is consistent with a model by which most of the key transcription factors during abscission may regulate downstream processes mostly related to ABA. The data provide the first thorough analysis available for a comprehensive picture of the array of cellular responses controlled by gene expression that lead to MFA in fleshy fruit.

Exogenous ethylene and some inhibitors of polyamine biosynthesis can induce mature-fruit abscission in olive, which could be associated with decreased nitric oxide production as a signaling molecule. Whether H₂O₂ also plays a signaling role in mature-fruit abscission is unknown. The possible involvement of H₂O₂ and polyamine in ethylene-induced mature-fruit abscission was examined in the abscission zone and adjacent cells of two olive cultivars. Endogenous H₂O₂ showed an increase in the abscission zone during mature-fruit abscission, suggesting that accumulated H₂O₂ may participate in abscission signaling. On the other hand, we followed the expression of two genes involved in the polyamine biosynthesis pathway during mature-fruit abscission and in response to ethylene or inhibitors of ethylene and polyamine. OeSAMDC1 and OeSPDS1 were expressed differentially within and between the abscission zones of the two cultivars. OeSAMDC1 showed slightly lower expression in association with mature-fruit abscission. Furthermore, our data show that exogenous ethylene or inhibitors of polyamine encourage the free putrescine pool and decrease the soluble-conjugated spermidine, spermine, homospermidine, and cadaverine in the olive abscission zone, while ethylene inhibition by CoCl₂ increases these soluble conjugates, but does not affect free putrescine. Although the impact of these treatments on polyamine metabolism depends on the cultivar, the results confirm that the mature-fruit abscission may be accompanied by an inhibition of S-adenosyl methionine decarboxylase activity, and the promotion of putrescine synthesis in olive abscission zone, suggesting that endogenous putrescine may play a complementary role to ethylene in the normal course of mature-fruit abscission.

No abstract

No abstract