Expression analysis of ethylene synthesis and signalling genes in kiwifruit stigmatic arms and their involvement in programmed cell death processes.

Kiwifruit (Actinidia chinensis var. deliciosa (A. Chev) A. Chev.) is a widely cultivated crop due to the nutritional value of its fruits. Its commercialization is related to the fruit size, which is directly linked with the number of seeds and, consequently, with pollination. In this dioecious species pollination is dependent on a short effective pollination period which is related to a Programmed Cell Death (PCD) process. At the same time, this PCD process allows the growth of many pollen tubes. Several studies suggest that ethylene can play an important role in PCD in a number of systems. In this report, we determined the full sequence of the AcACS gene, encoding the enzyme that catalyses a rate-limiting step of the ethylene synthesis. Next, we monitored the expression pattern of this gene as well as of other genes involved in ethylene synthesis (ACO2-5) and signalling (AdERS1a, AdERS1b, AdETR1, AdETR2, AdETR3, AdCTR1, AdCTR2, AdEIL1) in pollinated and non-pollinated stigmatic arms of kiwifruit female flowers. The relative expression patterns observed for AcACS, ACOs and ethylene perception and signalling genes (AdERS1, AdETR1, AdCTR1 and AdEIL1) showed that they are expressed before anthesis. After anthesis, expression of the studied genes was detected earlier in pollinated than in non-pollinated stigmatic arms, as it was previously determined for PCD hallmarks. In addition, the expression pattern of the studied genes showed a clear relationship with the PCD hallmarks described in a previous report in the secretory tissue both in non-pollinated stigmatic arms (related to the short EPP in this species) and in pollinated ones (related to the growth of many pollen tubes during progamic phase). Overall, these results suggest an involvement of ethylene with PCD contributing to the high reproductive success of this species.