Ethylene is involved in pistil fate by modulating the onset of ovule senescence and the GA-mediated fruit set in Arabidopsis

Pablo Carbonell-Bejerano 0 1 Cristina Urbez 0 Antonio Granell 0 Juan Carbonell 0 Miguel A Perez-Amador 0 0 Instituto de Biologia Molecular y Celular de Plantas (IBMCP), Universidad Politecnica de Valencia (UPV)-Consejo Superior de Investigaciones Cientificas (CSIC). Ciudad Politecnica de la Innovacion (CPI) , Ed. 8E, Ingeniero Fausto Elio s/n, 46022 Valencia , Spain 1 Centro Nacional de Biotecnologia (CNB) , Consejo Superior de Investigaciones Cientificas (CSIC), Cantoblanco, 28049 Madrid , Spain Background: Ovule lifespan is an important factor in determining the ability to set fruits and produce seeds. Once ovule senescence is established, fruit set capacity in response to gibberellins (GAs) is lost. We aimed to elucidate whether ethylene plays a role in controlling ovule senescence and the fruit set response in Arabidopsis. Results: Ethylene response inhibitors, silver thiosulphate (STS) and 1-methylcyclopropene (1-MCP), were able to delay the loss of pistil response to GA3. In addition, ethylene insensitive mutants ein2-5 and ein3-1 showed delayed loss of pistil response, as in plants treated with STS and 1-MCP, while constitutive mutant ctr1-1 displayed premature loss of response. The analysis of the expression of ethylene biosynthesis genes suggests that ethylene is synthesised in ovules at the onset of ovule senescence, while a transcriptional meta-analysis also supports an activated ethylene-dependent senescence upon the establishment of ovule senescence. Finally, a SAG12:GUS reporter line proved useful to monitor ovule senescence and to directly demonstrate that ethylene specifically modulates ovule senescence. Conclusions: We have shown that ethylene is involved in both the control of the ovule lifespan and the determination of the pistil/fruit fate. Our data support a role of the ovule in modulating the GA response during fruit set in Arabidopsis. A possible mechanism that links the ethylene modulation of the ovule senescence and the GA3-induced fruit set response is discussed. - Background The pistil is a highly specialised floral organ designed to facilitate fertilisation, seed development and dispersal. Pistils become mature fruits by following a complex developmental programme triggered by ovule fertilisation, and by the hormonal signal cascade that follows. In the absence of this triggering event, the pistils autonomous developmental programme leads to organ senescence after a few days [1-4]. Pistil senescence has been studied in pea (Pisum sativum) and Arabidopsis (Arabidopsis thaliana) plants. Unpollinated pea pistil senescence involves programmed cell death, which initiates at 2-3 days post-anthesis (DPA) [1,5,6]. Its onset correlates with both the expression of proteolytic activities [7-9] and the whole pistils cell degradation [2], including DNA fragmentation in specific cells at both the ovary wall and ovules [6]. More recently, we showed that the development of the Arabidopsis unfertilised pistil differs from that of pea since the Arabidopsis ovary wall shows developmental characteristics that are shared with a developing fruit, while senescence is specifically established first at the stigma, and then progresses from basal to apical ovules [4]. One physiological marker of pistil senescence in both pea and Arabidopsis is the loss of the pistils capacity to develop into a parthenocarpic fruit in response to exogenous gibberellic acid (GA3) [4,5]. The loss of pistil response to GA3 in Arabidopsis correlates with the onset of ovule senescence and its acropetal progression along the ovary [4]. In addition, several mutants with defects in ovule development showed a reduced fruit set response to GA3 [4]. Collectively, these data suggest that viable non-senescing ovules play a critical role in promoting fruit set in response to GA in Arabidopsis unfertilised pistils. The identification of the physiological and molecular factors regulating pistil/ovule senescence is important since the pistils capacity to develop as a fruit is lost when senescence is initiated. Therefore by delaying ovule senescence, pistil longevity is expected to increase. This can lead to important biotechnological applications because reduced pistil longevity can be a limiting factor for sexual reproduction and fruit production [10-13]. Ethylene is involved in the control of several terminal processes during vegetative and reproductive development, including senescence of leaves [14-16], senescence and abscission of floral organs [3,17-19] and ripening of fruits [20]. In pea, ethylene regulates both petal and unfertilised whole pistil senescence [6,21]. Ethylene production increases during pea flower senescence, and the inhibition of ethylene action with silver thiosulphate (STS) delays senescence symptoms, including a postponed loss of the capacity to set parthenocarpic fruits in response to GA3 [6]. Ethylene signalling has been extensively reviewed in recent years [22-25]. Briefly, ethylene is perceived by a s (...truncated)