Jens N. Wünsche

Isolation and characterization of FLOWERING LOCUS T subforms and APETALA1 of the subtropical fruit tree Dimocarpus longan

Longan (Dimocarpus longan Lour.) is a subtropical evergreen fruit tree, mainly cultivated in Asia. Two putative floral integrator genes, D. longan FLOWERING LOCUS T1 and 2 (DlFT1 and DlFT2) were isolated and both translated sequences revealed a high homology to FT sequences from other plants. Moreover, two APETALA1-like (DlAP1-1 and DlAP1-2) sequences from longan were isolated and characterized. Results indicate that the sequences of these genes are highly conserved, suggesting functions in the longan flowering pathway. Ectopic expression of the longan genes in arabidopsis resulted in different flowering time phenotypes of transgenic plants. Expression experiments reveal a different action of the longan FT genes and indicate that DlFT1 is a flowering promoter, while DlFT2 acts as flowering inhibitor. Overexpression of longan AP1 genes in transgenic arabidopsis results in a range of flowering time phenotypes also including early and late flowering individuals.

Influence of postharvest temperatures on leaf gas exchange, carbohydrate reserves and allocations, subsequent budbreak, and fruit yield of ‘Braeburn’ apple (Malus domestica) trees

Abstract Potted ‘Braeburn’ apple (Malus dom‐estica (Borkh.)) trees were grown after harvest in four controlled temperature conditions for 5 weeks to manipulate differences in carbohydrate reserves. Day/night temperatures ranged from 24/19 to 9/4°C. On several occasions, leaf gas exchange and soil respiration were measured and trees were destructively harvested before and after treatment to measure biomass of component parts. Samples were also taken for carbohydrate analysis. After treatment, the trees were returned outdoors and budbreak and fruit growth were measured in the following spring and crop load was measured at the next harvest. Trees at 24/19°C produced new leaves and flowered profusely whereas those at 9/4°C senesced rapidly but these trees grew new roots. Photosynthesis and respiration were highly dependent on temperature, and after 5 weeks of growth there were marked differences in rates between the various treatments. Non‐structural carbohydrate concentrations declined in trees at all treatments but mostly at 24/19°C. However, carbohydrate reserves increased in trees in all treatments except at 24/19°C. Budbreak was delayed by up to 16 days in the trees treated at 24/19°C compared to trees treated at 9/4°C but treatment had no affect on fruit growth. Crop yields were highest in those trees with highest carbohydrate reserves, providing some support for the conclusion that high apple yields in New Zealand are dependent on elevated carbohydrate reserves achieved by favourable photosynthetic conditions and the extended growing season between harvest and leaf senescence.

Late-season temperature effects on the carbon economy and tree performance of 'Royal Gala' apple (Malus domestica) trees

Abstract Potted ‘Royal Gala’ apple on M.9 rootstock (Malus domestica) trees were grown after fruit harvest at constant temperature conditions (18/ 8°C day/night) for 3, 6, and 9 weeks to manipulate their carbohydrate reserves. On several occasions, leaf gas exchange was measured and selected trees were destructively harvested before and after each treatment to measure the dry weight of the component parts. Samples were also taken for carbohydrate analysis. After treatment, the remaining trees were returned outdoors and, in spring, selected trees from each treatment were destructively harvested for dry weight and carbohydrate analysis. Budbreak was then measured and fruit weight and crop load determined next autumn and the trees destructively harvested for dry weight and carbohydrate analysis. There was no change in tree dry weight after each late‐season temperature treatment. Carbohydrate concentrations, averaged over the whole tree, increased by 10–15% compared to the pre‐treatment trees, consistent with a net increase in carbon acquisition. Over winter, the total carbohydrate concentration had declined by 25–40%. In spring, time of budbreak differed significantly; trees exposed for 3 weeks to 18/8°C in late‐season broke buds 9–19 days earlier than the other treatments in the following spring. The early budbreak was associated with not only the largest increase in carbohydrate concentration during treatment but also the greatest decrease thereafter. Although there is some support for the conclusion that high carbohydrate reserves confer a direct benefit on the budbreak process, it remains an open question if it was the increase in reserves, or their subsequent consumption that advanced budbreak.

Genetic variation in yield under hot ambient temperatures spotlights a role for cytokinin in protection of developing floral primordia

Unusually hot ambient temperatures (HAT) can cause pre-anthesis abortion of flowers in many diverse species, limiting crop production. This limitation is becoming more substantial with climate change. Flower primordia of passion fruit (Passiflora edulis Sims) vines exposed to HAT summers, normally abort. Flower abortion can also be triggered by gibberellin application. We screened for, and identified a genotype capable of reaching anthesis during summer as well as controlled HAT conditions, and also more resistant to gibberellin. Leaves of this genotype contained higher levels of endogenous cytokinin. We investigated a possible connection between higher cytokinin levels and response to gibberellin. Indeed, the effects of gibberellin application were partially suppressed in plants pretreated with cytokinin. Can higher cytokinin levels protect flowers from aborting under HAT conditions? In passion fruit, flowers at a specific stage showed more resistance in response to HAT after cytokinin application. We further tested this hypothesis in Arabidopsis. Transgenic lines with high or low cytokinin levels and cytokinin applications to wild-type plants supported a protective role for cytokinin on developing flowers exposed to HAT. Such findings may have important implications in future breeding programmes as well as field application of growth regulators.

Ethephon induced abscission in mango: physiological fruitlet responses

Fruitlet abscission of mango is typically very severe, causing considerable production losses worldwide. Consequently, a detailed physiological and molecular characterization of fruitlet abscission in mango is required to describe the onset and time-dependent course of this process. To identify the underlying key mechanisms of abscission, ethephon, an ethylene releasing substance, was applied at two concentrations (600 and 7200 ppm) during the midseason drop stage of mango. The abscission process is triggered by ethylene diffusing to the abscission zone where it binds to specific receptors and thereby activating several key physiological responses at the cellular level. The treatments reduced significantly the capacity of polar auxin transport through the pedicel at 1 day after treatment and thereafter when compared to untreated pedicels. The transcript levels of the ethylene receptor genes MiETR1 and MiERS1 were significantly upregulated in the pedicel and pericarp at 1, 2, and 3 days after the ethephon application with 7200 ppm, except for MiETR1 in the pedicel, when compared to untreated fruitlet. In contrast, ethephon applications with 600 ppm did not affect expression levels of MiETR1 in the pedicel and of MiERS1 in the pericarp; however, MiETR1 in the pericarp at day 2 and MiERS1 in the pedicel at days 2 and 3 were significantly upregulated over the controls. Moreover, two novel short versions of the MiERS1 were identified and detected more often in the pedicel of treated than untreated fruitlets at all sampling times. Sucrose concentration in the fruitlet pericarp was significantly reduced to the control at 2 days after both ethephon treatments. In conclusion, it is postulated that the ethephon-induced abscission process commences with a reduction of the polar auxin transport capacity in the pedicel, followed by an upregulation of ethylene receptors and finally a decrease of the sucrose concentration in the fruitlets.

Expression and interaction of the mango ethylene receptor MiETR1 and different receptor versions of MiERS1.

Different versions of the mango ethylene receptor MiERS1 were identified and the analysis indicates that, in addition to MiERS1, two short versions of this receptor (MiERS1m, MiERS1s), representing truncated proteins with central deletions of functional domains, are present in mango. The short receptor versions reveal a different expression pattern compared to MiERS1, and they are highly variably transcribed. With transient expression assays using fluorescent fusion proteins, the localisation and the interaction of the receptors were determined in leaf cells of the tobacco model. MiERS1, MiETR1, and the short MiERS1 receptor versions are anchored in the endoplasmic reticulum (ER) membrane and co-localise with each other and with an ER-marker. Furthermore, ectopic expression of the mango receptors appears to induce a re-organisation of the ER resulting in accumulation of ER bodies. Interaction assays suggest that both short MiERS1 receptor versions can bind to proteins located in the ER. Bi-molecular fluorescence complementation (BiFC) assays indicate, that MiERS1m may dimerise with itself and can also interact with MiERS1, but not with MiETR1. Further, it as found that MiETR1 can interact with MiERS1. Interaction of MiERS1s with the other ethylene receptors could not be detected, although it was located in the ER membrane system.