Martin Hegele

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.

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.

The Plant-Physiological Basis of Flower Induction in the Control of Fruit Production

In the last four years, research has focused on off-season flower induction of longan, lychee and mango trees (Chapter 3.3). In order to achieve control over the flower induction process of fruit trees, it is necessary to address the key factors responsible for the transition from vegetative to generative bud development. Various, partly competing theories have been developed in the past about the physiological ‘Who’s Who’ in flower induction (Bernier et al., 1993). One theory favours the role of carbohydrates, which need to be present in sufficient amounts as a prerequisite for flower induction (Sachs, 1977). Other theories of flower induction focus either on the genetic control of a developmental switch from vegetative to generative development (Levy and Dean, 1998), control by particular hormones (Bernier et al., 2002), the existence of specific promoting or inhibiting factors or a mixture of both. However these theories do not apply to adult perennial fruit trees (Goldschmidt and Samach, 2004). Knowledge and understanding of the hormonal changes associated with the treatments previously described (Chapter 3.3) can be beneficial for future trials to induce flowering in mango, lychee and other fruit trees.

Strategies for Flower Induction to Improve Orchard Productivity: From Compensation of Alternate Bearing to Off-Season Fruit Production

Due to alternate and irregular bearing of fruit trees, which occurs at various extent amongst different species and cultivars, the yield of many species of fruit tree is erratic. Uncertainties regarding the time of harvest and the quality and quantity of fruit can seriously affect the marketability of the product (Monselise and Goldschmidt, 1982; Westwood, 1995; Subhadrabandhu, 1999; Souza et al., 2004). Unfavourable climatic conditions during flower induction (FI) or the flowering period are amongst the most important causes of this phenomenon. Often large areas or even whole countries face the same problem simultaneously leading to overproduction and low prices in one year and a low return from fruit production the next. Equalising these fluctuations therefore would help to make fruit production more profitable and sustainable. Another option for raising the return from fruit production would be to extend or totally shift the harvest season by artificially influencing conventional and off-season flowering.