Different regulatory modules of two mango ERS1 promoters modulate specific gene expression in response to phytohormones in transgenic model plants.

Ethylene is a key element of plant physiology, thus ethylene research is important for both, fundamental research and agriculture. Previous work on ethylene receptors focused on expression level and protein interaction, but knowledge on regulation of gene transcription is scarce. Promoters of mango ethylene receptor genes (pMiERS1a, pMiERS1b) were analysed particularly regarding responsiveness to hormones. The promoter sequences reveal some variation and they were characterized by identifying functional regulatory candidate modules via truncated-promoter approach. Based on ectopic gene expression studies in transgenic Arabidopsis and Nicotiana it is demonstrated that both promoters are positively responsive to ethylene. For pMiERS1a the AHBP/DOFF1 module is linked to ethylene responsiveness, while for pMiERS1b it is the module MYBL/OPAQ1. A negative gene regulation in response to abscisic acid (ABA) is linked to MYBL/DOFF2. A positive response to indole-3-acetic acid (IAA) was found for GTBX/MYCL1, containing the motifs IBOX/IDDF/TEFB, which are present in this combination only in pMiERS1b, but not in pMiERS1a. Conclusively, the general response of the ethylene receptor genes is conserved, but similar regulation can be linked to different modules. Further, a minor variation in a transcription factor binding site (TFBS) motif within an overall conserved module type can lead to a different expression.

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.

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.

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.

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.

Root-zone temperatures affect phenology of bud break, flower cluster development, shoot extension growth and gas exchange of 'Braeburn' (Malus domestica) apple trees.

We investigated the effects of root-zone temperature on bud break, flowering, shoot growth and gas exchange of potted mature apple (Malus domestica (Borkh.)) trees with undisturbed roots. Soil respiration was also determined. Potted 'Braeburn' apple trees on M.9 rootstock were grown for 70 days in a constant day/night temperature regime (25/18 degrees C) and one of three constant root-zone temperatures (7, 15 and 25 degrees C). Both the proportion and timing of bud break were significantly enhanced as root-zone temperature increased. Rate of floral cluster opening was also markedly increased with increasing root-zone temperature. Shoot length increased but shoot girth growth declined as root-zone temperatures increased. Soil respiration and leaf photosynthesis generally increased as root-zone temperatures increased. Results indicate that apple trees growing in regions where root zone temperatures are < or = 15 degrees C have delayed bud break and up to 20% fewer clusters than apple trees exposed to root zone temperatures of > or = 15 degrees C. The effect of root-zone temperature on shoot performance may be mediated through the mobilization of root reserves, although the role of phytohormones cannot be discounted. Variation in leaf photosynthesis across the temperature treatments was inadequately explained by stomatal conductance. Given that root growth increases with increasing temperature, changes in sink activity induced by the root-zone temperature treatments provide a possible explanation for the non-stomatal effect on photosynthesis. Irrespective of underlying mechanisms, root-zone temperatures influence bud break and flowering in apple trees.

Physiological and biochemical leaf and tree responses to crop load in apple.

Seven-year-old apple (Malus x domestica Borkh.) trees cv. 'Braeburn' on rootstock M.26 were flower-thinned to establish four crop loads, resulting in final mean fruit numbers per tree of 0, 100, 225 and 400. Mean fruit mass decreased by about 35% with each decrease in cropping density. Fruit from light-cropping trees had significantly advanced maturity as indicated by the harvest management criteria of background color and starch/iodine score, and other fruit quality characteristics such as soluble solids. Flesh firmness and dry matter also increased with decreasing crop load. Compared with fruiting trees, mean leaf photosynthetic rates of non-cropping trees were significantly lower (40%) between 75 days after full bloom (dafb) and fruit harvest, with a maximum reduction of almost 60% at 118 dafb. Photosynthetic activity decreased linearly with increasing concentration of leaf starch, but was positively and significantly related to stomatal conductance. Consequently, the accumulation of nonstructural carbohydrates in leaves of light-cropping or non-cropping trees may have led to end-product inhibition of photosynthesis. Increases in xanthophyll cycle carotenoids mediated non-radiative thermal energy dissipation in non-cropping trees, providing increased capacity for photoprotection but reducing photochemical efficiency.