Nello Ceccarelli

Tomato fruit set driven by pollination or by the parthenocarpic fruit allele are mediated by transcriptionally regulated gibberellin biosynthesis

We investigated the role of gibberellins (GAs) in the phenotype of parthenocarpic fruit (pat), a recessive mutation conferring parthenocarpy in tomato (Solanum lycopersicum L.). Novel phenotypes that parallel those reported in plants repeatedly treated with gibberellic acid or having a GA-constitutive response indicate that the pat mutant probably expresses high levels of GA. The retained sensitivity to the GA-biosynthesis inhibitor paclobutrazol reveals that this condition is dependent on GA biosynthesis. Expression analysis of genes encoding key enzymes involved in GA biosynthesis shows that in normal tomato ovaries, the GA20ox1 transcript is in low copy number before anthesis and only pollination and fertilization increase its transcription levels and, thus, GA biosynthesis. In the unpollinated ovaries of the pat mutant, this mechanism is de-regulated and GA20ox1 is constitutively expressed, indicating that a high GA concentration could play a part in the parthenocarpic phenotype. The levels of endogenous GAs measured in the floral organs of the pat mutant support such a hypothesis. Collectively, the data indicate that transcriptional regulation of GA20ox1 mediates pollination-induced fruit set in tomato and that parthenocarpy in pat results from the mis-regulation of this mechanism. As genes involved in the control of GA synthesis (LeT6, LeT12 and LeCUC2) and response (SPY) are also altered in the pat ovary, it is suggested that the pat mutation affects a regulatory gene located upstream of the control of fruit set exerted by GAs.

Solid phase radioimmunoassay for the quantitation of abscisic acid in plant crude extracts using a new monoclonal antibody.

Summary A new mouse monoclonal antibody against abscisic acid (ABA) was produced and characterized. This antibody is highly specific for free (S)-ABA and shows a good affinity constant. A newly developed solid phase radioimmunoassay (RIA) for the quantitation of ABA content in plant extracts is described. This assay proved to be more sensitive and precise than the commonly used liquid phase RIA. Using this assay ABA was quantified in tomato and bean leaves and in Sechium endosperm. Results were compared with those obtained using the other two monoclonal antibodies produced up to now against ABA. Results obtained with RIA were validated by physicochemical methods.

Fruit-set and Early Fruit Growth in Tomato are Associated with Increases in Indoleacetic Acid, Cytokinin, and Bioactive Gibberellin Contents.

Fruit-set, defined as the activation of a developmental program which converts the ovary into a developing fruit, depends on the crosstalk among plant hormones. Here we show that in pollinated ovaries fruit-set is associated with an increase in indole-3-acetic acid (IAA) content and high transcript levels of ToFZY, a gene involved in the tryptophan-dependent auxin biosynthesis pathway. In unpollinated ovaries developed parthenocarpically in response to the synthetic auxin chlorophenoxyacetic acid (4-CPA), ToFZY mRNA levels and IAA content slightly increase. The most likely sequence of events after fertilization involves auxin-mediated activation of gibberellin (GA) synthesis. Fertilization events not only strongly increase SlGA20ox1 and SlGA20ox3 message levels but also increase SlGA2ox2, SlGA2ox4, and SlGA2ox5 mRNA levels, suggesting a concerted regulation to modulate the level of bioactive GAs, GA1 and GA3. 4-chlorophenoxyacetic acid was found to mimic the fertilization events in the stimulation of SlGA20ox1 and SlGA20ox3 mRNA levels, which were also enhanced and increased earlier, but in contrast with pollinated ovaries, SlGA2ox2, SlGA2ox4, and SlGA2ox5 mRNA levels were repressed leading to higher levels of bioactive GAs. We have also analyzed the content of abscisic acid (ABA) and its metabolites dihydrophaseic acid, phaseic acid, and ABA-glucosyl ester and the level of cytokinins (CKs) (free bases and their corresponding ribosides and ribotides) in pollinated and auxin-treated tomato fruits. We show that ABA levels decrease whereas the levels of free CKs increase immediately after pollination or auxin treatment.

Gibberellin Biosynthesis in Phaseolus coccineus Suspensor

Summary Ent(14C)-7-α-hydroxykaurenoic acid fed to a cell-free system of Phaseolus coccineus suspensors was incorporated into several products among which GA5, GA1 and GA8 were identified by GC-MS. Thus the suspensor has the enzymatic potential for the biosynthesis of the major gibberellins present in the very young whole seeds. Moreover these results clearly indicate that the suspensor is active in the biosynthesis of growth substances as well as in the transport of nutrients for the embryo (Yeung, 1980).

Nighttime Sugar Starvation Orchestrates Gibberellin Biosynthesis and Plant Growth in Arabidopsis

This work demonstrates that plants integrate the efficiency of photosynthesis over a period of days and transduce that information into a daily rate of gibberellin synthesis. This enables a plant to match its growth rate to its environment without unnecessary short-term fluctuations. A plant’s eventual size depends on the integration of its genetic program with environmental cues, which vary on a daily basis. Both efficient carbon metabolism and the plant hormone gibberellin are required to guarantee optimal plant growth. Yet, little is known about the interplay between carbon metabolism and gibberellins that modulates plant growth. Here, we show that sugar starvation in Arabidopsis thaliana arising from inefficient starch metabolism at night strongly reduces the expression of ent-kaurene synthase, a key regulatory enzyme for gibberellin synthesis, the following day. Our results demonstrate that plants integrate the efficiency of photosynthesis over a period of days, which is transduced into a daily rate of gibberellin biosynthesis. This enables a plant to grow to a size that is compatible with its environment.

Changes in free and conjugated IAA during dormancy and sprouting of potato tubers

The time course of free and conjugated indole-3-acetic acid (IAA) concentrations was investigated in three distinct parts (eyes, subeye tissues and pith) of potato (Solanum tuberosum L. cultivar Monalisa) tubers, from the last period of tuber growth to sprouting under two different storage temperatures (3 and 23˚C). The highest levels of both free and conjugated IAA were detected in eyes, which showed a significant increase of the free hormone concentration from harvest to the end of dormancy, regardless of storage temperature. The concentration of conjugates was higher than free IAA in eyes and subeye tissues at 23˚C, while at 3˚C conjugates prevailed only in eyes. Present results suggest the involvement of IAA in the control of potato tuber dormancy.

Changes in ultrastructure, protease and caspase-like activities during flower senescence in Lilium longiflorum

The last phase of flower development is senescence during which nutrients are recycled to developing tissues. The ultimate fate of petal cells is cell death. In this study we used the ethylene-insensitive Lilium longiflorum as a model system to characterize Lily flower senescence from the physiological, biochemical and ultrastructural point of view. Lily flower senescence is highly predictable: it starts three days after flower opening, before visible signs of wilting, and ends with the complete wilting of the corolla within 10 days. The earliest events in L. longiflorum senescence include a fall in fresh and dry weight, fragmentation of nuclear DNA and cellular disruption. Mesophyll cell degradation is associated with vacuole permeabilization and rupture. Protein degradation starts later, coincident with the first visible signs of tepal senescence. A fall in total protein is accompanied by a rise in total proteases, and also by a rise of three classes of caspase-like activity with activities against YVAD, DEVD and VEID. The timing of the appearance of these caspase-like activities argues against their involvement in the regulation of the early stages of senescence, but their possible role in the regulation of the final stages of senescence and cell death is discussed.

Nitric oxide and hydrogen peroxide involvement during programmed cell death of Sechium edule nucellus

The nucellus is a maternal tissue that feeds the developing embryo and the secondary endosperm. During seed development the cells of the nucellus suffer a degenerative process early after fertilization as the cellular endosperm expands and accumulates reserves. Nucellar cell degeneration has been characterized as a form of developmentally programmed cell death (PCD). In this work we show that nucellus PCD is accompanied by a considerable production of both nitric oxide and hydrogen peroxide (NO and H(2)O(2)). Interestingly, each of the two molecules is able to induce the production of the other and to cause cell death when applied to a living nucellus. We show that the induced cell death has features of a PCD, accompanied by profound changes in the morphology of the nuclei and by a massive degradation of nuclear DNA. Moreover, we report that NO and H(2)O(2) cause an induction of caspase-like proteases previously characterized in physiological nucellar PCD.

Role and metabolism of abscisic acid in potato tuber dormancy and sprouting

Summary Tubers cv. Monalisa were analysed to determine the content of abscisic acid (c-t-(+)ABA) and related metabolites in three tuber parts (eyes, sub-eye tissues and pith). Analyses were performed from the last period of tuber growth until sprouting under two different storage temperatures (3 °C and 23 °C). ABA content rises in eyes and sub-eye tissues as sprouting approaches, regardless of storage temperature. Dihydrophaseic acid is the main ABA metabolite while phaseic acid was not detected, probably because of its further metabolization to DPA. ABA-glucose ester concentrations are generally low in all tissues except in eyes before harvest. The results are discussed in relation to the current hypotheses on the role of ABA in the control of potato tuber dormancy.

Kaurene and kaurenol biosynthesis in cell-free system of Phaseolus coccineus suspensor

Abstract It is shown that suspensor tissue of Phaseolus coccineus can biosynthesize ent -kaur-16-ene and ent -kaur-16-en-19β-ol, two key precursors in the biosynthesis of gibberellins.