Fattori biochimici di regolazione dello sviluppo dello stelo fiorale della barbabietola da zucchero

Introduction. The development of the floral stem (bolting) in sugarbeet (Beta vulgaris L. subsp. vulgaris, Sugar Beet Group) is a problem that affects mostly the cultivations in southern Europe countries, where sowing is anticipated to early spring or autumn to allow plants to partly escape summer drought. One of the consequences of early sowing may be the exposure of the crop to low temperatures, resulting in plant vernalization and bolting induction. Such phenomenon is detrimental to crop yield because it lowers the sucrose reserves of the root and also enhances its fiber content, thus hampering processing quality. The physiology of bolting is regulated by several factors, both exogenous and endogenous. Among the latters, plant hormones and mainly gibberellins play a pivotal role, as it has been demonstrated for several species (Bernier et al., 1993; Blazquez et al., 1998; Gilmour et al., 1986; Zeevaart e Gage, 1993). According to our previous data, obtained during the first year of programmed work, gibberellins appear to stimulate floral stem elongation in sugarbeet. For this reason an investigation was undertaken, in order to deepen our knowledge on the identity and quantitative changes of the main gibberellin molecules characteristic of this species and to highlight correlations between gibberellins and floral stem development in two different plant genotypes. We then turned our attention to the physiological relations between two hormonal groups whose metabolism is strictly related in several species, i.e. auxins and gibberellins. We studied the putative role of the auxin indoleacetic acid in stimulating gibberellin biosynthesis and, as a consequence, its involvement in the control of bolting. To investigate more in depth the role of gibberellins in floral development, we studied the molecular regulation of their metabolism in two different plant genotypes, to assess its physiological relevance in the control of bioactive gibberellin levels and, consequently, in the regulation of bolting. Materials and methods. The concentration of the major gibberellins, both biologically active and biosynthetic precursors, was analyzed in apices and leaflets (30-60 mm length) of a vernalized (therefore committed to bolting) biennial cultivar ("Monodoro") and of an annual line ("CMS 7500"), both sampled before and at the starting of bolting. Concentration changes of GA19, GA20, GA5 and GA I were investigated. Changes of the concentrations of indoleacetic acid and of the aforesaid gibberellins were analyzed in sugarbeet samples analogous to those previously described. Analytical methods entailed HPLC purifications and hormone identification and quantification by GC-MS with the aid of deuterated internal standards. Molecular regulation of gibberellin metabolism was studied by analyzing the expression of the two sugarbeet genes encoding the main enzyme for gibberellin catabolism (GA 2-oxidase) in the apices from a commercial cultivar characterized by a high frequency of bolting and, subsequently, in a greenhouse-grown annual line, both sampled before and at the starting of bolting. Results and discussion. The most relevant results may be summarized as follows: it was confirmed that in sugarbeet the predominant branch of the biosynthetic pathway is the one involving early 13-hydroxylation of the precursor molecules; a correlation exists between the concentration of GA1 (which is known to enhance stem elongation) and bolting, because the hormone level is higher in bolting apices, particularly in the biennial cultivar; leaflets may be a source of gibberellin precursors, which could be translocated to apices; in these tissues, GA1 biosynthesis may be stimulated also by indoleacetic acid, whose concentration was 20-fold higher at the start of bolting than at the previous developmental stage; GA5 is the most abundant gibberellin in apices; although it does not directly stimulate stem elongation, the latter gibberellin might be involved in the precocious stage of flowering (floral induction), because of its high concentration in apices before bolting; in the latter stage, one of the two GA 2- oxidase genes is expressed at a lower level than at the beginning of bolting, which might be the cause for a lower catabolism (i.e. a rise of concentration) of those gibberellins putatively controlling floral stem development. Concerning GA5, several authors ascribe to this gibberellin, at least in a few species, a central role in floral induction: our data agree with such conclusions, because GA5 levels in apices are considerably high before bolting and drop thereafter. Conclusion. Present results highlight the relevance of gibberellins and of indoleacetic acid in bolting control of sugarbeet and suggest to develop our research to further investigate the physiological relevance of the control of metabolism of these growth regulators in floral development, with the aim to progress toward the comprehension of bolting control at both biochemical and molecular-genetic level.