Attempts to Isolate Transmissible Floral Regulators Have Been Unsuccessful

The many attempts to isolate and characterize the floral stimulus have been largely unsuccessful. The most common approach has been to make extracts from induced leaf tissue and test for their ability to elicit flowering in nonin-duced plants. In other experiments, investigators have extracted and analyzed phloem sap from induced plants. In some studies, extracts from one of these sources have induced flowering in test plants, but these results have not

Long days Short days

Long days Short days

FIGURE 24.31 Graft transmission of an inhibitor of flowering. Non-induced rosettes from the LDP Nicotiana sylvestris were grafted onto the day-neutral tobacco (Nicotiana tabacum, cv. Trapezond). Flowering of the day-neutral plant was suppressed under short days (left branch of plant on right), but not under long days (left branch of plant on left). Arrowheads indicate graft unions. (From Lang et al. 1977.)

FIGURE 24.31 Graft transmission of an inhibitor of flowering. Non-induced rosettes from the LDP Nicotiana sylvestris were grafted onto the day-neutral tobacco (Nicotiana tabacum, cv. Trapezond). Flowering of the day-neutral plant was suppressed under short days (left branch of plant on right), but not under long days (left branch of plant on left). Arrowheads indicate graft unions. (From Lang et al. 1977.)

been consistently reproduced. Most of these extractions have focused on small molecules.

Recent studies using fluorescent tracers have shown that in Arabidopsis there is actually a decrease in the movement of small molecules from the leaf-to-shoot apex via the sym-plast at the time of floral induction (Gisel et al. 2002). The lack of tracer movement from the leaf to the shoot apex may indicate either a reduction in overall symplastic transport to the shoot apex, or a change in the selectivity of the plasmodesmata during floral induction. There is increasing evidence that macromolecular traffic between cells via plasmodesmata plays essential roles in normal meristem development and function (see Chapter 16). Particles as large as viruses can move from cell to cell via plasmodes-mata, and throughout the plant via the phloem. Phloem translocation of small RNAs has recently been implicated in the spread of a viral resistance mechanism throughout plants (Hamilton and Baulcombe 1999). It is therefore possible that the floral stimulus is a macromolecule, such as RNA or protein, that is translocated via the phloem from the leaf to the apical meristem, where it functions as a regulator of gene expression (Crawford and Zambryski 1999).

However, thus far attempts to identify such a signal have been unsuccessful.

Efforts to isolate a specific, graft-transmissible inhibitor of flowering have also been unsuccessful. Thus, despite unequivocal data from grafting experiments showing that transmissible factors regulate flowering (see Table 24.2) (Zeevaart 1976), the substances involved remain elusive.

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