Abstract
Genetic studies in Arabidopsis established FLOWERING LOCUS T (FT) as a key flower-promoting gene in photoperiodic systems. Grafting experiments established unequivocal one-to-one relations between SINGLE FLOWER TRUSS (SFT), a tomato homolog of FT, and the hypothetical florigen, in all flowering plants. Additional studies of SFT and SELF PRUNING (SP, homolog of TFL1), two antagonistic genes regulating the architecture of the sympodial shoot system, have suggested that transition to flowering in the day-neutral and perennial tomato is synonymous with “termination.” Dosage manipulation of its endogenous and mobile, graft-transmissible levels demonstrated that florigen regulates termination and transition to flowering in an SP-dependent manner and, by the same token, that high florigen levels induce growth arrest and termination in meristems across the tomato shoot system. It was thus proposed that growth balances, and consequently the patterning of the shoot systems in all plants, are mediated by endogenous, meristem-specific dynamic SFT/SP ratios and that shifts to termination by changing SFT/SP ratios are triggered by the imported florigen, the mobile form of SFT. Florigen is a universal plant growth hormone inherently checked by a complementary antagonistic systemic system. Thus, an examination of the endogenous functions of FT-like genes, or of the systemic roles of the mobile florigen in any plant species, that fails to pay careful attention to the balancing antagonistic systems, or to consider its functions in day-neutral or perennial plants, would be incomplete.
Highlights
EVOLUTION OF THE FLORIGEN EXPERIMENTAL PLATFORM AND THE SINGLE FLOWER TRUSS (SFT)/SP REGULATORY PARADIGM IN TOMATO The florigen hypothesis emerged from elegant grafting experiments in photoperiod-sensitive plants (Chailakhyan, 1936a,b)
Extensive experiments in the following 40 years, using a variety of photoperiod-sensitive plants, supported the florigen paradigm and established its core physiological parameters. These were critically evaluated in the superb compendium directed by Zeevaart (1976) and can be summarized as follows: (A) Changing light regimes induces systemic florigenic signals in cotyledons and leaves, which are transported, primarily via the phloem, to the apical meristems, where they induce transition to flowering. (B) While the primary environmental inductive signals may vary, the final stimulus is universal and can be transmitted between species; long-day and short-day plants respond to the same florigenic signal. (C) The florigenic stimulus and the flowering response are quantitative. (D) The florigenic stimulus is balanced by systemic anti-florigenic agents
The SFT/SP ratio paradigm implies that genes belonging to the CEN, TFL1, and SP branch of the CETS may function as universal antagonists for florigen
Summary
EVOLUTION OF THE FLORIGEN EXPERIMENTAL PLATFORM AND THE SFT/SP REGULATORY PARADIGM IN TOMATO The florigen hypothesis emerged from elegant grafting experiments in photoperiod-sensitive plants (Chailakhyan, 1936a,b). A similar delay in the termination of the primary apices occurs in sft plants and results in an indeterminate inflorescence shoot with a mixture of leaves and solitary flowers that substitute for the regular compound inflorescence. Mobile florigen substituted for high endogenous SFT, inducing the collapse of the sympodial pattern and the simplification of the leaves in sp plants, conditioning slim stems in receptor WT shoots, and stimulating leaf simplification in tf plants.
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