Agriculture Reference
In-Depth Information
In the following sections, I describe how molecular genetic and cell biology based
approaches are being used to elucidate the molecular mechanisms underlying these
signaling processes.
7.2
Long-distance signaling during the induction to flowering
7.2.1
Discovery of a role for long-distance signaling in the induction
of flowering
The involvement of long-distance signaling in initiating the transition to flowering
was originally proposed based on the demonstration that differences in day length
are perceived in the leaves. The first experiments that demonstrated this were based
on exposure of different parts of the plant to distinct day lengths. James Knott
showed using spinach plants that exposing the whole plant or only the foliage to
days containing 15 h light would induce rapid flowering, while exposing the apex
of the shoot to such long day lengths while the foliage is exposed to shorter day
lengths would not induce flowering (Knott, 1934). He concluded that the role of the
leaves in the induction of flowering in response to day length is 'in the production
of some substance, or stimulus, that is transported to the growing point'.
The conclusion that leaves are the source of a floral stimulus was strengthened by
generating grafts between plants exposed to different day lengths (Zeevaart, 1976).
In particular, grafting of leaves of the short-day plant Perilla to shoots that had been
exposed only to long days was sufficient to induce flowering (King & Zeevaart,
1973). These experiments confirmed that leaves are the source of the floral stimulus,
and that a single leaf produces sufficient stimulus to induce floral development at
the apex of the plant. Indeed in Perilla , successively grafting the same leaves to
seven different shoots over a period of 97 days demonstrated that these leaves were
stably induced to produce floral stimulus and produced enough to trigger flowering
of all seven recipient shoots (Zeevaart, 1985).
Physiological experiments indirectly indicated that the stimulus is transported
from the leaf to the apex through the phloem. This was done in Perilla by compar-
ing the effect of the donor leaf on flowering with the movement of radioactively
labeled assimilates from the donor leaf through the phloem and into the axillary
shoot (King & Zeevaart, 1973). Assimilates and the floral stimulus are both trans-
ported over long distances in the shoot, and there is a close correlation between the
translocation of radioactively labeled assimilates into the axillary shoot of Perilla
and the movement of the floral stimulus into the shoot, as detected by the induction
of floral development.
Although transport of the floral stimulus across graft junctions could be followed
indirectly by its effect on flowering, its identity has been difficult to establish. In
the mustard Sinapis alba ,flowering can be induced by exposure to a single long
day (Bernier et al. , 1993). Sucrose was the earliest detected signal transported from
the leaves in response to long-day treatment, and was transported both up to the
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