Intercellular communication during floral initiation and development - Intercellular Communication in Plants - page 180

apex and down to the roots (Havelange et al. , 2000). Transport of sucrose to the
root was correlated with increased transport of cytokinin from the root up the shoot
to the apex. Similarly, nitrogenous compounds, such as the amino acids glutamine
and asparagine, were transported from the leaf up the shoot to the apex (Corbesier
et al. , 2001). When movement through the phloem from the shoot to the root was
prevented by removing living tissues, including phloem, from the surface of the
plant (girdling), or when movement from the root to the shoot through the xylem
was prevented by increasing relative humidity, the flowering response of the plant
to a single long day was reduced (Perilleux & Bernier, 2002). However, application
of sucrose and cytokinins to vegetative plants was not sufficient to induce flowering,
and no genetic evidence clearly indicates a signaling role for these substances in
the transition to flowering. This led to the suggestion that the floral stimulus may
be a complex mixture of compounds (Bernier et al. , 1993), and to the perception
that genetic-based approaches are required to supplement physiology to define the
floral stimulus (Colasanti & Sundaresan, 2000; Perilleux & Bernier, 2002).
7.2.2 Mutations that impair long-distance signaling in pea and maize
Mutations or natural-genetic variations that alter flowering-time have been described
in many species. Studies in pea plants were enhanced by the availability of extensive
genetic stocks, and the ability to readily graft different genotypes so that the effect of
this variation on long-distance signaling could be assessed (Weller et al. , 1997b). For
example, gigas ( gi ) mutants flower later than wild-type plants, but their flowering
is accelerated by grafting a gi shoot onto a wild-type stock (Beveridge & Murfet,
1996). This suggests that in wild-type plants the GI gene may be involved in the
synthesis or transport of the floral stimulus. Flowering of wild-type pea plants is
accelerated in response to long days and is delayed by exposure to short days. The gi
mutant flowers later under both conditions, and often never flowers under short days.
This suggests that the floral stimulus controlled by GI is not part of the response to
day length, but is expressed under all environmental conditions tested. The LATE
FLOWERING ( LF ) gene is proposed to act at the apex and encode a target of the
floral stimulus; dominant alleles at this gene delay flowering and the effect is not
influenced by grafting of an LF shoot onto a wild-type stock (Murfet, 1971, 1985). In
addition to the floral stimulus controlled by GI , there is evidence for a long-distance
inhibitory signal regulating flowering-time of pea plants. Mutations in the STERILE
NODES ( SNE ), DIE NEUTRALIS ( DNE )or PHOTOPERIOD ( PPD ) genes cause
early flowering, and flowering of the shoots of these plants can be delayed by grafting
onto a root stock of a wild-type plant (King & Murfet, 1985; Weller et al. , 1997a,b).
Plants in which these genes are mutated are almost day-length insensitive, flowering
at the same time under both long and short days, indicating that the photoperiod
response is largely caused by production of an inhibitor under short days. These
experiments suggested a model in which the timing of the transition to flowering at
the apex of pea plants is determined by a balance between long-distance promotive
and inhibitory signals, so that when the ratio of stimulus to inhibitor exceeds a certain