Apply What You’ve Learned

804

Review

37.2 Receptors for the photoperiodic signal for flowering are located in the leaf, and a signal travels to the apical meristem.

37.2 Although in terms of flowering, plants are classified as either short-day plants (SDPs) or long-day plants (LDPs), night length is actually the cue that controls flowering.

Original Papers: Beveridge, C. A. and I. C. Murfet. 1996. The gigas mutant in pea is deficient in the floral stimulus. Physiologia Plantarum 96: 637−645.

Hecht, V. et al. 2011. The pea GIGAS gene is a FLOWERING LOCUS T homolog necessary for graft-transmissible specification of flowering but not for response to photoperiod. The Plant Cell 23: 147−161.

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The garden pea (a long-day plant) is easy to graft, making it useful for studying the presence or absence of mobile signals. A mobile signal will diffuse from a wild-type root stock into a mutant graft that lacks the signal, restoring the signal in the graft. A nonmobile signal will not restore the signal. Grafting has been used to study genes involved in flowering in the pea.

The gigas (gi) allele in peas is a mutant version of an FT gene and encodes an FT-like protein. Table A compares the phenotypes of the wild type (WT) and the gi mutant at different long photoperiods. The results are given as the plant height (number of nodes) at first flowering. Presence of more nodes, or greater height, indicates delayed flowering (i.e., more vegetative growth before flowering occurs). “Veg.” at 24 hours indicates that all growth was vegetative; flowering did not occur.

Table A
Photoperiod (hours of light)
Strain 12 16 24
gi mutant 54 43 Veg.
WT 20 16 15

A grafting study was done to test whether the gi mutant was deficient in the ability to produce the floral stimulus molecule. Stems from 6-day-old plants were grafted, and plants were kept at a 24-hour photoperiod for 2–3 months. Grafts were either self (WT to WT, or gi to gi) or reciprocal (WT to gi, or gi to WT). Results are shown in Table B.

Table B
Graft partners (root/stem) Node at first flowering
WT/WT 15
gi/gi Veg.
gi/WT 17
WT/gi 16

The graphs (below) show the transcript levels (mRNA expression) of the WT allele (red circles) and the gigas allele (blue circles). Values are shown for both an open leaf and for the apex, or site of flowering. The tests were done on long-day plants.

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Questions

Question 1

Based on the data in Table A, what is the effect of the gi mutant on flowering in the pea plant?

The gi mutant inhibits or greatly delays flowering in the pea plant.

Question 2

Based on the data in Table B, is it likely that the gi mutant is deficient in a mobile floral stimulus molecule? Provide evidence for your conclusion.

Yes, the evidence suggests that the gi mutant is unable to produce the floral chemical. When only the gi mutant is present (gi/gi), flowering never occurs. But whenever the WT is present, flowering does occur. This suggests that only the WT contains the floral chemical and that it is able to move in both directions (from stock to scion, or vice versa), causing the signal to reach the apex of the plant and stimulate flowering.

Question 3

Develop a model describing the pathway of genetic control of flowering in long-day peas. Include the responses of both the normal (WT) FT gene and the mutant (gigas) FT gene.

Under long-day conditions, the WT gene is expressed. It stimulates production of the FT protein that acts as a floral signal, and the plant flowers after 15-20 vegetative nodes have formed. However, the gigas gene is a null, or inactive, gene; it does not stimulate production of floral signal. In cases where the gigas gene is expressed, the plant remains vegetative; it either does not flower at all or flowers only after a long delay.