Question

If you graft together a long-day rootstock plant and a short-day scion plant, what will induce flowering?

a. Long days, since the rootstock senses day length

b. Both long and short days due to the grafting

c. Short days, since the scion produces the leaves

d. Days and nights of exactly 12 hours each

e. Nothing; the plant will be unable to correctly sense day length.

Answer 1

Option C. Short days, since the scion produces the leaves

• Rootstock is the underground part of the plant or the stump which carries a healthy root system, upon which another plant can be grafted.
• Scion is the part of the plant which is grafted upon the root stock and has leaves, buds and flowers, i.e. the scion harbors the vegetative and reproductive properties.
• Flowering is induced by length of day (night) to which the plant remains exposed, which is precisely the effect of photoperiod. A short day plant flowers when it is exposed to day light for lengths of time shorter than the critical photoperiod, while they grow vegetatively for exposure more than this threshold. A long day plant, on the contrary, flowers when exposed to light more than the critical photoperiod and even can tolerate continuous exposure to light. The critical photoperiod for both long-day and short-day plants usually ranges between 12-14 hours and varies amongst species. Additionally, in order to induce flowering, the short day plant needs to be exposed to light more than the induction period which is the minimum period of exposure of a short-day or long-day plant to trigger flowering. So for a short day plant, the recommended period of exposure to light to trigger flowering should range between the induction period and the critical photoperiod.
• Photoperiod is induced by proteins called phytochromes, which exist in two interconvertible forms : P730 and P660. In presence of red light (660 mμ) or sunlight (which also contains red light), P660 is converted to P730. Therefore at the end of the day, P730 is the predominant form of phytochrome. In presence of far red light (730 mμ), P730 is quickly converted to P660, a process which occurs slowly during dark period or night. In a short day plant, flowering is triggered by accumulation of P660 phytochrome pigment. In this case since the scion is exposed to light, the flowering responses will be triggered once the requirements are fullfilled as for a short day plant.
• Scientists have demonstrated that leaves are the site of light perception for generating the floral stimulus. Therefore a plant which has been defoliated fails to trigger flowering even under inductive light conditions. Further experiments have also indicated that a floral stimulus from a leaf of an induced short or long-day plant can be transferred to another non-induced plant by grafting.
• Many notions regarding the nature of this floral stimulus point towards a hypothetical flowering hormone, "florigen". FLOWERING LOCUS T (FT) protein is recognized as a primary component of florigen and determines the onset of flowering. Research has shown that overexpression or inducible expression of FT activity can  promote flowering through a graft junction, indicating FT to be a mobile transmissible signal that can be transported over long distances. The FT gene is directly activated by the transcription factor CONSTANS (CO) that mediates the photoperiodic induction of flowering. Recent reports also suggest FT protein ( and not mRNA) is transmitted from the leaves to the shoot apex via the phloem, where it interacts with the transcription factor FD, that is preferentially expressed in the shoot apex and activates transcription of APETALA1 (AP1) and FRUITFULL (FUL), which are meristem identity genes. Fluorescent tagged fusion proteins of FT:GFP (green fluorescent protein) have been tracked in vascular tissues, shoot apices and its vicinity and phloem sap. Even transmission of FT:GFP proteins have been demonstrated through a graft junction from donor transgenic plants in the vasculature to the recipient plants which were null ft mutant background, leading to onset of flowering in the recipient plant. This takes place by transformation of the vegetative bud primordia into a floral bud.