PLS622 Plant Physiology I, Monday, October 2, 2006 Reproductive development:

Transition to flowering I:

The environmental cues controlling the transition to flowering:

Plants are extremely well attuned to their external environment. This is particularly evident in the decision to commence reproductive growth following a period of vegetative growth. Environmental cues such as temperature, daylength, indeed any reliable measure of the passage of time or seasonality, can serve to permit the synchronous flowering of a population of plants which is of crucial importance for outcrossing species. What environmental factor or combination of factors particular plants cue into to determine the switch to reproductive growth and the organ change depending on the habitat the plant is growing in. There are numerous examples of the same species of plant being say, biennial with vernalization requirements in one environment and a long day annual with no vernalization requirement in another. The signal that the environmental cue elicits must often travel a considerable distance in the plant to reach the vegetative meristem where it is to have its effect in converting the vegetative to an inflorescence meristem.

In order for a plant to respond to environmental stimula and change meristem determination from vegetative to inflorescent 1) the leaves that are to detect and/or transduce the signal must be competent to do so and, 2) the meristem must be competent to change to an inflorescent meristem. The inability of some long lived plant species to flower for the first few (or many) years is due to a lack of competence to respond to environmental cues that usually bring about flowering. This is termed juvenility and is a large thorn in the side of those wishing to improve attributes of wood through genetic engineering for use in the pulp and paper industry.

Vernalization: This term literally translated means “springization”. It is the ability of a period of low temperature to induce vegetative plants to flower. This can occur while the plant is at the cold temperature (nondelayed response) or after the plant has been removed to a higher, permissive temperature (delayed response). It can be necessary but not sufficient to induce flowering in plants requiring a second (or more) inductive environmental signal prior to/after vernalization. Some plants are responsive to vernalization as seeds, others as seedlings, or mature plants. Additionally, as in cereals, the vernalization response can be quantitative (facultative) where treatment simply shortens the vegetative growth phase or as with Lancer wheat where the response is qualitative (absolute) since flowering does not occur without vernalization.

Biennial plants often require vernalization to flower. The rosette of leaves of the first year’s growth protecting the shoot apical meristem at their base during a winter of cold temperatures which permits the production of a bolt and flowers the following spring and summer, thus completing the life-cycle.

Either high temperature or anoxia within 4 days of the completion of vernalization for plants showing delayed response can cause devernalization where the plant loses the effect of vernalization and fails to flower.

Photoperiod: Photoperiodism was the first clear-cut demonstration of a biological clock. Some plants will be induced to flower in response to day lengths in excess of a certain minimum length (critical day) so called long-day plants. Other plants respond to night lengths longer than some maximum (critical night), so called short-day plants. There are other plants that require long days followed by short days as occurs in late summer and early fall these are the long-short-day plants. Their counterparts are the short-long-day plants that require short days followed by long days as occurs during the early spring. Others flower only when day length is intermediate, remaining vegetative if the day length is too long or too short. Still others flower either under short or long days but not when day length is intermediate. The rest of the plants are placed in the day-neutral category.

Light quality: Many of the day neutral plants can be induced to flower due to changes in the intensity of the light they are grown under.

Whatever the plant, it is generally agreed that the signal to switch from vegetative to

reproductive growth is a transmissable signal.

There are three main hypotheses which attempt to explain the nature of this signal.

1) The florigen/antiflorigen hypothesis suggests that the plant produces either a yet to be discovered stimulatory or repressive compound in response to conditions favorable or unfavorable to flowering, respectively.

2) The nutrient diversion hypothesis stipulates that a favorable flowering environment is any environment that increases the amount of photoassimilate available to the shoot apical meristem, converting it to an inflorescence meristem.

3) Finally, the hypothesis of multifactorial control postulates that combinations of assimilate and known phytohormones act synergistically to induce flowering whenever environmental cues alter assimilate and hormonal ratios such that they are favorable for the transition to flowering.

Plant torture: gaining insight through surgical manipulation.

1) Plant-in-a-box: The fact that the signal to begin reproductive growth is, in the case of photoperiod, perceived by the leaves and transmitted to the shoot apical meristem has been exhibited in many species by physiological experiments dating back to the last century. The first such experiment involved inducing different parts of a plant with one long day while the preponderance of the plant resided in a light-tight container experiencing short days. When the exposed portion of the plant was a mature leaf and this was induced with a long day, the result was the shoot apical meristem took on the attributes of an inflorescence meristem.

2) Grafting: Stimulate one plant with long days then excise a mature leaf at different times and graft the leaf onto a plant that has been grown under non-inducing conditions. In Sinapis alba, the graft was often sufficient to induce the non-induced plant to flower.

3) Defoliating: The second such experiment is a “pulse chase” experiment where long day plants were induced to flower by prolonged (greater than 8 hours) illumination (A.K.A. 'a photoextended period') and subsequently defoliated at different hours after induction. This latter experiment when performed on Sinapis alba exhibited that the slowest mobilized floral stimulant exited the mature leaves between 12 and 16 hours after the commencement of the long day.

4) Stem ringing, stopping phloem transport: See diagram below.

5) 100% relative humidity, stopping xylem sap movement: Place un-induced plants in 100% relative humidity: A) 24 hours before a 24 hour constant light exposure; B) during a 24 hour constant light exposure; C) 24 hours after a 24 hour constant light exposure. A and C flowered while B did not. This means the signal is carried in the xylem at some point.

The possible nature of the floral stimulant:

Carbohydrates: By illuminating the long day plant Sinapis alba for 8 hours at light intensities 2.5 time greater than those usually used to induce flowering, the overall accumulated irradience was equal to or greater than the amount accumulated during long days. Yet the plant does not flower. This experiment was used to demonstrate that long day plants are not simply responding to accumulated illumination as the signal to flower. However, there were metabolic changes that accompanied the exposure to high illumination that are identical to those that take place upon the transition to flowering. The first of these was an increase in carbohydrate amounts and an increase in acid invertase activity in the apical meristem. This increase in sugar amounts, primarily sucrose, precedes increases in energy requiring events such as mitosis and so is not simply a reaction of the plant to provide more assimilate to a strong sink. This has brought many to conclude that sucrose itself may be a signal involved in the transition to flowering. The source of the carbohydrates translocated to the apical meristem in response to higher than usual illumination was stored reserves, primarily starch, in the mature leaves and stem.

Cytokinins: Like the illumination of long day plants with high intensity light of short duration (8 hours) a second treatment that fails to induce flowering completely but does induce a sub-set of events that are necessary for flowering is the application of cytokinin to the apical meristem. This application stimulates the rate and synchronicity of cell division along with some other cellular occurrences normally associated with the transition to flowering seen upon photoperiodic induction. Induction causes the concentration of cytokinins to increase in the mature leaves,

where both zeatin riboside (the major component) and isopentenyladenine riboside (minor component) accumulate. The supply of cytokinin has been traced to the roots of the plant and bark ringing experiments have been used to demonstrate that the mature leaves signal to the roots after induction and that preexisting cytokinin is released from the roots and transported to the mature leaves through the xylem. This process takes between 8 and 12 hours after the induction of flowering. The xylem has been implicated in the transport of cytokinin from the roots to the mature leaves by growing the plants for 24 hours in 100% relative humidity either before, during, or after the induction of flowering. By growing the plants in 100% relative humidity, among other things, the transpiration stream is much reduced, delaying xylem transport. Only plants grown in 100% relative humidity during the 24 hours which included the induction of flowering failed to flower signifying that the message from the roots (cytokinin) is transported in the xylem.

The mature leaves, upon receiving the cytokinin from the roots are stimulated to export isopentenyladenine (iP) primarily to the shoot apex. Levels of iP increase in the shoot apex and in the mature leaves at about 16 hours after induction of flowering. It is not known whether this increase in iP is due to the leaves metabolising the imported cytokinin from the roots or if the cytokinin from the roots is a stimulus used to promote the biosynthesis of iP in the leaves.

Auxins:

Little work has been done on the dynamics of auxin but it has been noted that there is a pronounced decline in auxin concentration in the apical meristem at 16 hours after induction of flowering, just when cytokinin concentrations are increasing in this plant part. The auxin-to-cytokinin ratio is a potent cue for many developmental processes in plants and the balance between them probably has implications for events leading to the transition to flowering.

Polyamines:

Photoinduction of flowering leads to a release of putrescine. Additional evidence implicating polyamines in the induction of flowering are the observations that inhibitors of polyamine biosynthesis, such as DL-a-difluoromethylornithine drastically reduce flowering response.

Calcium:

Calcium has been proposed to be a secondary messenger for flower induction in plants. The levels of calcium in root exudates increase dramatically following photoinduction. However, no increase in calcium amounts is found in mature leaves or in leaf exudates. Despite this fact, calcium amounts are seen to rise in the apical meristem approximately 40 hours after photoinduction. This calcium is supplied to the apical meristem from the apoplast of surrounding cells.

Synopsis:

The work presented above has been discovered using the long day plant Sinapis alba as a model. Other plants may utilize different signaling molecules. For example, the long day grass Lolium temulentum appears to use gibberellins to signal flower induction whereas they are not used by Sinapis alba. It does appear as though some short day plants (Xanthium strumarium) utilize sucrose and cytokinins as signals for inducing flowering as Sinapis alba does.