Floral Induction: Subject-Matter and Metabolism

In this article we will discuss about:- 1. Subject-Matter of Floral Induction 2. Changes in Metabolism in Leaves Associated with Floral Induction 3. Floral Induction, Determination and Specification in Arabidopsis Determination Experiments 4. Control of Flower Primordium Specification by LEAFY and APETALAI/CAULI FLOWER 6. Interactions between LFY, API and TERMINAL FLOWERI (TFLI) during the Transition to Flowering.

Subject-Matter of Floral Induction:

Floral induction is not a rapid process and the transformation of vegetative apex to flowering depends upon the intensity of stimulus (Fig. 22-12). Different plant species require different number of inductive photoperiods for complete flowering. The inductive stimulus has to be of specific intensity and the induction can be reversed.

We have already shown that leaf is the real site of perception of the light signals that induce or stop flowering. Obviously something must be translocated from the leaf to the stem tip. According to the Russian plant physiologist C. Kh. Chailakhyan, flowering response needs atleast four steps and these are perception of stumulus; the transformation of the perceiving organ; the translocation of stimulus; the translocation of a stimulator and finally a specific response in the growing tip and thus the formation of flower.

It has come to be realized that perception is through phytochrome. The stimulus takes several hours to move out of the leaf and then initiate flowering. L.T. Evans (1971) has also shown that rate of translocation of the flowering stimulus varies in the LD and SD plants. He also showed that the role of stimulus was different from photosynthate. Flowering inducing stimulus are possibly translocated through phloem (Fig. 22-12A).

Through series of grafting experiments involving LD and SD plants it has been suggested that floral stimulus is same in the two category or two different substances must produce the same result.

Thus differences in LD and SD plants could be due to the concentrations of flowering hormone or else secondary reactions may be induced in the two types as follows:

In general it is believed that florigens are the floral stimulating substances. Florigens is in fact a concept. Some experiments have indicated the occurrence of flowering inhibitors. This inhibitor effect is opposite to that of florigen.

Changes in Metabolism in Leaves Associated with Floral Induction:

During the past many decades several workers have undertaken studies on the metabolic changes during floral induction. However, the available information has not provided any definite clue since any change in photoperiodism or temperature causes several alternations which may not be related to flowering.

For instance, photoperiodic induction in crassulacean acid metabolism shows all the features of floral induction but the two processes are not casually similar. Despite different methodology it has not been possible to obtain any information on floral induction processes on the basis of biochemical techniques.

To date the chemical or other substances have not been identified which exclusively exist during floral induction. In the followings we shall mention some of the metabolic changes which operate in the leaves and are generally associated with flower induction.

These are as follows:

Evans (1961) reported that LD plants were much more dependent upon photosynthesis products than the SD plants. Inhoff (1971) demonstrated that floral induction was inhibited in Anagallis and Sinapsis when the leaves were treated with DCMU-potent inhibitor of light phase of photosynthesis.

Bodson (1971) showed that floral induction was inhibited in Arabiodosis when CO₂ from the air was removed. In Silene it has been observed that when CO₂ concentration was enhanced in the air, there was induction of flowering under SD conditions. Further, in Brassica sp. a light break in the dark inductive period induced flowering only when CO₂ was present in the air.

Recently Evans (1971) showed that LD plants were not dependent on photosynthesis for their floral induction. In Xanthium and Chenopodium when DCMU was applied to the leaves, there was no inhibition of flower induction. However, in Chenopodium DCMU inhibited formation of floral primordia. Moreover, in Xanthium and Pharbitis application of increased CO₂ inhibited flower induction.

It may be stated that the role of CO₂ in dark is not clear. For instance, in Perilla CO₂ had no effect either in the presence or the absence of CO₂. However, in Xanthium dark interruption with red light inhibited flowering only when CO₂ was present in the air.

Some plant physiologists have proposed that CO₂ affects through an antagonism with ethylene. For instance in Pharbitis action of ethylene on cotyledons has been observed during the last half of the 16hr of inductive period.

It has been reported that the levels of protein and nucleic acids increased in leaf during floral induction in Arabidopsis, Sinapsis, etc. In Arabidopsis, a cold requiring plant, when vernalized, it showed high levels of nucleic acids in leaf. When devernalized the protein content decreased up to 40%. In Chenopodium on the other hand, there was an increased protein content after photoperiodic induction.

In Hyoscymus when the leaves were treated with cycloheximide, floral formation was accelerated in non-induced conditions. The presence of untreated leaves did not hamper the induction. The interpretation offered is that it was likely that the chemical induced floral structures by inhibiting the formation of potent inhibitory substances. In Impatiens cycloheximide was not only active but also behaved like GA₃.

It may be added that with dual labelling technique no differences in protein structure were observed in induced and non-induced leaves of Xanthium. However, through gel electrophoresis technique difference in protein structure were made out in citrus leaves, one of which was treated with GA₃ and the other acting as a control. Histones or proteins did not differ in photoperiodically induced and non-induced conditions in wheat leaf.

On the other hand, distinct differences were observed in Silene leaves. In Lemnaperpisulla sugar was seen to inhibit floral induction when ammonia was present in the medium. When respiratory intermediates were added the inhibition could be overcome.

The inhibition could also be overcome through the application of some amino acids, adeniiderivatives, UTP, high CO₂ levels, etc. The precise mechanism of action of these compounds is not known. Serine and threonine both stimulated floral induction when leaves of Lemnaperpisulla were treated.

Recently some reports have appeared where sterols have been shown to inhibit flower induction in Xanthium and Pharbitis. These were applied to the leaves. It may be mentioned that application of other chemicals which inhibited sterol biosynthesis did not inhibit floral induction. Many of the sterols have been reported to be inactive in floral induction.

In conclusion it may be stated that floral induction is a complex biochemical process and some sort of organizations are required for floral induction. Perhaps it is extremely difficult to study these processes in the test tube.

Floral Induction, Determination and Specification in Arabidopsis Determination Experiments:

In Arabidopsis leaf-removal experiments are difficult to conduct due to its growth habit and architecture.

The data from simple photoperiodic determination assays has revealed:

i. The early flowering ecoypes can be irreversibly committed to flower within one day of the start of photo-induction

ii. That low R-FR light ratios strongly promote the commitment to flowering.

iii. That plants grown in LD photoperiods are determined to flower after nearly seven days, when the first two leaves are nearly the same size as the cotyledon.

iv. Developing primordia respond to floral induction signals over a period of time.

v. The determination of primordium identity is not instantaneous.

vi. It is not vivid whether meristems are florally determined or whether the leaves are committed to a perpetual production of floral stimulus.

vii. The available data suggest that the irreversible commitment to flowering is controlled outside the shoot meristem.

It is assumed that in the next few years combined biochemical and molecular studies of flowering- time genes and mutants will show whether primordia are committed to flower. It has been suggested that within the catalogue of genes that correspond to early and later flowering genes it should be possible to find genes concerned with floral stimulus production, floral stimulus transport and floral stimulus perception within the shoot apex.It will be interesting to locate genes for meristem determination and /or genes for irreversible commitment to floral stimulus production in the leaves.

Control of Flower Primordium Specification by LEAFY and APETALAI/CAULI FLOWER:

The knowledge regarding the mechanism by which flowers are specified on the flanks of the shoot apex is well understood though same cannot be said regarding mechanisms involved in the production of floral stimulus in the leaves.

The available information is:

i. Replacement of flowers with indeterminate shoots in lfy and apcal double mutants indicates that LFY and API/CAL are crucial for flower primordia specifications.

ii. In ecotypically expressed LFY and API these genes are sufficient to specify flowers when expressed in shoot primordia.

iii. Many other genes may be involved in the specifications of flowers along with LFY and API/CAL.

Hempel (2000) have discussed molecular interactions between LFY and MADS-BOX genes, API/CAL and AGAMOUS (AG). In wild type LFY is expressed throughout flower primordia early in their ontogeny. API and CAL expression also takes place throughout flower primordia, though the expression of these two genes occurs in primordia only after they have become distinct from the meristem.

The upregulation of API during floral induction treatments does not occur until several hours after LFY has been upregulated. In lfy mutants, API expression is weak and delayed whereas ecotypic expression of LFY induces the ecotypic expression of LFY induces the ecotypic expression of API in leaf primordia and in axillary flower primordia.

These data show that LFY is a formal regulator of API. The recent data also show that the API promoter is a direct target of LFY, and it has a LFY-responsive enhancer that is needed for its activity. Clearly LFY has direct and distinct roles in the specification of flowers and in the patterning of floral organs.

Interactions between LFY, API and TERMINAL FLOWERI (TFLI) during the Transition to Flowering:

It is the interactions between these genes (LFY and API/CAL) and TFLI that inflorescence is regulated. TFLI prevents the expression of floral meristem identity genes in the shoot meristem and promotes indeterminate growth.