Is There a Single Flower-Inducing Pathway?
But some plants, for example the “Night-Neutral” (a.k.a., “Day-Neutral”) plants, apparently initiate flowering because of factors other than night length.
In plants that flower independently of environmental factors, the flower-inducing pathway is currently called “autonomous”.
Such plants may flower after attaining a certain size or age, for example. Thus, floral induction in these plants may happen mainly in response to internal (endogenous) conditions rather than to environmental (external) conditions.
The stage of human life called “puberty” is quite well-known to most people, especially to those currently experiencing it.
But do plants go through puberty, too?
Strictly speaking, no. But most flowering plants do progress through developmental stages somewhat analogous to puberty.
After germination, many flowering plant species enter a juvenile phase in which they are not “competent” to flower. That is, even when experiencing conditions favorable to flowering, they lack the ability to flower.
This may be because some plants may not produce flowers until they are sufficiently robust enough to support the drain on resources required by flowering. In other words, a plant may not flower until it has enough leaves (photosynthetic sugar production) to build and support flowers.
This juvenile phase to adult phase transition, which affects many aspects of plant development, is the classic example of an “autonomous” flowering pathway.
The plant will become competent to flower after it makes a developmental transition to its adult phase, which may be determined primarily by the size of the plant. This size-related competency to flower may also be gauged by the plant’s age, presuming that the older a plant is, the bigger it is.
But if one proposes that some plants flower in response to size or age, important questions arise, such as:
How does a plant “know” how big or how old it is?
In plants that flower in response to internal cues (such as size or age), does florigen still play a primary role?
How Do Plants “Know” How Big They Are?
One way plants may be able to determine their relative size is by “node counting”. That is, the more nodes (stem buds/leaves) the plant has, the bigger (more productive) it is. (For all you scholars out there, Ref. 1 below provides an exhaustive review of “node counting”.)
A plant may also gauge its size by how far the shoot apical meristem (SAM) is from the roots. Or a plant may determine its overall size by how big a root system it has.
There is scientific evidence for all of these possibilities. However, the key to all of them is that the nodes, the roots, or both produce chemical signals (likely one or more of the common plant hormones) that travel via the phloem to the SAM. (The SAM is where the floral transition will take place.)
Thus, flowering may be triggered at the SAM by a threshold amount of – or ratio of – one or more plant hormones.
Aspen trees my not become competent to flower until they are over ten years old. But how does a tree “know” how many years have passed since it was a seedling?
It’s conceivable that a plant can obtain relative age info from the same ways it may estimate its size mentioned above.
It’s also been proposed that certain substances in plants (likely specific proteins) may start out at high levels in young seedlings, but then slowly decrease over the life of the plant (think sand through an hour-glass). Once the substance drops below a certain level in the SAM, the floral transition may then proceed.
Multiple Pathways Lead to Flowering
This, of course, is a big old subject in plant biology, with countless studies published over its hundred years of history.
The past few years, however, have yielded much genetic insight into how plants make flowers. (For an excellent review, please see Ref. 2 below)
From these genetic studies (mainly using the plant Arabidopsis thaliana) scientists have discovered the identity of florigen (much more on this here). These studies have revealed that the genetic mechanisms involved in floral induction are complex and are affected not only by florigen but by other plant chemical signals, such as gibberellins, as well as by environmental factors, such as temperature.
Indeed, a genetic study published in 2009 reported a signaling pathway that ensures that a plant flowers, no matter what.
Bottom Line: There is likely a central genetic mechanism, common to all flowering plants, that initiates flowering. This mechanism is triggered not only by florigen but is also affected by other endogenous and environmental factors.
1. Sachs, T. (1999) “‘Node counting’: an internal control of balanced vegetative and reproductive development.” Plant, Cell & Environment, Vol. 22, pp. 757-766. (Full Text)
2. Amasino, R. (2010) “Seasonal and developmental timing of flowering.” The Plant Journal, Vol. 61, pp. 1001-1013.(Full Text)
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