To Branch, Or Not To Branch – Plant Hormones Help Turn A Stem Into A Bush
When we hear the expression "stem cells", we tend to think of cells from animals or patients that are used to treat diseases or promote regeneration. However, stem cells are also present in plants. The growing tips of plants are called meristems and they are reservoirs of plant stem cells. A meristem is formed at the base of each leaf and can remain dormant as a small bud or be activated and give rise to a whole new branch. Gardeners know that pruning leaves can activate the buds and help transform a single stem plant into a multi-branched bush, but the exact mechanisms that govern branch formation are not fully understood.
The recent paper "Strigolactone Can Promote or Inhibit Shoot Branching by Triggering Rapid Depletion of the Auxin Efflux Protein PIN1 from the Plasma Membrane" published in PLOS Biology by Naoki Shinohara and colleagues has uncovered an important novel pathway that regulates the formation of branches in plants. The researchers based their work on an existing model which states that the plant growth hormone auxin is a central regulator of branch formation. Auxin levels are highest in activated buds because this is where auxin is produced. Auxin then flows to the roots, where auxin levels are low ("auxin sinks"). The removal of auxin from the activated bud allows for further auxin production and thus creates a continuous auxin flow pattern. This is thought to establish a positive feedback loop for the activated bud, which then ultimately results in the formation of branch emanating from the activated bud. This model is called the "auxin transport canalization" and is explained in an excellent accompanying article "Transforming a Stem into a Bush" by Amy Coombs, also published in PLOS Biology.
Once an activated bud initiates the positive auxin feedback loop, it also becomes necessary to inhibit the branch formation from other buds. If all the buds in a plant started making branches at once, the plant's resources would probably become depleted very quickly, possibly resulting in the chaotic formation of too many suboptimal branches. There is a clearly a need for a system that allows some activated buds to go on to make branches, while putting the brake on other buds so that they bide their time. The details of such a fine-tuned balance of selected activation and inhibition have been a bit of a mystery, but the work by Shinohara and colleagues is a major step forward in unraveling this puzzle.
The researchers show that the plant hormone strigolactone removes the auxin export protein PIN1 from the cell surface and increases its degradation. Therefore, a plant without strigolactone would have more PIN1, sustain greater auxin flux and thus increase branching. Genetically engineered plants that do not produce strigolactone did indeed show more branch formation. When the researchers added back synthetic strigolactone (called GR24), they were able to suppress the excessive branch formation. However, the researchers also obtained a somewhat counter-intuitive result: When they gave GR24 to plants with defective auxin transport, low doses of GR24 actually helped branch formation and only higher doses suppressed branch formation. The problem with these results is that the synthetic strigolactone also severely impacted the general growth of the plants (not just branch formation) in the auxin transport mutants, and it is difficult to interpret whether the subtle differences between low and high doses were just generalized effects due to reduced overall plant health or whether they were truly related to aberrant branching.
The oddly opposite results obtained with low dose and high dose GR24 treatment are probably going to raise some controversy, and as Amy Coombs pointed out, not all scientists agree with the auxin transport canalization theory of branch formation in plants. This is not the first study to propose an interaction between strigolactone and auxin as regulators of plant branch formation, but it is one of the most comprehensive papers in this area. It includes a mathematical model of the interaction between these two regulators, tests the model with experiments and identifies a novel cellular mechanism for how strigolactone reduces PIN1. These results do suggest that plants have a very finely-tuned system involving at least two hormones, auxin and strigolactone, that act together to promote branch formation in some buds, while suppressing bud formation in others. As a stem cell biologist who works with mammalian stem cells, I am quite intrigued by this fascinating interplay between activating and suppressing hormones in plants that permit a self-organized branch formation. In mammals, we still do not fully understand how during development, some embryonic stem cells commit to one lineage and form organs such as a heart, while also preventing other stem cells in the developing embryo to form a second or third heart in other areas. It is quite likely that developing mammalian embryonic stem cells also depend on positive feedback loops and inhibitory systems, similar to what the researchers found in the plants. Many major discoveries in cell biology and molecular biology are first made in plants and we then discover similar principles of regulation in animals and humans.
Image credit: Panel from Figure 5 of the PLOS Biology (2013) paper by Shinohara N., et al, Green indicates the PIN protein and magenta shows the autofluorescence of chloroplasts
Shinohara, N., Taylor, C., & Leyser, O. (2013). Strigolactone Can Promote or Inhibit Shoot Branching by Triggering Rapid Depletion of the Auxin Efflux Protein PIN1 from the Plasma Membrane PLoS Biology, 11 (1) DOI: 10.1371/journal.pbio.1001474