What if the roots of flowering plants produced chemical signals that regulated the branching and thickening of their shoots, i.e., secondary growth?
Chemical signals used by plants to regulate their development and physiology are called plant hormones.
Very small amounts of these compounds, acting alone or in tandem, often elicit dramatic effects on plant development.
For many years there were only five plant hormones known, but advances in plant sciences have recently revealed others.
Evidence in support of the discovery of another new class of plant hormones called strigolactones was reported in the September 11 2008 issue of Nature magazine (summarized here).
Briefly, an international team of scientists used several complementary experimental approaches to show that strigolactones produced in the roots travel up the plant in the xylem to the aerial parts of the plant and somehow suppress developmental pathways resulting in stem branching. (more info here)
(Strigolactones are likely chemically derived from carotenes, the orange-colored pigments in plants that give carrots their color, for example.)
Strigolactones are also suspected of being involved in mediating the formation of fungal symbioses in plants as well as promoting the germination of seeds of some plants (such as Striga ) that parasitize the roots of host plants and can cause significant crop damage.
Could there be any practical uses for strigolactones?
Conceivably, they may be agriculturally useful to induce premature germination of parasitic plants. By affecting shoot branching in flowering plants, and thus plant achitecture, strigolactones also may be useful to horticulturalists. (In a previous post, I discussed strigolactones as they may relate to apical dominance.)
In a recent report, summarized here, a new, more extensive role for strigolactones in mediating auxin signaling is proposed.
According to the authors of this paper: “Our results provide a model of how auxin-based long-distance signaling is translated into cambium activity and suggest that SLs [strigolactones] act as general modulators of plant growth forms linking the control of shoot branching with the thickening of stems and roots.” (from Ref 1 below)
Bottom line: Plants may use strigolactones as hormones produced in the roots to regulate not only the branching of shoots, but also secondary growth in general. (Moreover, some soil fungi and some parasitic plants use this chemical as a proximity signal in order for them to better colonize the roots of their plant hosts.)
1. Agusti, J., et al. (2011) “Strigolactone signaling is required for auxin-dependent stimulation of secondary growth in plants.” Proceedings of the National Academy of Sciences (USA), published online before print November 28, 2011, doi: 10.1073/pnas.1111902108.
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