Strigolactone, straight up

Plant growth is controlled by many different hormones (hormones being chemicals that influence the development and metabolism of both plants and animals). But recent research by the University of Queensland’s Dr Phil Brewer, among others, has shown that one hormone called strigolactone plays a number of important roles.

Parasitic plant Striga asiatica, or witchweed (click to embiggen)
Striga asiatica or witchweed, a parasitic plant, the seeds of which germinate in the roots of their host when triggered by the hormone strigolactone - hence the name (image by Florida Division of Plant Industry Archive, Florida Department of Agriculture and Consumer Services, Bugwood.org, via Wikimedia Commons)
Firstly, in a paper published three years ago, Dr Brewer and colleagues showed that strigolactone inhibits the growth of side-branches, making plants instead grow straight up (Gomez-Roldan V, Fermas S, Brewer PB, Puech-Pagès V, Dun EA, Pillot J-P, Letisse F, Matusova R, Danoun S, Portais J-C, Bouwmeester H, Bécard G, Beveridge CA, Rameau C & Rochange SF 2008, “Strigolactone inhibition of shoot branching”, Nature, no. 455, pp. 189-194 doi:10.1038/nature07271).

Levels of strigolactone increase when light or nutrients are limited, which is a good time to grow taller than your neighbours, or to quickly reproduce before the food runs out (by inhibiting side-shoots, more energy is available for making flowers and seeds for reproduction).

Conversely, when nutrients are plentiful strigolactone levels fall, the plant grows bushy and is able to take advantage of its environment.

Now a follow up paper has revealed that strigolactone helps plants grow tall in another way: by making their stem thicker and stronger and so able to support the weight. It does this in response to signals issued by another hormone, auxin (Agusti J, Herolda S, Schwarz M, Pablo Sancheza, Ljung K, Dun EA, Brewer PB, Beveridge CA, Sieberer T, Sehr EM & Greb T, “Strigolactone signaling is required for auxin-dependent stimulation of secondary growth in plants”, Proceedings of the National Academy of Sciences, published online 28 November 2011, doi:10.1073/pnas.1111902108).

Strigolactone also seems to encourage growth in other areas too, like tiny hairs on the roots needed to extract nutrients from the soil and encourage the growth of symbiotic fungi.

Of course, like anything it comes with negative effects too: presence of strigolactone in the root systems seems to stimulate the germination of seeds of some parasitic weeds, like those of the genus Striga (from which strigolactone gets its name).

It could also be used by foresters to make plantation trees grow faster with straighter trunks, which probably counts as a positive for both us and the plants; until the whole chopping down thing, of course. Despite their signalling hormones, the trees are a little quiet about whether it’s good or bad overall.

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