Plant & zo
The science of plants and more
The art of bending
How to bend, or more precisely, how do plants bend? This was the question Baral and his colleagues set to answer. The bending of plants occurs as a result of many stimuli, such as wind, the search of nutrients, or obstacles. These can all occur at various stages of plant development. However, there is type of bending that always takes place at the same moment, the formation of the apical hook through bending of the hypocotyl just after seedling germination. The apical hook is formed to protect the shoot meristem during the process of emerging from the soil. This process is so important for the young seedling that it even occurs when there is no soil to emerge from.
It is not the first time that researches looked at apical hook formation. In the past researches have looked at how cell elongation, hormones and gene transcription regulation affect hypocotyl bending. All in a laboratory stetting, growing seedlings on top of agar, thereby ignoring the potential influence that mechanical force might have.
For Baral and his colleagues it became clear that mechanical force was playing a role when they observed that in the absence of the protein katanin, which affects cortical microtuble organization, hypocotyl bending was absent when seedlings were grown on agar plates, but not when grown on soil.
Which aspects of bending need a mechanical cue?
Looking for which aspects need a mechanical cue, Baral and colleagues found that the auxin asymmetry required for bending, needed a mechanical cue. In line with this, PIN transporters (auxin efflux carriers) needed a mechanical cue to change their localization. The change of localization of the PIN transporters results in changes of the auxin flow, leading to asymmetric auxin distribution, what in turn leads to bending of the hypocotyl.
As auxin has a hand in almost every process of plant development, the next thing they looked at was how auxin was regulating the process of hypocotyl bending. There were two options
- The classical auxin response pathway via the transcription factors ARF7 and ARF19.
- The alternative auxin response pathway whereby the plasma membrane localized receptor TMK1 which upon perception of high auxin levels sent a C-terminal fragment of itself to the nucleus to stabilize specific AUX/IAA transcription factors.
While mechanical cues still caused hypocotyl bending in the arf7 arf19 mutant, no hypocotyl bending was observed for the tmk1 mutant. Indicating that it is the alternative auxin response pathway that is needed for the regulation of the process of hypocotyl bending.
To find out how precisely requires more research. But what the Baral and colleagues also show is that even for such a well studied process such as hypocotyl bending, there are still discoveries to be made that give new insights in how these processes are regulated.
Baral, A., Aryal, B., Jonsson, K., Morris, E., Demes, E., Takatani, S., Verger, S., Xu, T., Bennett, M., Hamant, O., & Bhalerao, R. P. (2021). External Mechanical Cues Reveal a Katanin-Independent Mechanism behind Auxin-Mediated Tissue Bending in Plants. Developmental cell, 56(1), 67–80.e3