Abstract
Plants are sessile organisms that are permanently restricted to their site of germination. To compensate for their lack of mobility, plants evolved unique mechanisms enabling them to rapidly react to ever changing environmental conditions and flexibly adapt their postembryonic developmental program. A prominent demonstration of this developmental plasticity is their ability to bend organs in order to reach the position most optimal for growth and utilization of light, nutrients, and other resources. Shortly after germination, dicotyledonous seedlings form a bended structure, the so-called apical hook, to protect the delicate shoot meristem and cotyledons from damage when penetrating through the soil. Upon perception of a light stimulus, the apical hook rapidly opens and the photomorphogenic developmental program is activated. After germination, plant organs are able to align their growth with the light source and adopt the most favorable orientation through bending, in a process named phototropism. On the other hand, when roots and shoots are diverted from their upright orientation, they immediately detect a change in the gravity vector and bend to maintain a vertical growth direction. Noteworthy, despite the diversity of external stimuli perceived by different plant organs, all plant tropic movements share a common mechanistic basis: differential cell growth. In our review, we will discuss the molecular principles underlying various tropic responses with the focus on mechanisms mediating the perception of external signals, transduction cascades and downstream responses that regulate differential cell growth and consequently, organ bending. In particular, we highlight common and specific features of regulatory pathways in control of the bending of organs and a role for the plant hormone auxin as a key regulatory component.
Highlights
To compensate for their sessile lifestyle, plants developed unique mechanisms that provide them with an unusual level of developmental plasticity
Efforts to characterize a functional link between the photoreceptor and ATPBINDING CASSETTE B19 (ABCB19) revealed that PHOT1 transiently interacts with ABCB19. Through this direct physical interaction PHOT1 promotes ABCB19 phosphorylation, which leads to the attenuation of its auxin transport activity. Based on these observations it has been proposed that PHOT1 restrains ABCB19 activity, thereby increasing the accumulation of auxin in and above the hypocotyl apex to prime lateral auxin fluxes that are channeled to the elongation zone by the auxin transporter PIN-like 3 (PIN3), hereby coordinating differential cell growth (Christie et al, 2011)
These results indicate that similar to the shoot, blue lightinduced PIN3 polarization contributes to the control of an asymmetric auxin distribution during the negative root phototropic response
Summary
Straeten D and Benková E (2015) Strategies of seedlings to overcome their sessile nature: auxin in mobility control. Plants are sessile organisms that are permanently restricted to their site of germination. To compensate for their lack of mobility, plants evolved unique mechanisms enabling them to rapidly react to ever changing environmental conditions and flexibly adapt their postembryonic developmental program. Plant organs are able to align their growth with the light source and adopt the most favorable orientation through bending, in a process named phototropism. We will discuss the molecular principles underlying various tropic responses with the focus on mechanisms mediating the perception of external signals, transduction cascades and downstream responses that regulate differential cell growth and organ bending.
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