Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism

Abstract

Gravity profoundly influences plant growth and development. Plants respond to changes in orientation by using gravitropic responses to modify their growth. Cholodny and Went hypothesized over 80 years ago that plants bend in response to a gravity stimulus by generating a lateral gradient of a growth regulator at an organ's apex, later found to be auxin. Auxin regulates root growth by targeting Aux/IAA repressor proteins for degradation. We used an Aux/IAAbased reporter, domain II (DII)-VENUS, in conjunction with a mathematical model to quantify auxin redistribution following a gravity stimulus. Our multidisciplinary approach revealed that auxin is rapidly redistributed to the lower side of the root within minutes of a 908 gravity stimulus. Unexpectedly, auxin asymmetry was rapidly lost as bending root tips reached an angle of 408 to the horizontal. We hypothesize roots use a "tipping point" mechanism that operates to reverse the asymmetric auxin flow at the midpoint of root bending. These mechanistic insights illustrate the scientific value of developing quantitative reporters such as DII-VENUS in conjunction with parameterized mathematical models to provide high-resolution kinetics of hormone redistribution.environmental sensing | systems biology R oot gravitropism has fascinated researchers since Knight (1) and Darwin (2). More recently, reorientation of Arabidopsis seedlings has been shown to trigger the asymmetric release of the growth regulator auxin from gravity-sensing columella cells at the root apex (Fig. 1A) (3-5). The resulting lateral auxin gradient is hypothesized to drive a differential growth response, where cell expansion on the lower side of the elongation zone is reduced relative to the upper side, causing the root to bend downward (6-8). Despite representing one of the oldest hypotheses in plant biology, key questions about auxin-regulated root gravitropism remain to be experimentally determined. How rapidly does the lateral auxin gradient form? Is this timescale consistent with the theory that auxin redistribution drives root bending? How long does the lateral auxin gradient persist? What triggers auxin redistribution to return to equal levels?Our understanding of gravity-induced auxin redistribution has been limited by the tools available to monitor auxin concentrations at high spatiotemporal resolution. Currently, the most widely used tools to follow auxin distribution in tissues are auxin-inducible reporters such as DR5::GFP (3, 4). However, as an output of the auxin response pathway (Fig. 1B), the activity of the DR5 reporter does not directly relate to endogenous auxin abundance, but also depends on additional parameters including local auxin signaling capacities and rates of transcription and translation (Fig. 1B). In practice, these intermediate processes confer a time delay of ∼1.5-2 h between changes in auxin abundance and DR5 reporter activity (9, 4), making it difficult to quantify the speed and magnitude of fold changes in auxin distribution during a root gravitropic response.Auxi...

Cellular Auxin Homeostasis under High Temperature Is Regulated through a SORTING NEXIN1–Dependent Endosomal Trafficking Pathway

“…To understand the consequences of high temperature in root development, we first monitored the change in the intracellular auxin level using a newly developed sensor DII-VENUS, which is capable of detecting intracellular auxin distribution at high spatio-temporal resolution (Brunoud et al, 2012). Consistent with previous results, we also found that DII-VENUS abundance is directly linked to the intracellular auxin level (see Supplemental Figure 1A online; Band et al, 2012) and responds to minute changes in auxin concentration (see Supplemental Figure 1A online). Comparison of DII-VENUS response at standard (23°C) and high (29°C) temperature conditions revealed a reduced DII-VENUS signal in high temperature-treated roots at all time points examined ( Figure 1; see Supplemental Figure 1B online), suggesting that high temperature affects the intracellular auxin response through stimulating the auxin level in root.…”

Shoot and root branch growth angle control—the wonderfulness of lateralness

“…In roots, this auxin accumulation inhibits cell elongation causing the root tip to grow downwards while in the shoot the reverse is true, auxin accumulation on the lower side of the organ drives cell elongation and so upward growth. Importantly, the magnitude of gravitropic response (as degrees of bending per unit time) increases as the organ is tilted further from the vertical [37,38]. This angle-dependent variation in the magnitude of graviresponse was first analysed by Sachs in the late 1800s [39].…”

From jellyfish to biosensors: the use of fluorescent proteins in plants

“…The initial characterisation showed that the DII-VENUS reporter is an ideal auxin sensor (Band et al, 2012, Brunoud et al, 2012, Vernoux et al, 2011. Firstly, it is broadly expressed, at least in root tissues, via the constitutive cauliflower mosaic virus (CaMV) 35S promoter, with fluorescence localised to the nucleus due to an NLS ( Fig.…”

“…2B lower panel). Using DII-VENUS, the dynamic redistribution of auxin during root gravitropism can thus be visualised even before the root starts to re-orientate its growth towards the gravity vector (Brunould et al, 2012;Band et al, 2012).…”

“…Despite these limitations this new generation of fluorescent biosensors have provided the opportunity to image the dynamics of hormone signalling in vivo at a resolution that has not been achieved before. To relate imaged DII-VENUS fluorescence to auxin abundance required the development of parameterised mathematical models as the relationship is non-linear (Band et al, 2012). In order to monitor plant roots responding to a gravistimulus in a realistic environment, the authors adapted an inverted confocal microscope to image roots arranged vertically on growth plates.…”