Aquaporins are membrane channels that facilitate water movement across cell membranes. In plants, aquaporins contribute to water relations. Here, we establish a new link between aquaporin-dependent tissue hydraulics and auxin-regulated root development in Arabidopsis thaliana. We report that most aquaporin genes are repressed during lateral root formation and by exogenous auxin treatment. Auxin reduces root hydraulic conductivity both at the cell and whole-organ levels. The highly expressed aquaporin PIP2;1 is progressively excluded from the site of the auxin response maximum in lateral root primordia (LRP) whilst being maintained at their base and underlying vascular tissues. Modelling predicts that the positive and negative perturbations of PIP2;1 expression alter water flow into LRP, thereby slowing lateral root emergence (LRE). Consistent with this mechanism, pip2;1 mutants and PIP2;1-overexpressing lines exhibit delayed LRE. We conclude that auxin promotes LRE by regulating the spatial and temporal distribution of aquaporin-dependent root tissue water transport.

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...

Abstract:The plant root cap, surrounding the very tip of the growing root, perceives and transmits environmental signals to the inner root tissues. In Arabidopsis thaliana, auxin released by the root cap contributes to the regular spacing of lateral organs along the primary root axis.Here, we show that the periodicity of lateral organ induction is driven by recurrent programmed cell death at the most distal edge of the root cap. We suggest that synchronous bursts of cell death in lateral root cap cells release pulses of auxin to surrounding root tissues, establishing the pattern for lateral root formation. The dynamics of root cap turnover may therefore coordinate primary root growth with root branching in order to optimize the uptake of water and nutrients from the soil.

Auxin is a key regulator of plant growth and development. Within the root tip, auxin distribution plays a crucial role specifying developmental zones and coordinating tropic responses. Determining how the organ-scale auxin pattern is regulated at the cellular scale is essential to understanding how these processes are controlled. In this study, we developed an auxin transport model based on actual root cell geometries and carrier subcellular localizations. We tested model predictions using the DII-VENUS auxin sensor in conjunction with state-of-the-art segmentation tools. Our study revealed that auxin efflux carriers alone cannot create the pattern of auxin distribution at the root tip and that AUX1/LAX influx carriers are also required. We observed that AUX1 in lateral root cap (LRC) and elongating epidermal cells greatly enhance auxin's shootward flux, with this flux being predominantly through the LRC, entering the epidermal cells only as they enter the elongation zone. We conclude that the nonpolar AUX1/LAX influx carriers control which tissues have high auxin levels, whereas the polar PIN carriers control the direction of auxin transport within these tissues.

2Plants can acclimate by using tropisms to link the direction of growth to 41 environmental conditions. Hydrotropism allows roots to forage for water, a process 42 known to depend on abscisic acid (ABA) but whose molecular and cellular basis 43 remains unclear. Here, we show that hydrotropism still occurs in roots after laser 44 ablation removed the meristem and root cap. Additionally, targeted expression 45 studies reveal that hydrotropism depends on the ABA signalling kinase, SnRK2.2, and 46 the hydrotropism-specific MIZ1, both acting specifically in elongation zone cortical 47 cells. Conversely, hydrotropism, but not gravitropism, is inhibited by preventing 48 differential cell-length increases in the cortex, but not in other cell types. We conclude 49 that root tropic responses to gravity and water are driven by distinct tissue-based 50 mechanisms. In addition, unlike its role in root gravitropism, the elongation zone 51 performs a dual function during a hydrotropic response, both sensing a water 52 potential gradient and subsequently undergoing differential growth. 53 3 Tropic responses are differential growth mechanisms that roots use to explore the 54 surrounding soil efficiently. In general, a tropic response can be divided into several steps, 55 comprising perception, signal transduction, and differential growth. All of these steps have 56 been well characterized for gravitropism, where gravity sensing cells in the columella of the 57 root cap generate a lateral auxin gradient, whilst adjacent lateral root cap cells transport 58 auxin to epidermal cells in the elongation zone, thereby triggering the differential growth that 59 drives bending [1][2][3][4] . In gravi-stimulated roots, the lateral auxin gradient is transported 60 principally by AUX1 and PIN carriers [3][4][5] . 61Compared with gravitropism, the tropic response to asymmetric water availability, i.e., 62 hydrotropism, has been far less studied. Previously, it was reported that surgical removal or 63 ablation of the root cap reduces hydrotropic bending in pea [6][7][8] and Arabidopsis thaliana 9 , 64suggesting that the machinery for sensing moisture gradients resides in the root cap. It has 65 also been reported that hydrotropic bending occurs due to differential growth in the 66 elongation zone 7,10 . However unlike gravitropism, hydrotropism in A. thaliana is independent 67 of AUX1 and PIN-mediated auxin transport 11,12 . Indeed, roots bend hydrotropically in the 68 absence of any redistribution of auxin detectable by auxin-responsive reporters 13,14 . 18,19 . 83However it is unclear whether this broad expression pattern is necessary for MIZ1's function 84 in hydrotropism or whether ABA signal transduction components in general have to be 85 expressed in specific root tip tissues for a hydrotropic response. The present study describes 86 a series of experiments in A. thaliana designed to identify the root tissues essential for a 87 hydrotropic response. We report that MIZ1 and a key ABA signal-transduction component 88SnRK2....