Root architectural traits play a critical role in crop adaptation; thus, they have started to be considered in breeding programs aimed at the release of new cultivars with improved soil exploration and water and nutrients absorption, lodging resistance, and yield. One of the most important root architectural traits is Root Growth Angle (RGA), namely the direction of root growth in respect of the gravity vector. RGA potentially affects the volume of the soil explored and mean and maximum root depth. For a given growing root tip at a given time, competing gravitropic versus antigravitropic offset mechanisms act to set RGA. However, only a few genes involved in controlling RGA have been discovered so far. The aim of this work is the characterization of a new hypergravitropic mutant and the identification of the gene or genes that control the phenotype. The TILLMore collection is an important genetic resource that allowed us to identify the first two genes in barley that controlled RGA, EGT1 and EGT2. By means of a new root phenotypic screening using mini-rhizotrones (based on CD cases), new root mutants were identified including some showing hypergravitropic root system. Genomic DNA of a new hypergravitropic root mutant underwent WGS sequencing using short reads (ILLUMINA). The new root mutant showed seminal and lateral root angle narrower than the wild-type. The phenotypic data of F1 population derived from cv. Morex × Mutant showed a wild-type root angle suggesting that the mutated allele is recessive. After WGS, no SNPs were found in previously identified EGT genes, suggesting that a mutation in a new root hypergravitropic gene is responsible for controlling the phenotype. Based on sequencing result, the mutation frequency is approximately 1/150 kb, with about 200 missense/non-sense mutation in the exome. Genetic mapping by bulk segregant analysis and genetic complementation analysis with known EGT mutants are in progress.

Water is essential for plants’ growth and development, but water in soils tends to be heterogeneously distributed. To acquire water, maize roots continuously explore the soil by building a highly complex, branched architecture; at the same time, they adjust their water transport capacity. However, the molecular underpinnings and coordinating the response to heterogeneous water distribution, especially the temporal response, are largely unknown. We developed a split-root hydroponic system and found that when subjected to local water deficit, maize lateral root growth is enhanced in the water-sufficient part and inhibited in the water-deficit part compared to their corresponding controls. To characterize the transcriptional mechanism underlying this response to heterogeneous water availability, we assayed how maize’s roots temporal transcriptome responds to local water deficit during the seedling stage. We treated hybrid maize B73h (B73-UH007) seedlings with Polyethylene glycol 8000 (PEG8000) at 150 g/L in the split-root hydroponic system (0/0 PEG, 0/150 PEG, 150/150 PEG) over 10 time points (0, 0.5, 1, 2, 4, 8, 12, 24, 48 and 96h). The lateral roots (LR) and axial root tips (Tips) were collected to conduct RNA-seq. DEGs were identified via fitting a spline to each gene, and comparing the expression with CK/PEG. kmeans clustering was used to identify identical temporal gene clusters. The result clearly identified an early and late molecular response to heterogeneous water availability for both LR and Tips. Compared to Tips, LR showed more massive and rapid response especially in the early time. This approach identified a LR specific gene cluster locally and systemically regulated within 30 min pointing out a transient signaling mechanism that will be further described.

Knowledge gap
Mechanistic representation of soil-root hydrodynamics is necessary for robust predictions of canopy flux limitation by soil moisture in Earth System Models (ESM). The mismatched scales of cause and effect make representing water limitation a central challenge in land surface modelling.

Objective
Our aim is to unify the description of soil-root water flows across scales to bridge this gap.

Material and Methods
We developed a new model formulation from analytical solutions to the differential equations for flows on root networks. By formulating the integrals in terms of mean water potentials over arbitrary root segments, we obtain a linear system directly without introducing a numerical approximation. Partial Gaussian elimination then yields a system of exact equations for mean water potentials in the root system at any chosen scale.

Results
The upscaled equations reproduce exact solutions for water potentials and flows on a single plant at any scale. Fitted to explicit stand-scale simulations, the model shows non-increasing error with the addition of further plants to the explicit simulation set. Proof-of-concept results show improved agreement with field data during a seasonal drought over previous models. The computational cost of these calculations is lower or equal to methods present in ESM and other upscaling methods. Code for producing the upscaled equations for any root hydraulic architecture is available for beta testing.

Discussion
Applications of this model formulation include connecting observations of plant hydrodynamic functioning across scales. We are currently collecting the data necessary to independently find model parameters for two species at single root, tree, and forest stand scales. By using the model to bridge across the scales of observation, we hope to disentangle cross-scale influences of individual mechanisms, such as the effect of root phenology on the seasonal variation in land-atmosphere hydrodynamics.

Defoliation triggers the remobilisation of plant reserves to generate new leaves, directing assimilates away from competing processes such as root growth. However, the duration of any root growth cessation and its impact on resource uptake potential is uncertain. Winter wheat was established in the 4 m high outdoor rhizobox facility equipped with imaging panels, sensors, and access points for tracer-labelling. The wheat was defoliated in autumn at early tillering and roots were imaged at a high time resolution and analysed for root depth/density by deep learning segmentation. The water and N uptake were measured using soil water sensors (TDR) and using 2H labelled water and 15N labelled fertilizer as tracers for uptake. Root growth of wheat paused for 269 ºC days (20 days) following defoliation after which the root depth penetration rate resumed at a similar rate to control plants (1.8 mm ºC days-1). Through this delay, defoliation caused a substantial reduction in root density with an associated reduction in water and N uptake at maturity, especially from deeper soil layers (>2 m). Our results have significant implications for managing the grazing of dual-purpose crops to balance the interplay between canopy removal and the capacity of deep roots to provide water and N for yield recovery.