TOR acts as a metabolic gatekeeper for auxin-dependent lateral root initiation in Arabidopsis thaliana.

Plant organogenesis requires matching the available metabolic resources to developmental programs. In Arabidopsis, the root system is determined by primary root-derived lateral roots (LRs), and adventitious roots (ARs) formed from non-root organs. Lateral root formation entails the auxin-dependent activation of transcription factors ARF7, ARF19, and LBD16. Adventitious root formation relies on LBD16 activation by auxin and WOX11. The allocation of shoot-derived sugar to the roots influences branching, but how its availability is sensed for LRs formation remains unknown. We combine metabolic profiling with cell-specific interference to show that LRs switch to glycolysis and consume carbohydrates. The target-of-rapamycin (TOR) kinase is activated in the lateral root domain. Interfering with TOR kinase blocks LR initiation while promoting AR formation. The target-of-rapamycin inhibition marginally affects the auxin-induced transcriptional response of the pericycle but attenuates the translation of ARF19, ARF7, and LBD16. TOR inhibition induces WOX11 transcription in these cells, yet no root branching occurs as TOR controls LBD16 translation. TOR is a central gatekeeper for root branching that integrates local auxin-dependent pathways with systemic metabolic signals, modulating the translation of auxin-induced genes.

Light sheet microscopy and live imaging of plants.

Light sheet fluorescence microscopy (LSFM) is increasingly used to investigate biological processes in animals as well as in plants. LSFM achieves optical sectioning by the selective illumination of a single plane of the sample with a sheet of laser light while simultaneously recording emitted fluorescence orthogonally to the illumination plane. A 3D image of the sample can then be generated with a temporal resolution ranging from seconds to several days, and at scales ranging from subcellular to whole organ. By design, LSFM provides fast imaging, and low phototoxicity, two key criteria for live imaging under physiological conditions. Despite its potential, LSFM remains underutilized in plant biology. This review aims to highlight challenges of live imaging in plants, to describe key steps in using LSFM on live plant samples and finally at providing an overview of published examples of applications of LSFM in plants.

Cell Death in Cells Overlying Lateral Root Primordia Facilitates Organ Growth in Arabidopsis.

Plant organ growth is widely accepted to be determined by cell division and cell expansion, but, unlike that in animals, the contribution of cell elimination has rarely been recognized. We investigated this paradigm during Arabidopsis lateral root formation, when the lateral root primordia (LRP) must traverse three overlying cell layers within the parent root. A subset of LRP-overlying cells displayed the induction of marker genes for cell types undergoing developmental cell death, and their cell death was detected by electron, confocal, and light sheet microscopy techniques. LRP growth was delayed in cell-death-deficient mutants lacking the positive cell death regulator ORESARA1/ANAC092 (ORE1). LRP growth was restored in ore1-2 knockout plants by genetically inducing cell elimination in cells overlying the LRP or by physically killing LRP-overlying cells by ablation with optical tweezers. Our results support that, in addition to previously discovered mechanisms, cell elimination contributes to regulating lateral root emergence.