Summary: Active polar transport establishes directional auxin flow and the generation of local auxin gradients implicated in plant responses and development. Auxin modulates gravitropism at the root tip and root hair morphogenesis at the differentiation zone. Genetic and biochemical analyses provide evidence for defective basipetal auxin transport in trh1 roots. The trh1, pin2, axr2 and aux1 mutants, and transgenic plants overexpressing PIN1, all showing impaired gravity response and root hair development, revealed ectopic PIN1 localization. The auxin antagonist hypaphorine blocked root hair elongation and caused moderate agravitropic root growth, also leading to PIN1 mislocalization. These results suggest that auxin imbalance leads to proximal and distal developmental defects in Arabidopsis root apex, associated with agravitropic root growth and root hair phenotype, respectively, providing evidence that these two auxin‐regulated processes are coupled. Cell‐specific subcellular localization of TRH1‐YFP in stele and epidermis supports TRH1 engagement in auxin transport, and hence impaired function in trh1 causes dual defects of auxin imbalance. The interplay between intrinsic cues determining root epidermal cell fate through the TTG/GL2 pathway and environmental cues including abiotic stresses modulates root hair morphogenesis. As a consequence of auxin imbalance in Arabidopsis root apex, ectopic PIN1 mislocalization could be a risk aversion mechanism to trigger root developmental responses ensuring root growth plasticity.

The wall is the last frontier of a plant cell involved in modulating growth, development and defense against biotic stresses. Cellulose and additional polysaccharides of plant cell walls are the most abundant biopolymers on earth, having increased in economic value and thereby attracted significant interest in biotechnology. Cellulose biosynthesis constitutes a highly complicated process relying on the formation of cellulose synthase complexes. Cellulose synthase (CesA) and Cellulose synthase-like (Csl) genes encode enzymes that synthesize cellulose and most hemicellulosic polysaccharides. Arabidopsis and rice are invaluable genetic models and reliable representatives of land plants to comprehend cell wall synthesis. During the past two decades, enormous research progress has been made to understand the mechanisms of cellulose synthesis and construction of the plant cell wall. A plethora of cesa and csl mutants have been characterized, providing functional insights into individual protein isoforms. Recent structural studies have uncovered the mode of CesA assembly and the dynamics of cellulose production. Genetics and structural biology have generated new knowledge and have accelerated the pace of discovery in this field, ultimately opening perspectives towards cellulose synthesis manipulation. This review provides an overview of the major breakthroughs gathering previous and recent genetic and structural advancements, focusing on the function of CesA and Csl catalytic domain in plants.