The GPAT4/6/8 clade functions in Arabidopsis root suberization nonredundantly with the GPAT5/7 clade required for suberin lamellae.

Lipid polymers such as cutin and suberin strengthen the diffusion barrier properties of the cell wall in specific cell types and are essential for water relations, mineral nutrition, and stress protection in plants. Land plant-specific glycerol-3-phosphate acyltransferases (GPATs) of different clades are central players in cutin and suberin monomer biosynthesis. Here, we show that the GPAT4/6/8 clade in Arabidopsis thaliana, which is known to mediate cutin formation, is also required for developmentally regulated root suberization, in addition to the established roles of GPAT5/7 in suberization. The GPAT5/7 clade is mainly required for abscisic acid-regulated suberization. In addition, the GPAT5/7 clade is crucial for the formation of the typical lamellated suberin ultrastructure observed by transmission electron microscopy, as distinct amorphous globular polyester structures were deposited in the apoplast of the gpat5 gpat7 double mutant, in contrast to the thinner but still lamellated suberin deposition in the gpat4 gpat6 gpat8 triple mutant. Site-directed mutagenesis revealed that the intrinsic phosphatase activity of GPAT4, GPAT6, and GPAT8, which leads to monoacylglycerol biosynthesis, contributes to suberin formation. GPAT5/7 lack an active phosphatase domain and the amorphous globular polyester structure observed in the gpat5 gpat7 double mutant was partially reverted by treatment with a phosphatase inhibitor or the expression of phosphatase-dead variants of GPAT4/6/8. Thus, GPATs that lack an active phosphatase domain synthetize lysophosphatidic acids that might play a role in the formation of the lamellated structure of suberin. GPATs with active and nonactive phosphatase domains appear to have nonredundant functions and must cooperate to achieve the efficient biosynthesis of correctly structured suberin.

Engineering Nicotiana tabacum trichomes for triterpenic acid production.

In this work, we aimed at implementing the biosynthesis of triterpenic acids in Nicotiana tabacum glandular trichomes. Although endogenous genes coding for enzymes involved in such biosynthetic pathway are found in the Nicotiana tabacum genome, implementing such pathway specifically in glandular trichomes required to boost endogenous enzymatic activities. Five transgenes coding for a farnesyl-diphosphate synthase, a squalene synthase, a squalene epoxidase, a beta-amyrin synthase and a beta-amyrin 28-monooxygenase were introduced in N.tabacum, their expression being driven by pMALD1, a trichome-specific transcriptional promoter. This study aimed at testing whether sinking isoprenoid precursors localized in plastids, by exploiting potential cross-talks allowing the exchange of terpenoid pools from the chloroplast to the cytosol, could be a way to improve overall yield. By analyzing metabolites extracted from entire leaves, a low amount of ursolic acid was detected in plants expressing the five transgenes. Our study shows that the terpene biosynthetic pathway could be, in part, redirected in N.tabacum glandular trichomes with no deleterious phenotype at the whole plant level (chlorosis, dwarfism,…). In light of our results, possible ways to improve the final yield are discussed.

Subtle interplay between trichome development and cuticle formation in plants.

Trichomes and cuticles are key protective epidermal specializations. This review highlights the genetic interplay existing between trichome and cuticle formation in a variety of species. Controlling trichome development, the biosynthesis of trichome-derived specialized metabolites as well as cuticle biosynthesis and deposition can be viewed as different aspects of a common defensive strategy adopted by plants to protect themselves from environmental stresses. Existence of such interplay is based on the mining of published transcriptomic data as well as on phenotypic observations in trichome or cuticle mutants where the morphology of both structures often appear to be concomitantly altered. Given the existence of several trichome developmental pathways depending on the plant species and the types of trichomes, genetic interactions between cuticle formation and trichome development are complex to decipher and not easy to generalize. Based on our review of the literature, a schematic overview of the gene network mediating this transcriptional interplay is presented for two model plant species: Arabidopsis thaliana and Solanum lycopersicum. In addition to fundamental new insights on the regulation of these processes, identifying key transcriptional switches controlling both processes could also facilitate more applied investigations aiming at improving much desired agronomical traits in plants.

Suberin plasticity to developmental and exogenous cues is regulated by a set of MYB transcription factors.

Suberin is a hydrophobic biopolymer that can be deposited at the periphery of cells, forming protective barriers against biotic and abiotic stress. In roots, suberin forms lamellae at the periphery of endodermal cells where it plays crucial roles in the control of water and mineral transport. Suberin formation is highly regulated by developmental and environmental cues. However, the mechanisms controlling its spatiotemporal regulation are poorly understood. Here, we show that endodermal suberin is regulated independently by developmental and exogenous signals to fine-tune suberin deposition in roots. We found a set of four MYB transcription factors (MYB41, MYB53, MYB92, and MYB93), each of which is individually regulated by these two signals and is sufficient to promote endodermal suberin. Mutation of these four transcription factors simultaneously through genome editing leads to a dramatic reduction in suberin formation in response to both developmental and environmental signals. Most suberin mutants analyzed at physiological levels are also affected in another endodermal barrier made of lignin (Casparian strips) through a compensatory mechanism. Through the functional analysis of these four MYBs, we generated plants allowing unbiased investigation of endodermal suberin function, without accounting for confounding effects due to Casparian strip defects, and were able to unravel specific roles of suberin in nutrient homeostasis.

The Root Cap Cuticle: A Cell Wall Structure for Seedling Establishment and Lateral Root Formation.

The root cap surrounding the tip of plant roots is thought to protect the delicate stem cells in the root meristem. We discovered that the first layer of root cap cells is covered by an electron-opaque cell wall modification resembling a plant cuticle. Cuticles are polyester-based protective structures considered exclusive to aerial plant organs. Mutations in cutin biosynthesis genes affect the composition and ultrastructure of this cuticular structure, confirming its cutin-like characteristics. Strikingly, targeted degradation of the root cap cuticle causes a hypersensitivity to abiotic stresses during seedling establishment. Furthermore, lateral root primordia also display a cuticle that, when defective, causes delayed outgrowth and organ deformations, suggesting that it facilitates lateral root emergence. Our results show that the previously unrecognized root cap cuticle protects the root meristem during the critical phase of seedling establishment and promotes the efficient formation of lateral roots.