Auxin triggers pectin modification during rootlet emergence in white lupin.

Emergence of secondary roots through parental tissue is a highly controlled developmental process. Although the model plant Arabidopsis has been useful to uncover the predominant role of auxin in this process, its simple root structure is not representative of how emergence takes place in most plants, which display more complex root anatomy. White lupin is a legume crop producing structures called cluster roots, where closely spaced rootlets emerge synchronously. Rootlet primordia push their way through several cortical cell layers while maintaining the parent root integrity, reflecting more generally the lateral root emergence process in most multilayered species. In this study, we showed that lupin rootlet emergence is associated with an upregulation of cell wall pectin modifying and degrading genes under the active control of auxin. Among them, we identified LaPG3, a polygalacturonase gene typically expressed in cells surrounding the rootlet primordium and we showed that its downregulation delays emergence. Immunolabeling of pectin epitopes and their quantification uncovered a gradual pectin demethylesterification in the emergence zone, which was further enhanced by auxin treatment, revealing a direct hormonal control of cell wall properties. We also report rhamnogalacturonan-I modifications affecting cortical cells that undergo separation as a consequence of primordium outgrowth. In conclusion, we describe a model of how external tissues in front of rootlet primordia display cell wall modifications to allow for the passage of newly formed rootlets.

Dynamic Development of White Lupin Rootlets Along a Cluster Root.

White lupin produces cluster roots in response to phosphorus deficiency. Along the cluster root, numerous short rootlets successively appear, creating a spatial and temporal gradient of developmental stages that constitutes a powerful biological model to study the dynamics of the structural and functional evolution of these organs. The present study proposes a fine histochemical, transcriptomic and functional analysis of the rootlet development from its emergence to its final length. Between these two stages, the tissue structures of the rootlets were observed, the course of transcript expressions for the genes differentially expressed was monitored and some physiological events linked to Pi nutrition were followed. A switch between (i) a growing phase, in which a normal apical meristem is present and (ii) a specialized phase for nutrition, in which the rootlet is completely differentiated, was highlighted. In the final stage of its determinate growth, the rootlet is an organ with a very active metabolism, especially for the solubilization and absorption of several nutrients. This work discusses how the transition between a growing to a determinate state in response to nutritional stresses is found in other species and underlines the fundamental dilemma of roots between soil exploration and soil exploitation.

Anatomical and hormonal description of rootlet primordium development along white lupin cluster root.

Cluster root (CR) is one of the most spectacular plant developmental adaptations to hostile environment. It can be found in a few species from a dozen botanical families, including white lupin (Lupinus albus) in the Fabaceae family. These amazing structures are produced in phosphate-deprived conditions and are made of hundreds of short roots also known as rootlets. White lupin is the only crop bearing CRs and is considered as the model species for CR studies. However, little information is available on CRs atypical development, including the molecular events that trigger their formation. To provide insights on CR formation, we performed an anatomical and cellular description of rootlet development in white lupin. Starting with a classic histological approach, we described rootlet primordium development and defined eight developmental stages from rootlet initiation to their emergence. Due to the major role of hormones in the developmental program of root system, we next focussed on auxin-related mechanisms. We observed the establishment of an auxin maximum through rootlet development in transgenic roots expressing the DR5:GUS auxin reporter. Expression analysis of the main auxin-related genes [TIR, Auxin Response Factor (ARF) and AUX/IAA] during a detailed time course revealed specific expression associated with the formation of the rootlet primordium. We showed that L. albus TRANSPORT INHIBITOR RESPONSE 1b is expressed during rootlet primordium formation and that L. albus AUXIN RESPONSE FACTOR 5 is expressed in the vasculature but absent in the primordium itself. Altogether, our results describe the very early cellular events leading to CR formation and reveal some of the auxin-related mechanisms.