Signaling for "the holes" in the cell wall through where lateral roots will emerge

In the model plant Arabidopsis, lateral roots are initiated in a process preceding by periodic gene expression known as the Root Clock. In a collaborative project between Dr. Benfey's (Duke University) and Dr. Moreno-Risueno (CBGP) laboratories, we have identified that the Root Clock requires the vesicular traffic regulator GNOM, as well as its suppressor ADENOSINE PHOSPHATE RIBOSYLATION FACTOR GTPase ACTIVATION OF PROTEIN DOMAIN 3 (Figure 1A). These proteins participate in the correct distribution of pectins, which mediate intercellular adhesion.

From RNA-seq data of the oscillation zone, where the oscillating signaling system of the Root Clock is located, and using reverse genetics, we have determined that the pectin esterification state is essential for a functional Root Clock; and thus the genes controlling pectin esterification regulate the Root Clock and lateral root initiation. These results indicate that the balance between esterified and de-esterified pectin states is essential for proper root clock function and the subsequent initiation of lateral root primordia. This was also corroborated by finding that in the sites of lateral root primordia emergence, both esterified and de-esterified pectin variants are differentially distributed (Figure 1B-C). We observed that de-esterified pectins were reduced in the cell walls near the lateral root primordium, while increased levels of esterified pectins associated to this region. As pectin de-esterification is usually associated with increased cell-cell adhesion, this differential distribution of esterified and de-esterified pectins would reduce cell wall siffness, facilitating lateral root emergence.

Taken together,our results indicate that the process of secreting and modifying pectin is critical to the function of the root clock and subsequent lateral root formation. Given that lateral roots emerge by pushing through overlying cell layers, pectin esterification state would provide competence to form lateral roots.