Cell wall remodeling and vesicle trafficking mediate the root clock in Arabidopsis

In Arabidopsis thaliana, lateral roots initiate in a process preceded by periodic gene expression known as the root clock. We identified the vesicle-trafficking regulator GNOM and its suppressor, ADENOSINE PHOSPHATE RIBOSYLATION FACTOR GTPase ACTIVATION PROTEIN DOMAIN3, as root clock regulators. GNOM is required for the proper distribution of pectin, a mediator of intercellular adhesion, whereas the pectin esterification state is essential for a functional root clock. In sites of lateral root primordia emergence, both esterified and de-esterified pectin variants are differentially distributed. Using a reverse-genetics approach, we show that 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.

Pectin Ultra-degradation Decreases the Force Required to Detach Ripe Fruit from the Calyx in Tabasco Pepper

Pectin metabolism was analyzed in tabasco pepper (Capsicum frutescens L.) to determine the metabolic process associated with the ease of fruit detachment from the calyx. The ease of fruit detachment (deciduous fruit) is a desirable trait in peppers that facilitates mechanical harvest. Two genotypes that differ in the fruit detachment force were used: `Easy Pick' (EZ), which requires a low force, and `Hard Pick' (HP), which requires higher force. Pectin dissolution in water from fresh-ripe EZ tissue was 20 times higher than from HP tissue. EDTA-soluble uronide from inactivated EZ cell wall, however, was only 1.8 times higher. Pectin dissolution was inversely correlated to the fruit detachment force and followed a sigmoidal curve during fruit ripening. Size-exclusion chromatography of EDTA-soluble polyuronides indicated that pectin was degraded in ripe fruit tissue from both genotypes. The degree of depolymerization, however, was more extensive in EZ fruit. Consequently, the ease of fruit detachment was attributed to pectin ultra-degradation. Total pectin content in dry tissue and ethanol/acetone-extracted cell wall was similar in both genotypes. Pectin content in dry tissue was maintained throughout ripening, while extracted cell wall pectin increased slightly. In contrast, the degree of pectin esterification of extracted cell wall decreased only in ripe EZ fruit. These results suggest that pectin de-esterification may have a role in the enhanced pectin depolymerization and consequently in the ease of fruit detachment of the EZ genotype.