A role for the root cap in root branching revealed by the non-auxin probe naxillin

Abstract

The acquisition of water and nutrients by plant roots is a fundamental aspect of agriculture and strongly depends on root architecture. Root branching and expansion of the root system is achieved through the development of lateral roots and is to a large extent controlled by the plant hormone auxin. However, the pleiotropic effects of auxin or auxin-like molecules on root systems complicate the study of lateral root development. Here we describe a small-molecule screen in Arabidopsis thaliana that identified naxillin as what is to our knowledge the first non-auxin-like molecule that promotes root branching. By using naxillin as a chemical tool, we identified a new function for root cap-specific conversion of the auxin precursor indole-3-butyric acid into the active auxin indole-3-acetic acid and uncovered the involvement of the root cap in root branching. Delivery of an auxin precursor in peripheral tissues such as the root cap might represent an important mechanism shaping root architecture.

Peroxisomal Ubiquitin-Protein Ligases Peroxin2 and Peroxin10 Have Distinct But Synergistic Roles in Matrix Protein Import and Peroxin5 Retrotranslocation in Arabidopsis

“…Similarly, an auxin precursor, indole-3-butyric acid (IBA), is b-oxidized to active indole-3-acetic acid (IAA) in peroxisomes (Zolman et al, 2000(Zolman et al, , 2007(Zolman et al, , 2008Strader et al, 2010;. IBA application enhances rooting in many plants (Woodward and Bartel, 2005a), and IAA produced from endogenous IBA promotes hypocotyl elongation, cotyledon expansion, root hair elongation, and lateral root proliferation in Arabidopsis seedlings (Zolman et al, 2001;Strader et al, 2010De Rybel et al, 2012).…”

Plant developmental biologists meet on stairways in Matera

“…Furthermore, local conversion of the auxin precursor indole-3-butyric acid into the bioactive indole acetic acid was shown by Tom Beeckman and his group (VIB-University Gent, Belgium) to occur preferentially in the root cap. They demonstrated further that this root cap-derived auxin source contributes, at least in part, to the initiation of lateral root primordia and, thus, root architecture (De Rybel et al, 2012). From these studies, it clearly emerges that fine-tuning of the local hormonal environment by transcriptional networks is a crucial element in plant development that needs further exploration.…”

Plant Chemical Genetics: From Phenotype-Based Screens to Synthetic Biology

“…In cases in which more specificity is desired in the screen, very often changes were monitored in either a particular reporter line or a specific mutant phenotype (Hayashi et al, 2001;De Rybel et al, 2012;Meesters et al, 2014). Examples of the latter are the discoveries of the compounds hyperphyllin and bubblin (Poretska et al, 2016;Sakai et al, 2017).…”

“…A screen for small molecules promoting lateral root development based on the ability to activate the B1-type cyclin (CYCB1)-GUS reporter specifically in xylem pole pericycle cells revealed the nonauxin small molecule naxillin. Naxillin enables the conversion of the auxin precursor indole-3-butyric acid (IBA) into IAA in the root cap and promotes lateral root formation (De Rybel et al, 2012). Screening for small molecules that restore the elongated hypocotyl phenotype in light caused by the ectopic expression of CONSTITUTIVE PHOTOMORPHOGENIC1 in the presence of SLs yielded a group of small molecules, designated as REDUCED GERMINATION (RG) compounds (Holbrook-Smith et al, 2016).…”