In plants, the developmental mechanisms that regulate the positioning of lateral organs along the primary root are currently unknown. We present evidence on how lateral root initiation is controlled in a spatiotemporal manner in the model plant Arabidopsis thaliana. First, lateral roots are spaced along the main axis in a regular left-right alternating pattern that correlates with gravity-induced waving and depends on AUX1, an auxin influx carrier essential for gravitropic response. Second, we found evidence that the priming of pericycle cells for lateral root initiation might take place in the basal meristem, correlating with elevated auxin sensitivity in this part of the root. This local auxin responsiveness oscillates with peaks of expression at regular intervals of 15 hours. Each peak in the auxin-reporter maximum correlates with the formation of a consecutive lateral root. Third, auxin signaling in the basal meristem triggers pericycle cells for lateral root initiation prior to the action of INDOLE-3-ACETIC ACID14(SOLITARY ROOT).

Roots are important to plants for a wide variety of processes, including nutrient and water uptake, anchoring and mechanical support, storage functions, and as the major interface between the plant and various biotic and abiotic factors in the soil environment. Understanding the development and architecture of roots holds potential for the exploitation and manipulation of root characteristics to both increase food plant yield and optimize agricultural land use. This theme issue highlights the importance of investigating specific aspects of root architecture in both the model plant Arabidopsis thaliana and (cereal) crops, presents novel insights into elements that are currently hardly addressed and provides new tools and technologies to study various aspects of root system architecture. This introduction gives a broad overview of the importance of the root system and provides a snapshot of the molecular control mechanisms associated with root branching and responses to the environment in A. thaliana and cereal crops.

Summary: Lateral root (LR) formation displays considerable plasticity in response to developmental and environmental signals. The mechanism whereby plants incorporate diverse regulatory signals into the developmental programme of LRs remains to be elucidated. Current concepts of lateral root regulation focus on the role of auxin. In this study, we show that another plant hormone, abscisic acid (ABA), also plays a critical role in the regulation of this post‐embryonic developmental event. In the presence of exogenous ABA, LR development is inhibited. This occurs at a specific developmental stage, i.e. immediately after the emergence of the LR primordium (LRP) from the parent root and prior to the activation of the LR meristem, and is reversible. Interestingly, this inhibition requires 10‐fold less ABA than the inhibition of seed germination and is only slightly reduced in characterised abi mutants, suggesting that it may involve novel ABA signalling mechanisms. We also present several lines of evidence to support the conclusion that the ABA‐induced lateral root inhibition is mediated by an auxin‐independent pathway. First, the inhibition could not be rescued by either exogenous auxin application or elevated auxin synthesis. Secondly, a mutation in the ALF3 gene, which is believed to encode an important component in the auxin‐dependent regulatory pathway for the post‐emergence LR development, does not affect the sensitivity of LRs to ABA. Thirdly, ABA and the alf3‐1 mutation do not act at the same developmental point. To summarise, these results demonstrate a novel ABA‐sensitive, auxin‐independent checkpoint for lateral root development in Arabidopsis at the post‐emergence stage. In addition, we also present data indicating that regulation of this developmental checkpoint may require novel ABA signalling mechanisms and that ABA suppresses auxin response in the LRPs.