LBD13 positively regulates lateral root formation in Arabidopsis.

MAIN CONCLUSION: Lateral Organ Boundaries Domain 13 (LBD13), which is expressed in emerged lateral roots and encodes a transcriptional activator, plays an important role in lateral root formation in Arabidopsis. Lateral roots (LRs) are major determinants of root system architecture, contributing to the survival strategies of plants. Members of the LBD gene family encode plant-specific transcription factors that play key roles in plant organ development. Several LBD genes, such as LBD14, 16, 18, 29, and 33, have been shown to play important roles in regulating LR development in Arabidopsis. In the present study, we show that LBD13 is expressed in emerged LRs and LR meristems of elongated LRs and regulates LR formation in Arabidopsis. Transient gene expression assays with Arabidopsis protoplasts showed that LBD13 is localized to the nucleus and harbors transcription-activating potential. Knock-down of LBD13 expression by RNA interference resulted in reduced LR formation, whereas overexpression of LBD13 enhanced LR formation in transgenic Arabidopsis. Analysis of ß-glucuronidase (GUS) expression under the control of the LBD13 promoter showed that GUS staining was detected in LRs emerged from the primary root, but not in LR primordia. Moreover, both the distribution of LR primordium number and developmental kinetics of LR primordia were not affected either by knock-down or by overexpression of LBD13. Taken together, these results suggest that LBD13 is a nuclear-localized transcriptional activator and controls LR formation during or after LR emergence.

LBD14/ASL17 Positively Regulates Lateral Root Formation and is Involved in ABA Response for Root Architecture in Arabidopsis.

The LATERAL ORGAN BOUNDARIES (LOB) DOMAIN/ASYMMETRIC LEAVES2-LIKE (LBD/ASL) gene family members play key roles in diverse aspects of plant development. Previous studies have shown that LBD16, 18, 29 and 33 are critical for integrating the plant hormone auxin to control lateral root development in Arabidopsis thaliana. In the present study, we show that LBD14 is expressed exclusively in the root where it promotes lateral root (LR) emergence. Repression of LBD14 expression by ABA correlates with the inhibitory effects of ABA on LR emergence. Transient gene expression assays with Arabidopsis protoplasts demonstrated that LBD14 is a nuclear-localized transcriptional activator. The knock-down of LBD14 expression by RNA interference (RNAi) resulted in reduced LR formation by delaying both LR primordium development and LR emergence, whereas overexpression of LBD14 in Arabidopsis enhances LR formation. We show that ABA (but not other plant hormones such as auxin, brassinosteroids and cytokinin) specifically down-regulated ß-glucuronidase (GUS) expression under the control of the LBD14 promoter in transgenic Arabidopsis during LR development from initiation to emergence and endogenous LBD14 transcript levels in the root. Moreover, RNAi of LBD14 enhanced the LR suppression in response to ABA, whereas LBD14 overexpression did not alter the ABA-mediated suppression of LR formation. Taken together, these results suggest that LBD14 promoting LR formation is one of the critical factors regulated by ABA to inhibit LR growth, contributing to the regulation of the Arabidopsis root system architecture in response to ABA.

Role of LBD14 during ABA-mediated control of root system architecture in Arabidopsis.

The LATERAL ORGAN BOUNDARIES DOMAIN/ASYMMETRIC LEAVES2-LIKE (LBD/ASL) gene family encode plant-specific transcription factors that regulate various aspects of plant growth and development. Arabidopsis genome has 42 LBD genes. Several LBD genes, such as LBD16, -18, -29, and -33, have been shown to function in lateral root (LR) development via auxin signaling. Although abscisic acid (ABA) is a well-known antistress plant hormone regulating various plant developmental processes, it also plays a role in LR growth regulation. Our recent study showed that LBD14 expression is downregulated by ABA during the entire steps of LR development. The RNAi-induced downregulation and overexpression of LBD14 indicated that LBD14 promotes LR formation. LBD14RNAi enhanced the ABA-induced suppression of LR density compared with the wild type, suggesting that LBD14 is involved in the ABA-mediated control of LR formation. Our study provides an insight into the signaling mechanism of developmental plasticity whereby ABA controls LR branching via LBD14 downregulation under abiotic stress conditions.