Lateral Organ Boundaries Domain Genes Research Articles

LATERAL ORGAN BOUNDARIES defines a new family of DNA-binding transcription factors and can interact with specific bHLH proteins

Conserved in a variety of evolutionarily divergent plant species, LOB DOMAIN (LBD) genes define a large, plant-specific family of largely unknown function. LBD genes have been implicated in a variety of developmental processes in plants, although to date, relatively few members have been assigned functions. LBD proteins have previously been predicted to be transcription factors, however supporting evidence has only been circumstantial. To address the biochemical function of LBD proteins, we identified a 6-bp consensus motif recognized by a wide cross-section of LBD proteins, and showed that LATERAL ORGAN BOUNDARIES (LOB), the founding member of the family, is a transcriptional activator in yeast. Thus, the LBD genes encode a novel class of DNA-binding transcription factors. Post-translational regulation of transcription factors is often crucial for control of gene expression. In our study, we demonstrate that members of the basic helix–loop–helix (bHLH) family of transcription factors are capable of interacting with LOB. The expression patterns of bHLH048 and LOB overlap at lateral organ boundaries. Interestingly, the interaction of bHLH048 with LOB results in reduced affinity of LOB for the consensus DNA motif. Thus, our studies suggest that bHLH048 post-translationally regulates the function of LOB at lateral organ boundaries.

Differential gene expression in Arabidopsis following infection by plant-parasitic nematodes Meloidogyne incognita and Heterodera schachtii.

SUMMARY Whole genome microarrays were used to study plant gene expression in mature Meloidogyne incognita-induced galls in Arabidopsis. We found 959 genes to be significantly differentially expressed, and two-thirds of these were down-regulated. Microarray results were confirmed by qRT-PCR. The temporal and spatial responses of four differentially expressed genes were analysed using GUS reporter plants following infection with M. incognita and the cyst nematode Heterodera schachtii. The ammonium transporter gene AtAMT1;2 was consistently and locally repressed in response to both nematodes at all developmental stages. The lateral organ boundary domain gene LBD41 showed up-regulation in the feeding sites of both nematode species, although there was variation in expression in saccate H. schachtii female feeding sites. Expression of an actin depolymerizing factor ADF3 and a lipid transfer protein was induced in feeding sites of both nematodes at the fusiform stage and this persisted in feeding sites of saccate M. incognita. These results contribute to the knowledge of how plant gene expression responds to parasitism by these nematodes as well as highlighting further differences in the mechanisms of development and maintenance of these feeding site structures.

Arabidopsis JAGGED LATERAL ORGANSIs Expressed in Boundaries and CoordinatesKNOXandPINActivity

Plant lateral organs are initiated as small protrusions on the flanks of shoot apical meristems. Organ primordia are separated from the remainder of the meristem by distinct cell types that create a morphological boundary. The Arabidopsis thaliana gain-of-function mutant jagged lateral organs-D (jlo-D) develops strongly lobed leaves, indicative of KNOX gene misexpression, and the shoot apical meristem arrests organ initiation prematurely, terminating in a pin-like structure. The JLO gene, a member of the LATERAL ORGAN BOUNDARY DOMAIN gene family, is expressed in boundaries between meristems and organ primordia and during embryogenesis. Inducible JLO misexpression activates expression of the KNOX genes SHOOT MERISTEMLESS and KNAT1 in leaves and downregulates the expression of PIN auxin export facilitators. Consequently, bulk auxin transport through the inflorescence stem is drastically reduced. During embryogenesis, JLO is required for the initiation of cotyledons and development beyond the globular stage. Converting JLO into a transcriptional repressor causes organ fusions, showing that during postembryonic development, JLO function is required to maintain the integrity of boundaries between cell groups with indeterminate or determinate fates.

Comparison and evolution analysis of two rice subspecies LATERAL ORGAN BOUNDARIES domain gene family and their evolutionary characterization from Arabidopsis

The plant specific LATERAL ORGAN BOUNDARIES (LOB) domain (LBD) gene family has a potential role in lateral organ development. Thirty-five LBD genes in a japonica rice (Nipponbare) (designated OsJLBD) and in an indica rice (9311) (designated OsILBD) were identified based on the current databases of the two rice subspecies. A new rice LBD gene with two LOB domains and two predicted coiled coil structures in both subspecies was found, which is not found in other plant species based on the current NCBI Genbank database. OsJLBD and OsILBD genes have similar chromosomal distribution pattern. Both OsJLBD and OsILBD genes can be divided into 7 subclasses (classes Ia–e, II and III (see Table 1)) and no subclass-specific expression pattern was observed. No introns have been predicted in all class Ie genes in both OsJLBD and OsILBD subfamilies. The genome and tandem duplication has contributed to the neofunctionalization and formation of new rice subclasses, but the mechanism of diploidization and limited tandem duplication have contributed to fewer LBD genes in rice than in Arabidopsis. Functional studies of genes in subclasses may help to determine whether special sequence structure (intron–exon, spacing characters of motifs) has caused special expression pattern of subclasses.

ASYMMETRIC LEAVES2-LIKE1gene, a member of the AS2/LOB family, controls proximal?distal patterning in Arabidopsis petals

The formation and the development of the floral organs require an intercalate expression of organ-specific genes. At the same time, meristem-specific genes are repressed to complete the differentiation of the organs in the floral whorls. In an Arabidopsis activation tagging population, a mutant affected in inflorescence architecture was identified. This gain-of-function mutant, designated downwards siliques1 (dsl1-D), has shorter internodes and the lateral organs such as flowers are bending downwards, similar to the loss-of-function brevipedicellus (bp) mutant. The affected gene in dsl1-D appeared to be ASYMMETRIC LEAVES2-LIKE1 (ASL1)/LATERAL ORGAN BOUNDARIES domain gene 36 (LBD36), which is a member of the ASYMMETRIC LEAVES2 (AS2)/LATERAL ORGAN BOUNDARIES (LOB) domain gene family. Analysis of the loss-of-function mutant asl1/lbd36 did not show morphological aberration. Double mutant analysis of asl1/lbd36 together with as2, the ASL1/LBD36 closest homologue, demonstrates that these two members of the AS2/LOB family act partially redundant to control cell fate determination in Arabidopsis petals. Moreover, molecular analysis revealed that overexpression of ASL1/LBD36 leads to repression of the homeobox gene BP, which supports the model that an antagonistic relationship between ASL/LBD and homeobox members is required for the differentiation of lateral organs.