The plant-specific LBD (Lateral Organ Boundaries Domain) gene family is essential in the regulation of plant lateral organ development and are involved in almost all aspects of plant development, including embryo, root, leaf, and inflorescence development, as the regulation of anthocyanin and nitrogen metabolism. These genes were found originally to participate in the definition of the boundary that separates lateral organs from adjacent tissue but others have roles in stress responses (Zhang et al., 2020).  There are about 135 LBD family members in the maize genome.

LBD proteins contain a characteristic LOB domain (PF00046) composed of a C-motif required for DNA-binding which contains four perfectly conserved cysteine (C) residues in a CX2CX6CX3C motif where X are residues that are not conserved, a conserved glycine residue, and a leucine-zipper-like sequence that includes five hydrophobic amino acids (valine, isoleucine, leucine) separated by six variable amino acid residues as a probable protein dimerization domain required for PPIs (protein protein interactions).

Phylogenetic reconstructions of the Arabidopsis, rice and maize LBD genes suggest two major classes of LBD genes which are characterized by the presence (class I) or absence (class II) of functional leucine-zipper-like domains. The majority of LBD genes belong to class I. LOB domain proteins are suggested to act as transcription factors based on their nuclear localization and their capacity to bind to the DNA motif 5' GCGGCG 3' (PMID:20961800).

The ramosa2 (ra2) mutant of maize has increased branching with short branches replaced by long, indeterminate ones. ra2 encodes a LATERAL ORGAN BOUNDARY domain transcription factor and is transiently expressed in a group of cells that predicts the position of axillary meristem formation in inflorescences. The early expression of ra2 suggests that it functions in the patterning of stem cells in axillary meristems. Alignment of ra2-like sequences reveals a conserved grass-specific domain in the C terminus that is not found in Arabidopsis thaliana (Bortiri et al., 2006, Gallavotti et al., 2010).

The ZmLBD2 protein exhibits localization to the cell nucleus and can specifically bind with inverted repeats of "GCGGCG". Heterologous overexpression of ZmLBD2 in Arabidopsis thaliana under drought stress resulted in better performance than the wild-type plants in terms of seed germination rates, root length, relative water content, fresh weight, chlorophyll content, proline content, and antioxidant enzyme content. Arabidopsis overexpressing ZmLBD2 contained less MDA, H2O2, and O2- than wild-type plants. This study suggested a role for ZmLBD2 in drought tolerance (Jiao et al., 2022)

The paralogous maize (Zea mays) LBD (Lateral Organ Boundaries Domain) genes rtcs (rootless concerning crown and seminal roots) and rtcl (rtcs-like) emerged from an ancient whole-genome duplication. RTCS is a key regulator of crown root initiation. The rtcs and rtcl genes display highly correlated spatio-temporal expression patterns in roots, despite the significantly higher expression of rtcs. Both RTCS and RTCL proteins bind to LBD downstream promoters and act as transcription factors. Although RTCS is instrumental in shoot-borne root initiation, RTCL controls shoot-borne root elongation early in development (Taramino et al., 2007, Xu et al., 2015).

Last updated June 2023 by John Gray

References:

Jiao P, Wei X, Jiang Z, Liu S, Guan S, Ma Y. ZmLBD2 a maize (Zea mays L.) lateral organ boundaries domain (LBD) transcription factor enhances drought tolerance in transgenic Arabidopsis thaliana. Front Plant Sci. 2022 Oct 13;13:1000149. doi: 10.3389/fpls.2022.1000149. PMID: 36311096; PMCID: PMC9612921.

Xu C, Tai H, Saleem M, Ludwig Y, Majer C, Berendzen KW, Nagel KA, Wojciechowski T, Meeley RB, Taramino G, Hochholdinger F. Cooperative action of the paralogous maize lateral organ boundaries (LOB) domain proteins RTCS and RTCL in shoot-borne root formation. New Phytol. 2015 Sep;207(4):1123-33. doi: 10.1111/nph.13420. Epub 2015 Apr 22. PMID: 25902765.

Taramino G, Sauer M, Stauffer JL Jr, Multani D, Niu X, Sakai H, Hochholdinger F. The maize (Zea mays L.) RTCS gene encodes a LOB domain protein that is a key regulator of embryonic seminal and post-embryonic shoot-borne root initiation. Plant J. 2007 May;50(4):649-59. doi: 10.1111/j.1365-313X.2007.03075.x. Epub 2007 Apr 8. PMID: 17425722.

Bortiri E, Chuck G, Vollbrecht E, Rocheford T, Martienssen R, Hake S. ramosa2 encodes a LATERAL ORGAN BOUNDARY domain protein that determines the fate of stem cells in branch meristems of maize. Plant Cell. 2006 Mar;18(3):574-85. doi: 10.1105/tpc.105.039032. Epub 2006 Jan 6. PMID: 16399802; PMCID: PMC1383634.

Gallavotti A, Long JA, Stanfield S, Yang X, Jackson D, Vollbrecht E, Schmidt RJ. The control of axillary meristem fate in the maize ramosa pathway. Development. 2010 Sep 1;137(17):2849-56. doi: 10.1242/dev.051748. PMID: 20699296; PMCID: PMC2938917.

Evans MM. The indeterminate gametophyte1 gene of maize encodes a LOB domain protein required for embryo Sac and leaf development. Plant Cell. 2007 Jan;19(1):46-62. doi: 10.1105/tpc.106.047506. Epub 2007 Jan 5. PMID: 17209126; PMCID: PMC1820972.

Zhang Y, Li Z, Ma B, Hou Q, Wan X. Phylogeny and Functions of LOB Domain Proteins in Plants. Int J Mol Sci. 2020 Mar 26;21(7):2278. doi: 10.3390/ijms21072278. PMID: 32224847; PMCID: PMC7178066.