Pivotal role of LBD16 in root and root-like organ initiation

Abstract

In the post-embryonic stage of Arabidopsis thaliana, roots can be initiated from the vascular region of the existing roots or non-root organs; they are designated as lateral roots (LRs) and adventitious roots (ARs), respectively. Some root-like organs can also be initiated from the vasculature. In tissue culture, auxin-induced callus, which is a group of pluripotent root-primordium-like cells, is formed via the rooting pathway. The formation of feeding structures from the vasculature induced by root-knot nematodes also borrows the rooting pathway. In this review, we summarize and discuss recent progress on the role of LATERAL ORGAN BOUNDARIES DOMAIN16 (LBD16; also known as ASYMMETRIC LEAVES2-LIKE18, ASL18), a member of the LBD/ASL gene family encoding plant-specific transcription factors, in roots and root-like organ initiation. Different root and root-like organ initiation processes have distinct priming mechanisms to specify founder cells. All these priming mechanisms converge to activate LBD16 expression in the primed founder cells. The activation of LBD16 expression leads to organ initiation via promotion of cell division and establishment of root-primordium identity. Therefore, LBD16 might play a common and pivotal role in root and root-like organ initiation.

Keywords: Adventitious root; Callus; LBD16; Lateral root; Root founder cell; Root-knot nematodes.

Fig. 1

Phylogeny of class-IB LBD genes in land plants. Phylogenetic analysis of protein sequences of class-IB LBDs from a bryophyte Physcomitrella patens (Pp, in blue), a lycophytes Selaginella kraussiana (Sk, in green), a dicot Arabidopsis thaliana (At, in red), and monocot Oryza sativa (Os, in black) and Zea mays (Zm, in black). Sequence of SkLBDA was obtained by analyzing RNA-seq and DNA-seq data published previously [91] (GenBank, MH107252). Other sequences were obtained from the published data [53]. Phylogenetic tree of full length of the LBD proteins was constructed using the maximum-likelihood method (the Poisson model) with MEGA7.0 and default parameters [92]

Fig. 2

Model of LBD16 in root and root-like organ formation. Model summarizing the common role of A. thaliana LBD16 in LR formation from primary root (a), AR formation from detached leaf explants (b), callus formation from detached leaf explant (c), and feeding structure formation from detached leaf explant (d). Cells and genes that are involved in the priming step are in blue, and those in the initiation step are in red