Phloem-associated auxin response maxima determine radial positioning of lateral roots in maize

Abstract

In Arabidopsis thaliana, lateral-root-forming competence of pericycle cells is associated with their position at the xylem poles and depends on the establishment of protoxylem-localized auxin response maxima. In maize, our histological analyses revealed an interruption of the pericycle at the xylem poles, and confirmed the earlier reported proto-phloem-specific lateral root initiation. Phloempole pericycle cells were larger and had thinner cell walls compared with the other pericycle cells, highlighting the heterogeneous character of the maize root pericycle. A maize DR5::RFP marker line demonstrated the presence of auxin response maxima in differentiating xylem cells at the root tip and in cells surrounding the proto-phloem vessels. Chemical inhibition of auxin transport indicated that the establishment of the phloem-localized auxin response maxima is crucial for lateral root formation in maize, because in their absence, random divisions of pericycle and endodermis cells occurred, not resulting in organogenesis. These data hint at an evolutionarily conserved mechanism, in which the establishment of vascular auxin response maxima is required to trigger cells in the flanking outer tissue layer for lateral root initiation. It further indicates that lateral root initiation is not dependent on cellular specification or differentiation of the type of vascular tissue.

To branch or not to branch: the role of pre-patterning in lateral root formation

“…Root pericycle cells at the poles of either xylem or phloem are further delineated in that they have distinct cellular morphology and transcriptional profiles and are the cells of origin for LRP (Brady et al, 2007;Jansen et al, 2012;Laskowski et al, 1995;Malamy and Benfey, 1997). In the Arabidopsis shoot, the XPP-like callus-forming cells are similar to root XPP cells in that they share marker gene expression and are associated with the vasculature, although presently this shoot tissue has not been specifically defined as XPP (Sugimoto et al, 2010).…”

Comparative Transcriptome Profiling of Maize Coleoptilar Nodes during Shoot-Borne Root Initiation

“…Auxin maxima are required for root primordia formation and outgrowth of root tips (Péret et al, 2009;Jansen et al, 2012). The rtcs gene is an auxin-inducible early auxin-responsive gene that controls seminal and shoot-borne root formation (Taramino et al, 2007).…”

Repression of early lateral root initiation events by transient water deficit in barley and maize

“…The LRIS appears to be the only system that has been validated in maize [12]. Owing to the small LR spacing in maize and other cereals, the gravitropic stimulus-based system may be inappropriate as several LRs are likely to appear in sequence in the outer side of the bend.…”

“…For the same reason, it is virtually impossible to sample for negative control root segments where LRs are absent. Treating roots with N-1-naphthylphthalamic acid prevents LR initiation on the whole root [12], but this is likely to affect many other developmental processes as well. Given the complex nature of the regulatory processes of LR formation, which involves an interplay of intrinsic and response pathways [13], experimental systems manipulating LR formation in different ways are certainly needed to reach a more integrated understanding of the molecular mechanisms that regulate the sites of LRs.…”