New Paradigms in Brassinosteroids, Strigolactones, Sphingolipids, and Nitric Oxide Interaction in the Control of Lateral and Adventitious Root Formation

Abstract

The root system is formed by the primary root (PR), which forms lateral roots (LRs) and, in some cases, adventitious roots (ARs), which in turn may produce their own LRs. The formation of ARs is also essential for vegetative propagation in planta and in vitro and for breeding programs. Root formation and branching is coordinated by a complex developmental network, which maximizes the plant's ability to cope with abiotic stress. Rooting is also a response caused in a cutting by wounding and disconnection from the donor plant. Brassinosteroids (BRs) are steroid molecules perceived at the cell surface. They act as plant-growth-regulators (PGRs) and modulate plant development to provide stress tolerance. BRs and auxins control the formation of LRs and ARs. The auxin/BR interaction involves other PGRs and compounds, such as nitric oxide (NO), strigolactones (SLs), and sphingolipids (SPLs). The roles of these interactions in root formation and plasticity are still to be discovered. SLs are carotenoid derived PGRs. SLs enhance/reduce LR/AR formation depending on species and culture conditions. These PGRs possibly crosstalk with BRs. SPLs form domains with sterols within cellular membranes. Both SLs and SPLs participate in plant development and stress responses. SPLs are determinant for auxin cell-trafficking, which is essential for the formation of LRs/ARs in planta and in in vitro systems. Although little is known about the transport, trafficking, and signaling of SPLs, they seem to interact with BRs and SLs in regulating root-system growth. Here, we review the literature on BRs as modulators of LR and AR formation, as well as their crosstalk with SLs and SPLs through NO signaling. Knowledge on the control of rooting by these non-classical PGRs can help in improving crop productivity and enhancing AR-response from cuttings.

Keywords: 24-epibrassinolide; MAX2; PUCHI; auxin; nitric oxide signaling; post-embryonic root development; root formation in cuttings; stress response; synthetic strigolactones; very long chain fatty acids (VLCFAs).

Figure 3

Model explaining the hypothesis of a central role of nitric oxide (NO) in the modulation of promotive/antagonistic actions of brassinosteroids (BRs), strigolactones (SLs), and sphingolipids (SPLs) in the control of the auxin-induced lateral and adventitious (LR/AR) root formation. The double arrow between BRs and NO indicates how the two compounds mutually influence their levels and activity. Indole-3-acetic acid (IAA), and its natural precursor indole-3-butyric acid (IBA), cause LR and AR formation through the activity of the auxin influx (e.g., AUX1) and efflux (e.g., PIN1) cellular carriers. NO is a by-product of IBA to IAA conversion and positively modulates auxin levels at various steps, including auxin synthesis and transport by AUX1/PIN1 carriers (arrows in Figure). SLs induce NO, which in turn negatively affects their levels. Thus, NO modulates SLs through a negative interaction. GSNOR (S-nitrosoglutathione reductase) is needed to control NO levels to allow SL functioning in LR/AR formation [88]. Moreover, SLs directly inhibit auxin efflux by PIN, thus inhibiting LR/AR formation (arrow), with a role for MAX2/D3 (MORE AXILLARY GROWTH2) F-box type protein [107,112]. Even if not shown in the Figure, a positive relationship between NO and SLs has been also reported (see the text). SLs may also induce the production of VLCFAs (very long-chain fatty acids) components of sphingolipids (SPLs) through the activities of KCS (ketoacyl-CoA synthase) genes, as in other systems [149] (dashed arrow). The accumulation of SPLs, possibly those containing VLCFAs, might positively affect BR signaling (dashed arrow) through BAK1 (BRASSINOSTEROID INSENSITIVE 1 -ASSOCIATED RECEPTOR KINASE 1 [152], but this hypothesis needs to be confirmed. PUCHI, an auxin regulated AP2/ERF transcription factor, might target the spatial expression of SPL-VLCFA genes [144], which regulate the activity of AUX1/PIN1 auxin carriers [136,144]. In addition, SPLs might positively affect NO (dashed arrow), essential to LR/AR formation. (See the text for further explanations).

Figure 1

Histological images of adventitious root primordia (ARP, (A,C,E–G,I)) lateral root primordia (LRP, (B,D)) and apices of adventitious root (AR, (H)) formed following either the anticlinal proliferation of the hypocotyl pericycle (ARPs) or the anticlinal proliferation of the primary root pericycle (LRPs) in Arabidopsis thaliana seedlings (Col ecotype) cultured for nine days with continuous darkness followed by seven days under 16 h light/8 h darkness photoperiod, according to [27]. (A,B) regular ARP (A) and regular LRP (B) at stage VII of development, formed on the control medium, i.e., with no exogenous phytohormone (Control). The regular quiescent center (QC) presence is shown by the asterisks. (C,D), ARP (C) and LRP (D) at stage VII or soon after, showing irregular apical structure under the treatment with 60 µM of CdSO₄ (Cd). (E,G) ARPs at different developmental stages showing regular QC definition at stage VII (F). Treatments with either 1 nM of 24-epibrassinolide (eBL) (E) or 10 nM eBL (F,G). Arrows in (G) show ARPs located very near each other. The image is representative of the high density of roots obtained with 10 nM eBL [27]. (H) AR apex with regular QC (showed by asterisks) under 10 nM eBL. (I) young regular ARP (stage of dome definition, [8]) formed in the combined presence of Cd and 10 nM eBL. Longitudinal radial sections stained with toluidine blue and presented in black and white. Data from Della Rovere et al. [27]. Bars = 10 µm.

Figure 2

Adventitious root primordia (ARPs) (A,C), lateral root primordia (LRPs) (B,D), and apices of adventitious roots (ARs) (E) of Arabidopsis thaliana seedlings grown for 16 days in the absence (Control, (A,B)) or in the presence of 10 nM 24-epibrassinolide (eBL) (C–E) and then treated with the NO-specific probe 4-Amino-5-Methylamino-2’,7’-Difluorofluorescein Diacetate (DAF-FM DA). The fluorescence signal was visualized using a DMRB microscope (Leica, Wetzlar, Germany). Arrows show the inception of the dome of an ARP (A) and of an LRP (B). Bars: (A,C–E) = 40 µm; (B) = 20 µm. Data from Della Rovere et al. [27].