Table 1.
Hormones and their involvement in root growth and development.
| Hormone | Chemical compounds | Function | Hormone source | Species | Reference |
|---|---|---|---|---|---|
| Auxin | IAA | Promotes lateral root initiation by specifying lateral root founder cells | Endogenous, root tip | At | Casimiro et al. (2001), De Smet et al. (2007), Dubrovsky et al. (2008) |
| IAA | Promotes lateral root emergence | Endogenous, shoot | At | Bhalerao et al. (2002) | |
| NAA | Increases lateral root primordia initiation and outgrowth | Exogenous | Os, Nt | Campanoni and Nick (2005), Sreevidya et al. (2010) | |
| 2,4-D | Increases lateral root primordia initiation through cell division (but does not promote cell elongation and root outgrowth) | Exogenous | Os, Nt | Campanoni and Nick (2005), Sreevidya et al. (2010) | |
| IAA | Promotes primary root elongation by facilitating the response of root cells to GA3 | Exogenous | At | Fu and Harberd (2003) | |
| Cytokinins | Kinetin, BAP | Inhibits lateral root primordia formation by perturbing PIN gene expression and disrupting formation of a RAM auxin gradient controlling cell division to maintain the QC and neighboring initials | Increased endogenous | At | Laplaze et al. (2007), Dello Ioio et al. (2008); reviewed in Péret et al. (2009b) |
| Kinetin, trans-zeatin | Stimulates lateral root elongation | Exogenous | Os | Rani Debi et al. (2005), Laplaze et al. (2007), Dello Ioio et al. (2008), reviewed in Bishopp et al. (2009) | |
| Kinetin, trans-zeatin | Stimulates crown root primordia formation | Exogenous | Os | Rani Debi et al. (2005), Hirose et al. (2007), Zhao et al. (2009) | |
| Zeatins, other endogenous cytokinins | Inhibits primary root elongation by reducing cell division in RAM, thus regulating RAM size | Increased endogenous | At | Kuderová et al. (2008), Ruzicka et al. (2009) | |
| Gibberellins | GA3 | Interacts with ethylene to promote crown root primordia outgrowth and elongation | Exogenous | Os | Steffens et al. (2006) |
| GA3 | Promotes primary root elongation in the presence of auxin by repressing growth-repressing DELLA proteins | Decreased endogenous and increased exogenous | At | Fu and Harberd (2003) | |
| GA3 | Inhibits lateral root primordia initiation | Exogenous | Pt | Gou et al. (2010) | |
| Ethylene | Ethylene | Promotes crown root formation at submerged nodes | Internode | Os | Lorbiecke and Sauter (1999) |
| Ethylene | Promotes crown root emergence at submerged nodes through induction of epidermal cell death over sites of lateral root primordia formation | Internode | Os | Mergemann and Sauter (2000) | |
| Ethylene | |||||
| Jasmonates | MeJA | Promotes lateral root formation through interaction with auxin pathway | Increased endogenous | At, Gm | Xue and Zhang (2007), Sun et al. (2009) |
| MeJA | Inhibits primary root growth | Increased endogenous | Gm | Xue and Zhang (2007) | |
| Abscisic acid | ABA | Induces lateral root primordia formation under non-stress conditions by modulating the auxin response | Endogenous | At | Brady et al. (2003) |
| ABA | Maintains primary root elongation under drought stress | Endogenous | Zm | Saab et al. (1990) | |
| ABA | Inhibits lateral root outgrowth prior to lateral root meristem formation under non-stress conditions | Exogenous | At | De Smet et al. (2006) | |
| Brassinosteroids | BL | May induce lateral root initiation in the presence of auxin, through modulating auxin signaling | Exogenous | At | Bao et al. (2004) |
| BL | Induces primary root elongation in the presence of exogenous auxin (IAA) by affecting ethylene biosynthesis and the gravitropic response | Exogenous | Zm, At | Chang et al. (2004), Kim et al. (2007), Yun et al. (2009) | |
| HBR | Induces primary and crown root elongation possibly through modulating auxin signaling | Exogenous | Hv | Kartal et al. (2009) | |
| Strigolactone | GR24 (synthetic strigolactone analog) | May either inhibit primary root elongation in low concentrations, or stimulate primary root growth in high concentrations, in the presence of auxin, by putative regulation of auxin efflux carriers | Exogenous | At | Koltai et al. (2010), Kapulnik et al. (2011), Ruyter-Spira et al. (2011) |
| GR24 (synthetic strigolactone analog) | Induces primary root curving in high concentrations, in the presence of no-low auxin by inducing asymmetric cell elongation | Exogenous | At | Koltai et al. (2010) | |
| SLs | Promote crown root elongation by inducing meristematic cell division, possibly through the modulation of local auxin concentrations that regulate meristem cell number | Endogenous | Os | Arite et al. (2012) | |
| SLs | Putatively modulates auxin sensitivity by downregulating auxin efflux carrier expression to inhibit lateral root formation under low auxin levels by reducing auxin accumulation in roots, or inducing lateral root formation under high auxin concentrations by allowing optimal auxin levels to be met | Endogenous | At | Kapulnik et al. (2011), Ruyter-Spira et al. (2011) |
2,4-D, 2,4-dichlorophenoxyacetic acid; ABA, abscisic acid; At, Arabidopsis thaliana; BAP, 6-benzylaminopurine; BL, brassinolide; GA3, gibberellic acid; Gm, Glycine max; HBL, homobrassinolide; Hv, Hordeum vulgare; IAA, indole-3-acetic acid; MeJA, methyl jasmonate; NAA, 1-naphthaleneacetic acid; Nt, Nicotiana tabacum; Os, Oryza sativa; Pt, Populus tremulus; QC, quiescent center; RAM, root apical meristem; SLs, endogenous strigolactones; Zm, Zea mays.