4.1. Hormones in primary, lateral and crown roots development
Auxin is the leading actor of root development. Whatever the root’s type is considered, all always start in the cells that will initiate a root by establishing an auxin maximum via auxin efflux transport PIN FORMED (PIN) proteins, stabilized by the CROWN ROOTLESS4/OsGNOM1. Auxin is perceived by the auxin receptor TRANSPORT INHIBITOR RESPONSE 1(TIR1)/AUXIN SIGNALING F-BOX (AFB). The transduction of the signal induces the ubiquitination of the AUXIN (Aux)/INDOLE-3-ACETIC ACID (IAA) proteins by the SCFTIR1/AFB complex and degradation by the 26S proteasome, releasing AUXIN RESPONSE FACTOR (ARF) proteins. ARF act as repressors or activators of transcription. The signal transduction downstream of Aux/IAA and ARF proteins activates or represses many auxin-induced responses, including cell wall loosening, cell elongation, cell cycle, hormone homeostasis and root meristem patterning. As the ARFs and the AUX/IAA are members of multi-genes families, the transcriptional effects of auxin depend on its concentration and the combinatorial expression of AUX/IAA and ARFs. The first committed step in crown root initiation is controlled by the CROWN ROOTLESS1 (CRL1)/ADVENTITIOUS ROOTLESS1 (ARL1) and CROWN ROOTLESS5 (CRL5), a plant-specific LATERAL BOUNDARIES DOMAIN (LBD) protein and an APETALA2(AP2)/ETHYLENE RESPONSIVE FACTOR (ERF), respectively. In rice, disrupting the TRYPTOPHAN AMINOTRANSFERASE 1 (TAA1 ), which functions upstream of YUCCA genes in the auxin biosynthesis, reduces crown root development. The regulatory role of auxin in crown root initiation has been further emphasized in studies ofarl1 and crl1 mutant plants. The arl1/crl1 mutants, devoid of crown roots, carry one primary root and fewer lateral roots and show an abnormal gravitropism response at the root level.
Cytokinins are antagonists to auxin. Whereas auxin stimulates root initiation but inhibits elongation, CKs inhibit initiation but support elongation. In cereals, the silencing of gene encoding proteins involved in the CK metabolism affects root development. In rice, the hk5 hk6 double mutant with a defect in CK signalling presents a severely reduced root growth and an enlarged root cap. It is interesting to note that the crown rootless phenotype of taa1 mutants was partially rescued by the overexpression of the WUSCHEL-related Homeobox (WOX) 11 transcription factor. OsWOX11 is part of the CK signalling pathway and directly represses OsRR2, a type-A cytokinin-responsive gene. Interestingly, OsWOX11 is induced by both auxin and CK, affecting both auxin and cytokinin-responsive gene expression.
In flooding conditions, ethylene is produced and promotes crown root emergence at submerged nodes through induction of epidermal cell death. Ethylene stimulates auxin biosynthesis and basipetal auxin transport toward the elongation zone, where it activates a local auxin response leading to inhibition of cell elongation. In rice, the crown root primordia development requires the transcription factors CRL5 andCRL1 , both being ethylene- and an auxin-responsive gene. In rice, the 1-aminocyclopropane-1-carboxylic acid synthase (ACS), the rate-limiting enzyme in ethylene biosynthesis, is involved in the control of RSA. A stimulatory role for ethylene in lateral root development under Pi-deficient conditions has been suggested. IndeedOsacs mutants produce shorter roots and almost fail to promote lateral root growth in response to Pi deficiency.
In maize, ABA altered the polar localization of ZmPIN1 , disrupted the distribution of auxin and inhibited lateral root initiation and development. In general, ABA has an antagonistic effect on lateral root primordial formation and emergence of auxin, which initiates lateral roots, finally affecting the size and architecture of the root system.
The role of strigolactones in root development has been demonstrated in several plant species. They promote crown root elongation and repress the formation of lateral roots. Rice dwarf mutants, impaired in strigolactone biosynthesis or signalling, showed shorter crown roots than the wild type. Exogenous application of GR24, a synthetic analogue of strigolactones, rescued the phenotype in biosynthetic mutants in a concentration-dependent fashion. On the other hand, signalling mutants were insensitive to it, proving that strigolactones – or their derivatives – have a direct function in crown root elongation. Lateral roots of dwarf mutants did not present a higher density than the wild-type one, contrary to what was observed in Arabidopsis strigolactone mutants. In both cases, though, treatment with exogenous GR24 decreased the density in wild-type and SL-deficient plants but not in strigolactone-insensitive mutants. The interaction between strigolactones and auxin seems to be crucial for lateral root development. Strigolactones probably inhibit lateral root formation by reducing PIN protein levels.