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.