Agriculture Reference
In-Depth Information
occurs only after the differentiation of a critical number of phytomers, which is
relatively low in the case of annual plants but much higher in woody species, where
juvenile phase may last for several years. The juvenile phase length is a polygenic
trait controlled by several QTLs, which appear unstable throughout development.
Pioneer work in
Arabidopsis
demonstrated that the length of the juvenile phase
is controlled by miRNA 156 and 172 targeting the transcription factors SPL and
AP2-like, respectively (Bäurle and Dean
2006
; Wu et al.
2009
). These genes could
be used to overcome juvenility. Recently, through the over-expression of the
FT
gene controlling phase transition, it was possible to shorten dramatically the juve-
nile phase in apple seedling (Tränkner et al.
2010
).
It is clear that the presence of a long juvenile stage is a big constraint in breed-
ing programs focused to improve fruit quality traits, since the progeny needs to be
maintained up to the attainment of the physiological maturity. The cost reduction of
such programs may be pursued by accelerating the seedling development by keep-
ing them under continuous growth conditions. By means of this technique, it was
possible to attain apple seedling maturity in 20-30 months. An additional system
for reducing the costs would rely on the isolation and characterization of gene sets
related to the traits of interest. These genes could be used as genetic markers in
Marker Assisted Selection (MAS) programs allowing the evaluation of the progeny
already at seed or early seedling level.
Maturity
The gaining of the physiological maturity, related to the accumulation of a critical
ontogenetic experience, allows the shoot meristem to become responsive to ex-
ternal and internal factors that may induce its transition from the vegetative to the
reproductive phase (Poethig
2003
). In case of monopodial branching, the apical
meristem is the earliest to reach maturity. Within the canopy, maturation is estab-
lished according to a hierarchy reflecting the age of the meristem and moving from
external to inner shoots, according to a basipetal gradient. The attainment of the
physiological maturity appears to be an irreversible process, maintained throughout
development even though the meristem is grafted on a different rootstock. This
behaviour is exploited in clone propagation through the grafting of mature buds on
rootstocks which are usually in a juvenile phase. A regression of the shoot meristem
from the adult phase to juvenility may occur as a consequence of traumatic events
or chemical treatment. For example, thiazols sprayed at high concentrations may re-
vert adult meristem to the juvenile phase, inducing the formation of juvenile leaves
and modifying laterals in thorn-like structures.
The adult phase lasts for several years and is characterized by a balance between
vegetative and reproductive activity. The latter consists of two overlapping indepen-
dent reproductive cycles: one is related to the pending crop and the other one to the
opening of the reproductive cycle that will be concluded in the following growing
season. The concurrent presence of these reproductive cycles may generate prob-
lems since a balance between the two activities has to be achieved to avoid biannual
