© Benaki Phytopathological Institute
Margaritopoulou & Milioni
44
involved in development and homeosta-
sis. It has been recently demonstrated that
a change in expression of a key component
of the RNA silencing pathway is associat-
ed with both vegetative phase change and
shifts in epigenetic regulation of a maize
transposon (Li
et al.,
2010).
RNA interference (RNAi) [RNA-mediated
gene silencing by sequence-specific degrada-
tion of homologous mRNA triggered by dou-
ble-stranded RNA (dsRNA)].
The RNAi system
was used to improve resistance to maize
dwarf mosaic virus on transgenic maize
(Zhang
et al.,
2011). Maize lines expressing
RNAi to chromatin remodeling factors were
shown to be similarly hypersensitive to UV-B
radiation but exhibit distinct transcriptome
responses (Casati and Walbot 2008). By us-
ing near infrared reflectance spectrosco-
py (NIRS), a set of 39 maize mutants with al-
tered spectral phenotypes (‘spectrotypes’)
have been identified (Vermerris
et al.,
2007).
A number of these mutants were shown to
have altered lignin-to-carbohydrate ratios
(Penning
et al.,
2009). Sequence- specific
DNA binding Transcription Factors (TFs) are
key molecular switches that control or in-
fluence many biological processes, such as
development or response to environmen-
tal changes. The Maize Transcription Factor
Database provides a comprehensive collec-
tion of 764 predicted transcription factors
from maize with available links to informa-
tion on mutants, map positions or puta-
tive functions for these transcription factors
(MaizeTFDB) (
family.html?species=Maize
). Information
resources related to metabolomics can play
major role not only in metabolomics re-
search but also in synergistic integration
with other omics data. MaizeCYc is a bio-
chemical pathway database that provides
manually curated or reviewed information
about metabolic pathways in maize.
Molecular breeding for current needs
Molecular breeding, including both
transgenic approach and marker-assist-
ed breeding, is primary associated with the
challenges for developing cultivars with
combinations of adaptive traits (Brown
et al.,
2011; Varshney
et al.,
2011). For making mo-
lecular marker-assisted breeding success-
ful, marker-trait associations are now known
for almost all important economic traits, in-
cluding thousands of mapped microsatel-
lite or SSR markers, and additional recently,
SNPs, and insertion-deletion (InDel) mark-
ers. For maize, there is an updated compila-
tion of mapped QTL for abiotic stress resis-
tance
; http://
;
.
org
). Additionally, a large number of genes
controlling various aspects of plant devel-
opment, biotic and abiotic stress resistance,
quality characters, etc. have been cloned
and characterized in maize, which are ex-
cellent assets for molecular marker- assist-
ed breeding (Aslam and Ali 2018; Prasanna
et al.,
2010).
Tolerance against drought.
Since drought
is considered to be the most important con-
straint across all areas where maize is culti-
vated, and global warming is predicted to
further exacerbate drought’s impact, a to-
tal management plan is necessary for in-
creasing maize yield in stress-prone envi-
ronments (Fig. 2). The high variability to
drought stress and also the uncontrollable
fact that drought response has great fluc-
tuations across environments, have made
it difficult to spot specific metabolic path-
ways which limits breeding efforts towards
drought tolerance (Collins
et al.,
2008). A
Marker-Assisted BackCross (MABC) selec-
tion approach meant for improving grain
yield under water limited conditions in trop-
ical maize, was successfully conducted at
CIMMYT (Ribaut and Ragot 2006) and more
recently at sub-Saharan Africa (Beyene
et al.,
2016). However, this approach delivers a re-
stricted level of improvement in drought
tolerance since it provides an improved ver-
sion of an existing genotype (Ribaut
et al.,
2009). Nevertheless, a molecular breeding
approach-marker-assisted recurrent selec-
tion (MARS) can be used to overcome this
problem. MARS studies exploit association
mapping and can effectively double the
rate of yield gain compared to conventional
