Agriculture Reference
In-Depth Information
possible between marker and trait. Full resequencing of the genome to
capture all allelic variants is a not-too-distant scenario for very precise
GWA scans. Generally in AM, the cereals have led the way with the
development of methods and examples of successful CG and GWA
studies.
This is because of the greater investment in cereal crops compared
with noncereals in general and the early use of comparative genomics
and a switch from model species to crop plants in the monocots, not
because of any inherent difference in the maximum potential of AM in
either group of crops. Given the history of AM, we will start with the
monocotyledonous cereals in the examples we give next and followwith
dicotyledenous plants, especially legumes and root or tuber crops with a
focus on crops important for developing countries. We will point out the
current challenges to implementing AM in certain clonal and long-lived
crops within these two major orders of plans as well as in certain poorly
studied tropical crops that are important to developing countries.
A. Cereals
The majority of AM studies performed so far for plants have been in the
cereals (barley, maize, rice, pearl millet, sorghum, and wheat; Table 2.2).
The
first studies performed with CG analyses were in maize. Here, the
CG approach was very useful for evaluating the importance of genes
involved in starch accumulation and composition evaluating six maize
candidate genes for starch biosynthesis (Wilson et al. 2004). In a later
study, at least
five alleles were found for sugar accumulation in sweet
corn controlled by the Su1 gene that encodes an isoamylase (Tracy et al.
2006). Likewise, the CG approach was used for evaluating carotenoid
biosynthesis in maize (Harjes et al. 2008; Yan et al. 2010) locating
β
-carotene hydroxylase 1 as an instrumental gene in controlling the
accumulation of
-carotene and the yellow color of maize kernels. In a
study of many candidate physiological genes, Setter et al. (2010) found
three important genes associated with carbohydrate accumulation and
ABA metabolism during drought stress in maize using a panel of 350
tropical and subtropical maize lines and 540 candidate genes evaluated
with 1229 SNP markers. These included a pyruvate dehydrogenase
kinase, an aldehyde oxidase, and a MADS box gene. In wheat, CG
studies began quite late, perhaps due to the complex genetics of hexa-
ploidy. However, one recent study found some association between
β
flowering time and its components (vernalization sensitivity and earli-
ness) with six previously studied genes from various cereals (Rousset
et al. 2011).
