Principles of Plant Genetics and Breeding

394 CHAPTER 21
type of sorghum that was eaten in Egypt over 4,000 years ago. I knelt in the dust to examine the soil, making sure this plot wasn’t
receiving any additional water. Like the other plots, deep cracks snaked through the soil indicating the severity of drought. Now it
was time to check the other replicates. A critical component of experimental design is the existence of replicates to verify outcomes.
Excitedly, we made our way over to the next replicate of B35. Before we even arrived I could see it standing out from its
lifeless neighbours. Yes! Quickly we headed to the third and fourth replicates. The same again! I was amazed at the resilience of
B35, and determined to find out what drought adaptation mechanisms contributed to its remarkable survival. For me, the adventure
of cracking the stay-green phenomenon had just begun.
Andrew Borrell
Introduction
Producing more grain with less water is one of the greatest challenges facing crop scientists in the 21st century. Globally, the
availability of fresh water per capita has declined 37% since 1970 as population growth and degradation of water supplies has
surpassed the capacity to develop new sources (Downer 2000). Governments all over the world are choosing carefully how they
allocate water between agricultural, urban, and industrial uses. In a contest between these three, agriculture is often the loser
because water used for irrigation generally produces a smaller economic return than water diverted to industry (Dupont 2000),
with urban requirements being even more important for many governments. Yet in the face of diminishing water resources, the
world is expected to consume twice as much food in the next 50 years as it has in the past 10,000 years. To meet this demand,
world grain production will have to increase 40% by 2020 (Dupont 2000).
The case study above describes how a multidisciplinary team of Australian and US scientists are collaborating to discover
genes for drought adaptation in sorghum. The potential to utilize these genes in the world’s other major cereals is also discussed.
Sorghum is a repository of drought-resistance mechanisms, and has developed biochemical, physiological, and morphological
characteristics such as C 4 photosynthesis, deep roots, and thick leaf wax that enable growth in hot and dry environments.
Sorghum is the dietary staple of more than 500 million people in over 30 countries, making it the world’s fifth most important crop
for human consumption after rice, wheat, maize, and potatoes (Miller 1996).
Multidisciplinary approach
In many areas of human endeavor, it is often the integration of fields of knowledge that proves to be the fertile ground for innovation.
So it is with “gene discovery” in the world’s most important cereal crops. The pursuit of drought-resistance genes in sorghum
is a multidisciplinary effort involving plant breeders, crop physiologists, molecular biologists, biometricians, functional genomicists,
and simulation modelers. Scientists from Australia and the USA are collaborating in the search for genes (Stg1, Stg2, Stg3,
and Stg4) associated with the “stay-green” trait in grain sorghum. Keeping leaves alive for longer is a fundamental strategy for
increasing crop production, particularly under water-limited conditions. During postanthesis drought, genotypes possessing the
stay-green trait maintain more photosynthetically active leaves than genotypes not possessing the trait. The broad objective of
this research is to identify and understand the function of the genes and gene networks that contribute to improved plant drought
resistance under water-limited conditions.
Approaches to gene discovery
There are two general approaches to identifying and isolating genes involved in drought resistance (Mullet et al. 2001). First,
genes are targeted that show relatively rapid changes in expression at the RNA level in response to water limitation. Second,
sorghum genes involved in drought adaptation are identified and isolated using map-based gene discovery. The current staygreen
project primarily utilizes map-based gene discovery undertaken by scientists at Texas A&M University, although microarray
analysis is being used simultaneously to assist in gene discovery.
Phenotyping, genotyping, and physiological characterization
Phenotyping driving genotyping
Map-based cloning requires the accurate screening of the phenotype and genotype of large segregating populations (Tanksley
et al. 1995), highlighting the need for collaboration between plant breeders, crop physiologists, and molecular biologists.
Typically, plant breeders develop a range of populations for mapping (e.g., recombinant inbred lines), fine mapping (e.g., segregating
populations with breakpoints across the loci of interest), and physiological dissection (e.g., near-isogenic lines). Such
populations are systematically phenotyped and genotyped by crop physiologists and molecular biologists, respectively, resulting
in the identification of regions of genomes (trait loci) that modulate the expression of traits such as stay-green.
Genotyping driving phenotyping
Following the mapping of drought-resistance loci, efficient map-based cloning requires the availability of a high-resolution integrated
genetic and physical map, large populations, and careful phenotyping (Mullet et al. 2001). The construction of an integrated