Can Progress Towards Stress Resistant Crops be Accelerated?.

Broadly speaking there are two categories of stress resistance: (i) those under “simple” control of one or two genes, and (ii) those under complex, polygenic control. Some forms of salinity stress, disease and insect resistance, and P efficiency are in the “simple” category, while drought, N, and heat stress are complex traits. While recent advances in genetics, bioinformatics, and measurement provide powerful tools to hasten progress on improvement of simple stress resistance, these advances have been less successful for complex stresses due to scarcity of “tradeoff-free” targets as noted by Ford Denison. Approaches successful in improving complex stress resistance include: (i) a brute force breeding “numbers game” within a target environment, and (ii) hypothesis breeding based on physiological insight that identifies a selectable phenotype. The thousands of on-farm strip trials used by commercial seed companies indirectly select for resistance to common stresses in the target environment exemplify the brute force approach. Carbon isotope discrimination identified by Graham Farquar and colleagues and the maize anthesis-silking interval targeted by Ken Fischer and colleagues at CIMMYT are examples of phenotypic proxies for drought resistance. Unfortunately, identifying proxy traits takes time, from hypothesis to selection tool, as seen in the field work and detailed measurement of Sadras and Lawson to identify plasticity in grain size as a trait contributing to wheat yield gain in harsh rainfed environments of S Australia. And they propose that plasticity of stress resistance traits per se could be a useful generic selection target. But how to accelerate progress on complex stresses? (1) development and use of crop models to evaluate impact of putative stress traits; (2) rigorous testing of promising lines in highly favorable, high-yield environments to ensure that stress resistance does not come at the expense of yield potential, and (3) there are no nearby silver bullets.