Novel approaches in plant breeding for rhizosphere-related traits

Selection of modern varieties has typically been performed in standardized, high fertility systems with a primary focus on yield. This could have contributed to the loss of plant genes associated with efficient nutrient acquisition strategies and adaptation to soil-related biotic and abiotic stresses if such adaptive strategies incurred a cost to the plant that compromised yield. Furthermore, beneficial interactions between plants and associated soil organisms may have been made obsolete by the provision of nutrients in high quantity and in readily plant available forms. A review of evidence from studies comparing older traditional varieties to modern high yielding varieties indeed showed that this has been the case. Given the necessity to use scarce and increasingly costly fertilizer inputs more efficiently while also raising productivity on poorer soils, it will be crucial to reintroduce desirable rhizosphere-related traits into elite cultivars. Traits that offer possibilities for improving nutrient acquisition capacity, plant-microbe interactions and tolerance to abiotic and biotic soil stresses in modern varieties were reviewed. Despite the considerable effort devoted to the identification of suitable donors and of genetic factors associated with these beneficial traits, progress in developing improved varieties has been slow and has so far largely been confined to modifications of traditional breeding procedures. Modern molecular tools have only very recently started to play a rather small role. The few successful cases reviewed in this paper have shown that novel breeding approaches using molecular tools do work in principle. When successful, they involved close collaboration between breeders and scientists conducting basic research, and confirmation of phenotypes in field tests as a 'reality check'. We concluded that for novel molecular approaches to make a significant contribution to breeding for rhizosphere related traits it will be essential to narrow the gap between basic sciences and applied breeding through more interdisciplinary research that addresses rather than avoids the complexity of plant-soil interactions.