Breeding the plants for a sustainable future will need to exploit their genotype-specific differential interactions with plant-probiotic microorganisms.

Among the genetic traits of plants, the ability to interact with plant-probiotic microorganisms (PPM, sensu Haas and Keel, 2003) is very well conserved in natural populations. In fact, during their evolution (more than 400 M years) terrestrial plants have not achieved the ability to grow independently of their own PPM, and most plant genotypes still show the biological need to find such microorganisms in their habitat to successfully complete their growth cycle. Indeed, well established knowledge on PPM, such as plant-growth-promoting-rhizobacteria (PGPR) (Kloepper et al., 1980) or arbuscular mycorrhizal fungi (AMF), tells how they positively and directly affect plant growth through several mechanisms, such as N2 fixation, P, K, microelements dissolution, biosynthesis of siderophores, plant hormones, and plant hormone regulators (Picard and Bosco, 2006). PPM can also indirectly promote plant growth by antagonizing the action of weeds, pests, and phytopathogenic organisms. All these mechanisms are regulated by the population density and biodiversity of PPM that are in direct contact with plant surfaces, or in the close rhizosphere, a habitat where there is the maximum microbial activity due to the release of organic compounds and microbial-growth regulators from plant roots. During the last decade, scientific literature has reported how breeding for conventional agriculture often resulted in modern crop varieties which are much less sensitive to PPM than needed for low-input and organic agriculture. Cultivar development in fully fertilized and parasite-protected environments has been proven to result in selection against genotypes that fully interact with AMF (Hetrick et al., 1995; Scholten et al., 2006), or PGPR (Smith et al., 1999; Engelhard et al., 2000; Germida and Siciliano, 2001; Mazzola et al., 2004). Since AMF actually reduce plant growth in environments where biotic and abiotic stresses are limited (i.e., when the cost of maintaining the AMF exceeds the benefit to the host), it is logical that selection under high input levels was highly biased towards non-mycorrhizal genotypes. In the same way, conventional breeding and crop improvement have resulted in the loss of host genes important in the PGPR-legume interaction, and thus in varieties that don’t maximize the benefits of root symbioses. As is already known for Rhizobium and AMF (Smith and Goodman, 1999), we recently proved the genetic basis of PGPR interactions with cereals (Picard and Bosco, 2006). Thus, future efforts of crop breeding in low-input and organic environments would indeed be able to take into account the biological need of plants to have broad-spectrum interactions with PPM. Here we propose and support the theoretical development, and the laboratory, field, and biostatistic assessment of a European breeding programme for crop genotypes genetically able to support broad populations of probiotic microorganisms (Picard et al., 2007). This could be done at different and parallel levels, by exploiting existing European-based knowledge on natural, local, and commercial (Bosco et al., 2006) PPM.