Gene expression or functionality

Apart from the evolutionary forces influencing population diversity, climate change may induce phenotypic change leading to differences in gene expression or functionality, which also tend to modify pathosystems. Increases in temperature can modify host physiology and resistance by changing gene expression and activity. For example, temperatures above 20°C deactivate temperature-sensitive resistance to stem rust in oat cultivars with Pg3 and Pg4 genes (Maertens et al., 1967). In tetraploid wheat, lines carrying Yr36, a previously unidentified stripe rust resistance gene from Triticum turgidum ssp. dicoccoides located on chromosome arm 6BS, are susceptible to almost all stripe rust resistance races of P. striiformis tested at the seedling stage, but show adult-plant resistance to the prevalent races in California at high diurnal temperatures (Uauy et al., 2005). This high temperature adult-plant resistance (HTAPR) is closely linked to the grain protein content locus and has proven to be more durable than seedling resistance due to its non-race-specific nature (Uauy et al., 2005). Temperature was also shown to influence the resistance gene Xa7 in rice against bacterial blight caused by Xanthomonas oryzae (Garrett et al., 2006). Other environmental conditions are also likely to alter the physiology and functionality of resistance genes. In barley, Newton and Young (1996) showed that the mechanisms of Mlo-resistance, an important powdery mildew resistance source, could be disrupted following drought stress as cells undergo expansion once water supply is restored. The positive effect of the elevation of the CO2 concentration on plant growth is now well recognized (Drake et al., 1997), but the interference with pathogen development will also influence the evolution of pathosystems. Kobayashi et al. (2006) observed that rice plants grown in an elevated atmospheric CO2 concentration showed more leaf blast (Magnaporthe oryzae) lesions than those in ambient CO2. A relationship with leaf silicon content, lower at high CO2 concentration, and plant susceptibility was suggested. Malmström and Field (1997) showed that barley yellow dwarf (BYD) infection on Avena sativa influenced plant response to CO2 enrichment by increasing root biomass response, photosynthesis and water-use efficiency. A change in the epidemiology of BYD could occur at high CO2 content by increasing the persistence of infected plants.

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