Significant roles of Kasalath allele in Nipponbare background on the root development of rice plant under soil moisture fluctuation stress

Soil moistures in the field inevitably tend to fluctuate from wet to dry and vice versa. Rice genotypes greatly differ in their ability to maintain root growth and functions under such condition, which could lead to reduction of dry matter production and yield. The roles of L-type lateral root (LR) plasticity in plant adaptation under such environment was evaluated using the 54 chromosome segment substitution lines (Nipponbare/Kasalath, CSSL). Quantitative trait locus (QTL) that regulates LR plasticity was previously detected on the short-arm of chromosome 12 regions, whose effect was contributed by substituted Kasalath allele. In this study, the authors used CSSL52 which has one substituted segment of Kasalath on chromosome 12 region to validate the functions of the plasticity of LR development. Plants were subjected to well-watered, fluctuating soil moisture and progressive drought conditions for 38 days. No significant difference was observed among genotypes in shoot growth and root development under well-watered conditions. In contrast, under fluctuating soil moisture conditions, CSSL52 genotype showed greater shoot dry weight by 31%, which was associated with larger root system. This larger and longer root system development of CSSL52 genotype was attributed to faster 1-type LB development by 95% relative to Nipponbare. However, under simple drought condition CSSL52 genotype developed a smaller-sized root system that was associated with reduced SDW, relative to those grown under fluctuating soil moisture and well-watered conditions. These results indicate that the introgressed segment of Kasalath on chromosome 12 region promotes production of L-type LR, and has contributed to a greater root system development, which led to effective water and nutrient uptake specifically under fluctuating soil moisture conditions. This further suggests that the expression of this QTL is unique and triggered only when plants are exposed to fluctuating moisture stress but not to simple drought condition.