Physiological and functional root traits that are related to drought tolerance and their genetic bases in crop plants

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Root signal productionA series of pot and field experiments showed that in rice plants, the decrease in stomatal conductance and the increase in ABA concentration in leaf and xylem exudates were detected earlier than apparently drop of leaf water potential under the conditions where a part of root system was exposed to drying conditions while the rest was accessible to water. Severing the roots in the drying soil resulted in recovery of the stomatal conductance as well as leaf water potential. These facts strongly suggest that ABA was produced in the roots in the dry soil and worked as root chemical signal, and regulated stomatal opening together with hydraulic signal that was also coming from the roots.Functional significance of root plasticity in drought adaptationFifty four Chromosome Segment Substitution Lines (CSSL) derived from Nipponbare and Kasalath created by the National Institute of Agrobiological Sciences were used to examine developmental and physiological responses to drought stress, and we screened several lines that showed significant roles of plastic development of root system in plans' adaptation to the stress conditions. Those lines were backcrossed with Nipponbare to prepare materials for further genetic analysis and breeding. Further, several QTLs that are responsible for the lateral root plasticity were identified, and root osmotic adjustment as physiological basis for the plasticity was also studied. It was also found that there were NERICA (New Rice for Africa) lines in which such plastic root responses played key roles in the plants' growth under drought.Ability of oxygen transport in rootsBy using the CSSLs, we examined the rice plant adaptation to soil moisture fluctuations and screened several lines that showed adaptability through high ability in aerenchyma formation, oxygen transport to root tip, and plastic lateral root development under hydroponic, soil culture and field conditions.