In rice, it has beenshown that drou

In rice, it has beenshown that drought affects the expression of aquaporins in root, but not in leaf [30].In Arabidopsis, simultaneous downregulation of PIP1 (PLASMA MEMBRANEINTRINSCI PROTEIN1) and PIP2 aquaporins using an antisense techniquereduced hydraulic conductivity by 5- to 30-fold. When control and antisensetransgenic lines were subjected to water stress and rewatered, the leaf waterpotential was higher in control plants, which proved that PIP1 and PIP2 playimportant roles in water uptake [75]. Yu et al. proposed a possible role foraquaporins in plant chilling stress [76]. When they subjected two rice cultivars tochilling stress and after subsequent recovery, they found higher expression of someaquaporins in the shoot and root of the chilling-tolerant Somewake cultivar than inthe chilling-sensitive Wasetoitsu cultivar [76]. The expression levels of OsPIP1;1and OsPIP2;1 were significantly higher in roots of Somewake [76]. Lian et al. founddifferential expression of OsPIP genes between upland and lowland rice cultivarsunder drought and ABA treatment [77]. They found OsPIP1;2, Os-PIP1;3,OsPIP2;1, and OsPIP2;5 were more upregulated in the upland cultivar than thelowland cultivar, where they were unchanged or downregulated [77]. Transgenicrice plants overexpressing OsPIP1 were shown to have increased rice seed yield,salt resistance, root hydraulic conductivity, and seed germination rate [65]. RWC3,an aquaporin that was upregulated in an upland cultivar under stress conditions,was introduced into a lowland cultivar and the transgenic lowland cultivar hadbetter water potential than non-transgenic plants [77]. It was reported thataquaporins HvPIP2;2, HvPIP2;5, and HvTIP1;1 contribute mostly for water uptakeby roots in barley [78]. Transgenic tobacco plants, overexpressing TdPIP1;1 andTdPIP;2 genes from durum wheat, were resistant to drought and salinity stresses[79]. All this evidence suggests that aquaporins are a major component of the stressavoidance mechanism that primarily involves the root system. Lorenzo et al. haveidentified a salt stress-induced leucine-rich repeat receptor-like kinase (LLR-RLK)gene Srlk in Medicago truncatula, a legume model crop. Downregulation of Srlkusing antisense technology resulted in transgenics with better root growth undersalt stress [80]. Regulation of lateral root emergence under stress conditions is oneof the important adaptation mechanisms for plants. In M. truncatula, salt-inducedHD-ZIP1 transcription factor HB1 was shown to regulate root architecture understress conditions via LBD1 [81]. de Z
elicourt et al. reported a NAC transcriptionfactor, MtNAC969, which is involved in root development and salt stress tolerance[82].