The role of ethylene (ET) and interactions between ET and abscisic acid (ABA) in coordinating the response of roots to salt stress was investigated. A positive regulatory role for ET was established by (1) the salt-induced expression of genes encoding enzymes and transcription factors that mediate ET biosynthesis and responses and (2) the enhanced production of ET in salt-stressed roots. ABA accumulated in salt-stressed roots and was a “dominant” signal that negatively regulated genes encoding ET biosynthetic enzymes and transcription factors. The regulation of root system architecture by ABA and ET was explored together with auxin. Salt stress reduced root growth and ET and auxin contributed to the reduction in growth. However, ABA suppressed ET/auxin responses allowing for growth maintenance. Salt stress reduced the production of lateral roots in an ABA-dependent manner, which allows resources to be dedicated for continued root elongation. Keywords: tomato; roots; salt stress; ethylene; abscisic acid
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