A novel function of abscisic acid in the regulation of rice (Oryza sativa L.) root growth and development.

Plant roots retain developmental plasticity and respond to environmental stresses or exogenous plant growth regulators by undergoing profound morphological and physiological alteration. In this study, we investigated the effects of exogenous ABA on root growth and development in Taichung native 1 (TN1) rice. Exogenous application of 10 microM ABA leads to swelling, root hair formation and initiation of lateral root primodia in the tips of young, seminal rice roots. Cortex cells increased in size and were irregularly shaped. ABA treatment significantly increased 2, 3, 5-triphenyl tetrazolium chloride (TTC) reductase ability in the root tips and the exudation rate of xylem sap. In addition, the K(+) ion content in xylem sap increased nearly 2-fold, but not that of Ca(2+) or Mg(2+). Analysis of proteins expressed in the root tips identified several ABA-induced or -repressed proteins, including actin depolymerization factor (ADF), late embryo abundant protein (LEA), putative steroid membrane-binding protein, ferredoxin thionine reductase and calcium-binding protein. The effects of ABA on root morphogenesis change were Ca(2+) dependent and required the participation of calmodulin and de novo protein synthesis. A model is presented that illustrates how ABA acts through a potential cellular and signal transduction mechanism to induce morphological and physiological changes in rice roots.