The nucleotide positions of the 35S motif-1 in the 35S promoter are from 139 to 110. petiole growth inArabidopsis thaliana, governed by the NAC transcription factor SPEEDY HYPONASTIC GROWTH (SHYG). Overexpression ofSHYGin transgenicArabidopsis thalianaenhances waterlogging-triggered hyponastic leaf movement and cell expansion in abaxial cells of the basal petiole region, while both responses are largely diminished inshygknockout mutants. Expression of severalEXPANSINandXYLOGLUCAN ENDOTRANSGLYCOSYLASE/HYDROLASEgenes encoding cell wallloosening proteins was enhanced inSHYGoverexpressors but lowered inshyg. We identifiedACC OXIDASE5(ACO5), encoding a key enzyme ofETbiosynthesis, as a direct B-Raf-inhibitor 1 transcriptional output gene of SHYG and found a significantly reduced leaf movement in response to root flooding inaco5T-DNA insertion mutants. Expression ofSHYGin shoot tissue is triggered by root flooding and treatment withET, constituting an intrinsic ET-SHYG-ACO5activator loop for rapid petiole cell expansion upon waterlogging. == INTRODUCTION == Root flooding (waterlogging) in rosette plants likeArabidopsis thalianacauses an upward leaf movement, called hyponastic growth, to reestablish B-Raf-inhibitor 1 contact with air and photosynthetic gas exchange (Pierik et al., 2005;Jackson, 2008;Vashisht et al., 2011). The upward leaf movement is triggered by unequal cell elongation at the basal (proximal to the shoot) petiole region, where abaxial cells extend more than adaxial cells, resulting in increased petiole angle. The gaseous phytohormone ethylene (ET; C2H4), which regulates many plant developmental processes (Schaller and Kieber, 2002;Lin et al., 2009;Stepanova and Alonso, 2009;Schaller, 2012), has been indicated as the primary trigger of several waterlogging or whole-plant submergence-induced physiological B-Raf-inhibitor 1 and morphological acclimations in plants, including hyponastic growth (Cox et al., 2003;Millenaar et al., 2005;Voesenek et al., 2003,2006;Bailey-Serres and Voesenek, 2008;Jackson, 2008). In the semiaquatic dicotRumex palustris, theETlevel increases 20-fold within the first hour of waterlogging (Banga et al., 1996). The hyponastic growth response to waterlogging is notably fast inR. palustriswith a lag phase of only 1 1.5 to 3 h and the response being completed after 7 h, depending on the initial leaf angle (Cox et al., 2003). A comparably fast hyponastic growth response to flooding was observed in the Columbia-0 (Col-0) accession ofArabidopsis(Millenaar et al., 2005). Recently, a study byPolko et al. (2012)showed thatET-mediated hyponasty inArabidopsisinvolves the reorientation of cortical microtubules at the abaxial side of the petiole from longitudinal to transverse; thus,ETis associated with tissue-specific changes in the arrangement of cortical microtubules along the petiole and most likelyETalso triggers local stimulation of cell expansion upon waterlogging. ROTUNDIFOLIA3(ROT3) inArabidopsisencodes the cytochrome P450 enzyme CYP90C1, which catalyzes the C-23 hydroxylation of various brassinosteroids (BRs).ROT3is involved in polar cell elongation, and it has recently been shown thatrot3mutants have reduced hyponastic growth uponETtreatment, as well as low-light treatment and heat treatment, which both also induced hyponastic growth. Treatment with brassinazole, an inhibitor ofBRbiosynthesis, reduces theET-induced increase of the petiole angle, revealing a modulatory role ofBRsin petiole angle establishment (Polko et al., 2013). In addition Rabbit Polyclonal to CaMK1-beta toBRs, other phytohormones like auxin and gibberellins can act as positive regulators of hyponastic leaf growth, while abscisic acid functions as a negative regulator (Polko et al., 2011). Furthermore, the defense-related hormones methyl jasmonate and salicylic acid have been shown to act as positive and negative modulators, respectively, ofET-induced hyponastic leaf growth (van Zanten et al., 2012). Flooding triggers and accelerates leaf senescence in many plant species, including tobacco (Nicotiana tabacum), tomato (Solanum lycopersicum), sunflower (Helianthus annuus), and maize (Zea mays), possibly because of the reduced production of cytokinin, a phytohormone that delays senescence, in flooded roots (Burrows and Carr, 1969;Trought and Drew, 1980;VanToai et al., 1994;Huynh et al., 2005). In line with this model, expression of the rate-limiting enzyme of cytokinin biosynthesis (i.e., isopentenyl transferase) from a senescence-induced promoter enhanced flooding tolerance inArabidopsis(Zhang et al., 2000). Taken together, although various molecular players affecting the adaptive leaf growth response to waterlogging have been identified in recent years, an integrated view of the underlying regulatory networks is currently missing. ETregulates two important molecular processes during hyponastic leaf growth, namely, rapid acidification of the apoplast to reduce cell wall rigidity and.