A NAC transcription factor for flooding: SHYG helps plants keep their leaves in the air.

Like struggling swimmers trying to keep their heads above water, rosette plants lift up their leaves when faced with flooding. Instead of flailing arms and legs, plants use hyponastic petiole growth to improve their chances of keeping leaves in contact with the air. During this hyponastic growth, cells on the abaxial side of the petiole elongate more than those on the adaxial side, changing the angle of the petiole (see figure). Ethylene is a major regulator of this process (reviewed in Polko et al., 2011), and new work from Rauf et al. (pages 4941–4955) describes a NAC transcription factor that mediates floodinginduced hyponastic growth via regulation of an ethylene biosynthesis enzyme. Rauf et al. characterized SPEEDY HYPONASTIC GROWTH (SHYG; ANAC047), which encodes a group III NAC transcription factor family member and is induced upon waterlogging (i.e., root submergence) inArabidopsis thaliana. Whereas shyg mutants had much reduced hyponastic petiole growth in response to waterlogging, SHYG overexpression lines had increased responsiveness, consistent withSHYGmediation of floodinginduced hyponastic growth. In addition, more than a dozen genes related to cell expansion responded to the level of SHYG expression, providing further support for the idea that SHYG is an upstream regulator of the process. One of the early effects of flooding is the accumulation of ethylene in the plant. Treatment of plants with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) led to increased induction of SHYG expression as well as increased hyponastic petiole growth in response to waterlogging. Interestingly, Rauf et al. identified the ethylene biosynthesis gene ACC OXIDASE5 (ACO5) as a downstream target activated by SHYG and demonstrated that ACO5 is necessary for SHYG’s induction of hyponastic growth upon flooding. This suggests that a positive feedback loop involving ethylene functions in the flooding response. The authors went on to examine natural variation of the SHYG-ACO5 regulatory module, finding little induction of SHYG and ACO5 in an Arabidopsis accession that is less responsive to waterlogging. Together, this work represents a solid advance in our understanding of flooding biology, placing ethylene and SHYG in a regulatory cascade leading to localized cell expansion in the petiole. It also emphasizes that plants use NAC transcription factors to respond to a host of biotic and abiotic stresses (reviewed in Nuruzzaman et al., 2013), in this case to save themselves from drowning.