A Flooding-Induced Xyloglucan Endo-Transglycosylase Homolog In Maize Is Responsive To Ethylene And Associated With Aerenchyma
Development of aerenchyma (soft cortical tissue with large intercellular air spaces) in flooded plants results from cell-wall hydrolysis and eventual cell lysis and is promoted by endogenous ethylene. Despite its adaptive significance, the molecular mechanisms behind aerenchyma development remain unknown. We recently isolated a flooding-induced maize (Zea mays L.) gene (wusl1005[gfu]; abbreviated as 1005) encoding a homolog of xyloglucan endo-transglycosylase (XET), a putative cell-wall-loosening enzyme active during germination, expansion, and fruit softening. XET and related enzymes may also be involved in cell-wall metabolism during flooding-induced aerenchyma development. Under flooding, 1005 mRNA accumulated in root and mesocotyl locations that subsequently exhibited aerenchyma development and reached maximum levels within 12 h of treatment. Aerenchyma development was observed in the same locations by 48 h of treatment. Treatment with the ethylene synthesis inhibitor (aminooxy)acetic acid (AOA), which prevented cortical air space formation under flooding, almost completely inhibited 1005 mRNA accumulation in both organs. AOA treatment had little effect on the accumulation of mRNA encoded by adh1, indicating that it did not cause general suppression of flooding-responsive genes. Additionally, ethylene treatment under aerobic conditions resulted in aerenchyma development as well as induction of 1005 in both organs. These results indicate that 1005 is responsive to ethylene. Treatment with anoxia, which suppresses ethylene accumulation and aerenchyma development, also resulted in 1005 induction. However, in contrast to flooding, AOA treatment under anoxia did not affect 1005 mRNA accumulation, indicating that 1005 is induced via different mechanisms under flooding (hypoxia) and anoxia.