Induced elongation responsive cells

Sequential Interaction. Synergism between two hormones is sometimes more clearly observed in sequential treatments of the two hormones. Thus, GA pretreatment always results in synergistic enhancement of IAA-induced elongation of hypocotyl sections (37). Sequential interaction between BR and IAA is very similar to that between GA and IAA (3). Sections pretreated with BR for 2 h respond to IAA synergistically. The reverse order treatment is rather inhibitory as is the case for GA. Therefore, both BR and GA seem to sensitize hypocotyl sections to a later response by IAA. These facts suggest that cells which have already been initiated to elongation by IAA are not the target cells of BR/GA. 

Both stem cell elongation and xylem differentiation are auxin-mediated processes. There has long been speculation that BRs act through alterations in the auxin response [109]. This is certainly not always the case, as BR induces elongation of soybean hypocotyls without activating any of the auxin-induced genes such as the SAUR genes [112]. On the other hand, one of the effects of BR in tomato hypocotyls appears to be to increase the sensitivity of the tissue to auxin [113]. Thus some BR effects may actually be mediated via auxin, while others are independent of auxin. 

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