Fig. 1 | Auxin response. ARFs are transcription factors that regulate gene promoters that have auxin response elements. There are three different groups of ARF proteins: class A ARFs are considered mainly transcriptional activators on their own, while classes B and C are thought to mostly repress transcription. Under low auxin concentrations, Aux/IAA proteins bind to class A ARF proteins and repress ARF target gene expression by recruiting histone deacetylases. Most Aux/IAA proteins are short-lived, and their auxin-induced degradation releases ARF proteins to activate gene expression. Thus, auxin switches class A ARFs from a repressing to an activating complex. Aux/IAA degradation is mediated by a conserved domain termed DII, to which auxin receptors from the TIR1/AFB family bind in the presence of auxin. Class B and C ARFs can repress gene expression on their own and generally do not bind strongly to Aux/IAA proteins. Thus, class A ARFs are the main mediators of auxin response by this mechanism. Evolutionarily, this auxin-responsive machinery derived from an ancestral gene-regulation programme, which likely did not respond to auxin, and involved an ARF-like protein most closely related to class C ARFs. Some non-canonical auxin response mechanisms have also been discovered. For example, auxin can bind to the class B ARF ETTIN (ETT)/AtARF3 to regulate tissue patterning, and auxin perception by a plasma membrane receptor kinase can stabilize rather than destabilize some Aux/IAA proteins lacking domain II.

演化过程中基因家族的扩张会导致成员之间存在功能冗余，也会出现功能特化，这是复杂生物中信号通路的一个主要特征。比如，介导植物激素生长素响应的转录激活因子与抑制因子基因家族就在演化过程中出现了极大的扩张，不同成员之间即存在功能冗余，也存在功能特化。尽管有关生长素响应的调控因子早在20年前就已经鉴定，但是功能冗余以及补偿性的负反馈调节使得这些基因的功能研究十分困难。通过对基础陆地植物功能缺失突变以及被子植物功能获得突变研究，能够部分克服上述的难题，但是仍然缺乏比较全面、完整的理解。本文中，作者提出对于多个突变及物种的遗传学研究对于理解介导生长素响应调控基因家族的功能或许能提供新的视野。通过结合生长素响应激活子和抑制子的功能缺失突变体研究，可能会有助于理解扩张后的基因家族的复杂输出，比如精确的发育事件和鲁棒性调控。类似的方法和概念可能有助于研究其它同样由庞大基因家族所介导的调控途径。