背景回顾：Root hair (RH) growth to increase the absorptive root surface area is a key adaptation of plants to limiting phosphate availability in soils.

提出问题：Despite the importance of this trait, especially for seedling survival, little is known about the molecular events connecting phosphate starvation sensing and RH growth regulation. 

研究基础：KARRIKIN INSENSITIVE2 (KAI2), an α/β-hydrolase receptor of a yet-unknown plant hormone (“KAI2-ligand” [KL]), is required for RH elongation. KAI2 interacts with the F-box protein MORE AXILLIARY BRANCHING2 (MAX2) to target regulatory proteins of the SUPPRESSOR of MAX2 1 (SMAX1) family for degradation. 

结果1-磷饥饿-KAI2/MAX2-根毛伸长：Here, we demonstrate that Pi starvation increases KL signaling in Arabidopsis roots through transcriptional activation of KAI2 and MAX2. Both genes are required for RH elongation under these conditions, while smax1 smxl2 mutants have constitutively long RHs, even at high Pi availability. 

结果2-kai2突变体-AUX1/PIN2-生长素：Attenuated RH elongation in kai2 mutants is explained by reduced shootward auxin transport from the root tip resulting in reduced auxin signaling in the RH zone, caused by an inability to increase localized accumulation of the auxin importer AUXIN TRANSPORTER PROTEIN1 (AUX1) and the auxin exporter PIN-FORMED2 (PIN2) upon Pi starvation. 

结果3-低磷-KAI2-乙烯-AUX1/PIN2：Consistent with AUX1 and PIN2 accumulation being mediated via ethylene signaling, expression of 1-AMINOCYCLOPROPANE-1-CARBOXYLATE SYNTHASE 7(ACS7) is increased at low Pi in a KAI2-dependent manner, and treatment with an ethylene precursor restores RH elongation of acs7, but not of aux1 andpin2. 

结论：Thus, KAI2 signaling is increased by phosphate starvation to trigger an ethylene- AUX1/PIN2-auxin cascade required for RH elongation.

 摘 要 

根毛生长以增加根吸收的表面积，这是植物适应于低磷土壤环境的关键特征。尽管根毛生长具有非常重要的作用，尤其是对于幼苗的存活，但是对于植物中磷饥饿感知和根毛生长调控之间的分子关联还不清楚。KAI2是一个未知植物激素的α/β-水解酶受体，是根毛伸长所必需的。KAI2能够与F-box蛋白MAX2互作，靶向MAX2抑制蛋白SMAX1家族的调控蛋白，使之降解。本文中，作者发现磷饥饿通过转录激活KAI2和MAX2基因，增加了拟南芥根中的KAI2配体信号转导。KAI2和MAX2基因是拟南芥在低磷条件下保证根毛伸长的必需条件，而smax1 smxl2双突植株甚至是在高磷条件下都具有更长的根毛。kai2突变体在磷饥饿处理下，无法增加生长素输入蛋白AUX1和输出蛋白PIN2的局域积累，从而导致由根尖向地上组织的生长素转运减少，最终导致了kai2突变体中根毛伸长减缓。与AUX1和PIN2蛋白积累受到乙烯信号转导介导一致，低磷条件下ACS7基因的表达上调，并且依赖于KAI2；另外，乙烯前体处理能够拯救acs7突变体的根毛伸长缺陷，但是却不能恢复aux1和pin2突变体的缺陷。因此，KAI2信号转导受到磷饥饿的诱导增强，从而诱导了一个乙烯-AUX1/PIN2-生长素的级联，最终作用于根毛伸长。

 通讯作者 

** Caroline Gutjahr **