Garcia Mary Emily, Lynch Tim, Peeters Julian, Snowden Chris, Finkelstein Ruth
Molecular, Cellular, and Developmental Biology Department, University of California at Santa Barbara, Santa Barbara, CA 93106, USA.
Plant Mol Biol. 2008 Aug;67(6):643-58. doi: 10.1007/s11103-008-9344-2. Epub 2008 May 17.
The transcription factor ABA-Insensitive5 (ABI5) is a key regulator of ABA signaling and stress response in Arabidopsis seeds and seedlings. Potential ABI5-interacting proteins were identified by a yeast two-hybrid screen; the most prevalent interactors were a family of four highly conserved plant-specific proteins with no domains of known function, but homology to a previously characterized ABI Five Binding Protein (AFP). This study compares expression and function of the family members. The AFPs are induced by ABA and/or dehydrating stresses in young seedlings, but the developmental timing of their induction differs. Mutations in AFP1 or AFP2 result in increased sensitivity to ABA and salt, whereas afp4 mutants are mildly ABA-resistant. AFP2, like AFP1, acts epistatically to ABI5. Reduced germination or seedling growth of the mutants under stress correlates with a higher level of ABI5 protein when compared to wild-type seedlings, but it is not clear whether this is a cause or effect of the reduced growth. Although both ABI5 and the AFPs are ABA-induced, the ABI5:AFP ratio increases at high ABA concentrations, maintaining growth inhibition under severe stress. An AFP2:GFP fusion, which complements the afp2 mutation, is nuclear-localized in seedlings exposed to stress, but becomes delocalized before being degraded following removal of stress. The AFPs may also interact to varying extents with many ABI5-related bZIP transcription factors. This study suggests that germination and seedling growth are regulated by antagonistic interactions among at least two functionally redundant families, the AFPs and the ABI5-related proteins, providing a mechanism to fine-tune seedling stress responses.
转录因子脱落酸不敏感5（ABI5）是拟南芥种子和幼苗中脱落酸信号传导及应激反应的关键调节因子。通过酵母双杂交筛选鉴定出了潜在的与ABI5相互作用的蛋白；最常见的相互作用蛋白是一个由四个高度保守的植物特异性蛋白组成的家族，这些蛋白没有已知功能的结构域，但与先前鉴定的ABI5结合蛋白（AFP）具有同源性。本研究比较了该家族成员的表达和功能。AFP在幼苗中受脱落酸和/或脱水胁迫诱导，但诱导的发育时间不同。AFP1或AFP2的突变导致对脱落酸和盐的敏感性增加，而afp4突变体对脱落酸有轻度抗性。AFP2与AFP1一样，在功能上位于ABI5之上。与野生型幼苗相比，突变体在胁迫下种子萌发或幼苗生长减少与ABI5蛋白水平升高相关，但尚不清楚这是生长减少的原因还是结果。虽然ABI5和AFP均受脱落酸诱导，但在高脱落酸浓度下，ABI5:AFP比值增加，在严重胁迫下维持生长抑制。一个补充afp2突变的AFP2:GFP融合蛋白在受胁迫的幼苗中定位于细胞核，但在胁迫解除后降解之前会发生去定位。AFP也可能在不同程度上与许多与ABI5相关的bZIP转录因子相互作用。本研究表明，种子萌发和幼苗生长受至少两个功能冗余家族（AFP和与ABI5相关的蛋白）之间的拮抗相互作用调节，为微调幼苗应激反应提供了一种机制。
