Guo Jianjun, Chen Jin-Gui
Department of Botany, University of British Columbia, Vancouver, BC, Canada.
BMC Plant Biol. 2008 Oct 23;8:108. doi: 10.1186/1471-2229-8-108.
RACK1 is a versatile scaffold protein in mammals, regulating diverse developmental processes. Unlike in non-plant organisms where RACK1 is encoded by a single gene, Arabidopsis genome contains three RACK1 homologous genes, designated as RACK1A, RACK1B and RACK1C, respectively. Previous studies indicated that the loss-of-function alleles of RACK1A displayed multiple defects in plant development. However, the functions of RACK1B and RACK1C remain elusive. Further, the relationships between three RACK1 homologous genes are unknown.
We isolated mutant alleles with loss-of-function mutations in RACK1B and RACK1C, and examined the impact of these mutations on plant development. We found that unlike in RACK1A, loss-of-function mutations in RACK1B or RACK1C do not confer apparent defects in plant development, including rosette leaf production and root development. Analyses of rack1a, rack1b and rack1c double and triple mutants, however, revealed that rack1b and rack1c can enhance the rack1a mutant's developmental defects, and an extreme developmental defect and lethality were observed in rack1a rack1b rack1c triple mutant. Complementation studies indicated that RACK1B and RACK1C are in principle functionally equivalent to RACK1A. Gene expression studies indicated that three RACK1 genes display similar expression patterns but are expressed at different levels. Further, RACK1 genes positively regulate each other's expression.
These results suggested that RACK1 genes are critical regulators of plant development and that RACK1 genes function in an unequally redundant manner. Both the difference in RACK1 gene expression level and the cross-regulation are likely the molecular determinants of their unequal genetic redundancy.
RACK1是哺乳动物中一种多功能的支架蛋白，参与调控多种发育过程。与非植物生物中由单个基因编码RACK1不同，拟南芥基因组包含三个RACK1同源基因，分别命名为RACK1A、RACK1B和RACK1C。先前的研究表明，RACK1A功能缺失等位基因在植物发育中表现出多种缺陷。然而，RACK1B和RACK1C的功能仍不清楚。此外，三个RACK1同源基因之间的关系也未知。
我们分离出了RACK1B和RACK1C功能缺失突变的等位基因，并研究了这些突变对植物发育的影响。我们发现，与RACK1A不同，RACK1B或RACK1C功能缺失突变在植物发育中并未导致明显缺陷，包括莲座叶的产生和根的发育。然而，对rack1a、rack1b和rack1c双突变体和三突变体的分析表明，rack1b和rack1c可以增强rack1a突变体的发育缺陷，并且在rack1a rack1b rack1c三突变体中观察到极端的发育缺陷和致死性。互补研究表明，RACK1B和RACK1C在功能上原则上等同于RACK1A。基因表达研究表明，三个RACK1基因表现出相似的表达模式，但表达水平不同。此外，RACK1基因正向调控彼此的表达。
这些结果表明，RACK1基因是植物发育的关键调节因子，并且RACK1基因以不等同冗余的方式发挥作用。RACK1基因表达水平的差异和交叉调控都可能是其不等同遗传冗余的分子决定因素。
