Bieri Stéphane, Mauch Stefan, Shen Qian-Hua, Peart Jack, Devoto Alessandra, Casais Catarina, Ceron Francesca, Schulze Sabine, Steinbiss Hans-Henning, Shirasu Ken, Schulze-Lefert Paul
Max-Planck-Institut für Züchtungsforschung, Department of Plant Microbe Interactions, D-50829 Köln, Germany.
Plant Cell. 2004 Dec;16(12):3480-95. doi: 10.1105/tpc.104.026682. Epub 2004 Nov 17.
The polymorphic barley (Hordeum vulgare) Mla locus harbors allelic race-specific resistance (R) genes to the powdery mildew fungus Blumeria graminis f sp hordei. The highly sequence-related MLA proteins contain an N-terminal coiled-coil structure, a central nucleotide binding (NB) site, a Leu-rich repeat (LRR) region, and a C-terminal non-LRR region. Using transgenic barley lines expressing epitope-tagged MLA1 and MLA6 derivatives driven by native regulatory sequences, we show a reversible and salt concentration-dependent distribution of the intracellular MLA proteins in soluble and membrane-associated pools. A posttranscriptional process directs fourfold greater accumulation of MLA1 over MLA6. Unexpectedly, in rar1 mutant plants that are compromised for MLA6 but not MLA1 resistance, the steady state level of both MLA isoforms is reduced. Furthermore, differential steady state levels of MLA1/MLA6 hybrid proteins correlate with their requirement for RAR1; the RAR1-independent hybrid protein accumulates to higher levels and the RAR1-dependent one to lower levels. Interestingly, yeast two-hybrid studies reveal that the LRR domains of RAR1-independent but not RAR1-dependent MLA isoforms interact with SGT1, a RAR1 interacting protein required for the function of many NB-LRR type R proteins. Our findings implicate the existence of a conserved mechanism to reach minimal NB-LRR R protein thresholds that are needed to trigger effective resistance responses.
多态性大麦（大麦）的Mla基因座含有对白粉病真菌大麦白粉菌的等位基因种族特异性抗性（R）基因。高度序列相关的Mla蛋白包含一个N端卷曲螺旋结构、一个中央核苷酸结合（NB）位点、一个富含亮氨酸重复序列（LRR）区域和一个C端非LRR区域。利用由天然调控序列驱动表达表位标记的Mla1和Mla6衍生物的转基因大麦品系，我们展示了细胞内Mla蛋白在可溶性和膜相关组分中可逆的、依赖盐浓度的分布。转录后过程导致Mla1的积累量比Mla6高四倍。出乎意料的是，在对Mla6抗性有缺陷但对Mla1抗性无缺陷的rar1突变体植物中，两种Mla同工型的稳态水平均降低。此外，Mla1/Mla6杂交蛋白的不同稳态水平与其对RAR1的需求相关；不依赖RAR1的杂交蛋白积累到较高水平，而依赖RAR1的杂交蛋白积累到较低水平。有趣的是，酵母双杂交研究表明，不依赖RAR1但不依赖RAR1的Mla同工型的LRR结构域与SGT1相互作用，SGT1是许多NB-LRR型R蛋白功能所需的一种与RAR1相互作用的蛋白。我们的发现暗示存在一种保守机制，以达到触发有效抗性反应所需的最小NB-LRR R蛋白阈值。
