Department of Plant Molecular Biology, University of Lausanne, Switzerland.
Plant J. 2012 Mar;69(5):906-20. doi: 10.1111/j.1365-313X.2011.04842.x. Epub 2011 Dec 15.
The central structure of the symbiotic association between plants and arbuscular mycorrhizal (AM) fungi is the fungal arbuscule that delivers minerals to the plant. Our earlier transcriptome analyses identified two half-size ABCG transporters that displayed enhanced mRNA levels in mycorrhizal roots. We now show specific transcript accumulation in arbusculated cells of both genes during symbiosis. Presently, arbuscule-relevant factors from monocotyledons have not been reported. Mutation of either of the Oryza sativa (rice) ABCG transporters blocked arbuscule growth of different AM fungi at a small and stunted stage, recapitulating the phenotype of Medicago truncatula stunted arbuscule 1 and 2 (str1 and str2) mutants that are deficient in homologous ABCG genes. This phenotypic resemblance and phylogenetic analysis suggest functional conservation of STR1 and STR2 across the angiosperms. Malnutrition of the fungus underlying limited arbuscular growth was excluded by the absence of complementation of the str1 phenotype by wild-type nurse plants. Furthermore, plant AM signaling was found to be intact, as arbuscule-induced marker transcript accumulation was not affected in str1 mutants. Strigolactones have previously been hypothesized to operate as intracellular hyphal branching signals and possible substrates of STR1 and STR2. However, full arbuscule development in the strigolactone biosynthesis mutants d10 and d17 suggested strigolactones to be unlikely substrates of STR1/STR2. Interestingly, rice STR1 is associated with a cis-natural antisense transcript (antiSTR1). Analogous to STR1 and STR2, at the root cortex level, the antiSTR1 transcript is specifically detected in arbusculated cells, suggesting unexpected modes of STR1 regulation in rice.
植物与丛枝菌根（AM）真菌共生的中心结构是真菌丛枝，它向植物输送矿物质。我们之前的转录组分析确定了两个半大小的 ABCG 转运蛋白，它们在菌根根中的 mRNA 水平增强。我们现在显示这两个基因在共生过程中在丛枝细胞中的特定转录积累。目前，单子叶植物的丛枝相关因子尚未报道。拟南芥 ABCG 转运蛋白的突变体阻止了不同 AM 真菌的丛枝生长在一个小而发育不良的阶段，再现了 Medicago truncatula 发育不良的丛枝 1 和 2（str1 和 str2）突变体的表型，这些突变体缺乏同源的 ABCG 基因。这种表型相似性和系统发育分析表明 STR1 和 STR2 在被子植物中具有功能保守性。由于缺乏野生型护士植物对 str1 表型的互补，基础真菌营养不良导致的有限丛枝生长被排除在外。此外，发现植物 AM 信号是完整的，因为在 str1 突变体中，丛枝诱导的标记转录积累不受影响。Strigolactones 先前被假设为作为细胞内菌丝分枝信号和 STR1 和 STR2 的可能底物。然而，在 strigolactone 生物合成突变体 d10 和 d17 中，完全的丛枝发育表明 strigolactones不太可能是 STR1/STR2 的底物。有趣的是，水稻 STR1 与一个顺式天然反义转录本（antiSTR1）相关。类似于 STR1 和 STR2，在根皮层水平，antiSTR1 转录本特异性地在丛枝细胞中检测到，表明在水稻中存在意想不到的 STR1 调控模式。
