Comparative Phylogenetic and Expression Analysis of Small GTPases Families in Legume and Non-Legume Plants

<b>Background:</b> Small monomeric GTPases act as molecular switches in several processes that involve polar cell growth, participating mainly in vesicle trafficking and cytoskeleton rearrangements. This gene superfamily has largely expanded in plants through evolution as compared with other Kingdoms, leading to the suggestion that members of each subfamily might have acquired new functions associated to plant-specific processes. Legume plants engage in a nitrogen-fixing symbiotic interaction with rhizobia in a process that involves polar growth processes associated with the infection throughout the root hair. To get insight into the evolution of small GTPases associated with this process, we use a comparative genomic approach to establish differences in the Ras GTPase superfamily between legume and non-legume plants. <b>Results:</b> Phylogenetic analyses did not show clear differences in the organization of the different subfamilies of small GTPases between plants that engage or not in nodule symbiosis. Protein alignments revealed a strong conservation at the sequence level of small GTPases previously linked to nodulation by functional genetics. Interestingly, one Rab and three Rop proteins showed conserved amino acid substitutions in legumes, but these changes do not alter the predicted conformational structure of these proteins. Although the steady-state levels of most small GTPases do not change in response to rhizobia, we identified a subset of Rab, Rop and Arf genes whose transcript levels are modulated during the symbiotic interaction, including their spatial distribution along the indeterminate nodule <b>Conclusions:</b> This study provides a comprehensive study of the small GTPase superfamily in several plant species. The genetic program associated to root nodule symbiosis includes small GTPases to fulfill specific functions during infection and formation of the symbiosomes. These GTPases seems to have been recruited from members that were already present in common ancestors with plants as distant as monocots since we failed to detect asymmetric evolution in any of the subfamily trees. Expression analyses identified a number of legume members that can have undergone neo- or sub-functionalization associated to the spatio-temporal transcriptional control during the onset of the symbiotic interaction.

**背景：** 小分子单体GTP酶（small monomeric GTPases）在诸多涉及细胞极性生长的生物学过程中充当分子开关，主要参与囊泡运输与细胞骨架重排。相较于其他生物界，该基因超家族在植物演化进程中发生了显著扩增，这提示每个亚家族的成员可能获得了与植物特有生物学过程相关的新功能。豆科植物可与根瘤菌建立固氮共生互作，该过程包含与根毛侵染相关的极性生长事件。为解析与此共生过程相关的小分子GTP酶的演化机制，本研究采用比较基因组学方法，明确豆科与非豆科植物间Ras GTP酶超家族（Ras GTPase superfamily）的差异。
**结果：** 系统发育分析显示，是否具备根瘤共生能力的植物间，小分子GTP酶各亚家族的组织模式并无显著差异。蛋白质序列比对结果表明，此前通过功能遗传学研究被证实与结瘤作用相关的小分子GTP酶，在序列水平上呈现高度保守性。值得注意的是，豆科植物中1个Rab蛋白与3个Rop蛋白存在保守的氨基酸替换，但此类替换并未改变这些蛋白的预测构象结构。尽管多数小分子GTP酶的稳态水平不会因根瘤菌侵染发生改变，但我们鉴定到一组Rab、Rop及Arf基因，其转录水平在共生互作过程中受到调控，包括它们在不定型根瘤中的空间分布模式。
**结论：** 本研究对多个植物物种的小分子GTP酶超家族开展了全面分析。与根瘤共生相关的遗传程序会招募小分子GTP酶，以在侵染过程与共生体形成阶段发挥特定功能。由于我们未在任何亚家族的系统发育树中检测到不对称进化现象，提示这些GTP酶是从与单子叶植物等远缘植物共享的共同祖先中已存在的成员招募而来。表达分析鉴定出一批豆科植物成员，它们可能在共生互作起始阶段的时空转录调控过程中发生了新功能化或亚功能化。