Department of Cell Biology and Molecular Genetics, Maryland Agricultural Experiment Station, University of Maryland College Park, MD, USA.
Front Plant Sci. 2012 Feb 14;3:25. doi: 10.3389/fpls.2012.00025. eCollection 2012.
All organisms have evolved strategies to regulate ion and pH homeostasis in response to developmental and environmental cues. One strategy is mediated by monovalent cation-proton antiporters (CPA) that are classified in two superfamilies. Many CPA1 genes from bacteria, fungi, metazoa, and plants have been functionally characterized; though roles of plant CPA2 genes encoding K(+)-efflux antiporter (KEA) and cation/H(+) exchanger (CHX) families are largely unknown. Phylogenetic analysis showed that three clades of the CPA1 Na(+)-H(+) exchanger (NHX) family have been conserved from single-celled algae to Arabidopsis. These are (i) plasma membrane-bound SOS1/AtNHX7 that share ancestry with prokaryote NhaP, (ii) endosomal AtNHX5/6 that is part of the eukaryote Intracellular-NHE clade, and (iii) a vacuolar NHX clade (AtNHX1-4) specific to plants. Early diversification of KEA genes possibly from an ancestral cyanobacterium gene is suggested by three types seen in all plants. Intriguingly, CHX genes diversified from three to four members in one subclade of early land plants to 28 genes in eight subclades of Arabidopsis. Homologs from Spirogyra or Physcomitrella share high similarity with AtCHX20, suggesting that guard cell-specific AtCHX20 and its closest relatives are founders of the family, and pollen-expressed CHX genes appeared later in monocots and early eudicots. AtCHX proteins mediate K(+) transport and pH homeostasis, and have been localized to intracellular and plasma membrane. Thus KEA genes are conserved from green algae to angiosperms, and their presence in red algae and secondary endosymbionts suggest a role in plastids. In contrast, AtNHX1-4 subtype evolved in plant cells to handle ion homeostasis of vacuoles. The great diversity of CHX genes in land plants compared to metazoa, fungi, or algae would imply a significant role of ion and pH homeostasis at dynamic endomembranes in the vegetative and reproductive success of flowering plants.
所有生物体都进化出了调节离子和 pH 值稳态的策略，以响应发育和环境线索。一种策略是由单价阳离子-质子反向转运体（CPA）介导的，这些转运体分为两个超家族。已经对来自细菌、真菌、后生动物和植物的许多 CPA1 基因进行了功能表征；尽管编码 K(+)外排反向转运体（KEA）和阳离子/H(+)交换体（CHX）家族的植物 CPA2 基因的作用在很大程度上尚不清楚。系统发育分析表明，从单细胞藻类到拟南芥，CPA1Na(+)-H(+)交换体（NHX）家族的三个分支已经保守。这些分支是（i）质膜结合的 SOS1/AtNHX7，与原核生物 NhaP 有共同的祖先，（ii）内体的 AtNHX5/6，是真核生物 Intracellular-NHE 分支的一部分，以及（iii）植物特有的液泡 NHX 分支（AtNHX1-4）。从所有植物中看到的三种类型表明，KEA 基因可能从祖先蓝细菌基因中早期多样化。有趣的是，CHX 基因在早期陆地植物的一个亚科中从三种多样化到四个成员，在拟南芥的八个亚科中多样化到 28 个基因。来自水绵或石松的同源物与 AtCHX20 具有高度相似性，表明保卫细胞特异性的 AtCHX20 及其最接近的亲属是该家族的创始人，并且在单子叶植物和早期真双子叶植物中出现了花粉表达的 CHX 基因。AtCHX 蛋白介导 K(+)转运和 pH 值稳态，并被定位到细胞内和质膜。因此，KEA 基因从绿藻到被子植物都是保守的，它们在红藻和二次内共生体中的存在表明它们在质体中具有作用。相比之下，AtNHX1-4 亚型在植物细胞中进化，以处理液泡的离子稳态。与后生动物、真菌或藻类相比，陆地植物中 CHX 基因的巨大多样性意味着离子和 pH 值稳态在有花植物的营养和生殖成功中的动态内膜中的重要作用。
