Department of Plant Physiology and Biochemistry, Biology Section, University of Konstanz, Universitätsstrasse 10, 78464 Konstanz, Germany.
BMC Plant Biol. 2010 Apr 19;10:70. doi: 10.1186/1471-2229-10-70.
Proline (Pro) accumulation is a widespread response of prokaryotic and eukaryotic cells subjected to osmotic stress or dehydration. When the cells are released from stress, Pro is degraded to glutamate by Pro-dehydrogenase (ProDH) and Pyrroline-5-carboxylate dehydrogenase (P5CDH), which are both mitochondrial enzymes in eukaryotes. While P5CDH is a single copy gene in Arabidopsis, two ProDH genes have been identified in the genome. Until now, only ProDH1 (At3g30775) had been functionally characterised.
We demonstrate vasculature specific expression of the Arabidopsis ProDH2 gene (At5g38710) as well as enzymatic activity and mitochondrial localisation of the encoded protein. Expression levels of ProDH2 are generally low, but increased in senescent leaves and in the abscission zone of floral organs. While sucrose represses ProDH2 expression, Pro and NaCl were identified as inducers. Endogenous ProDH2 expression was not able to overcome Pro sensitivity of ProDH1 mutants, but overexpression of a GFP-tagged form of ProDH2 enabled the utilisation of Pro as single nitrogen source for growth. Amongst two intronic insertion mutants, one was identified as a null allele, whereas the other still produced native ProDH2 transcripts.
Arabidopsis possesses two functional ProDHs, which have non-redundant, although partially overlapping physiological functions. The two ProDH isoforms differ with respect to spatial, developmental and environmental regulation of expression. While ProDH1 appears to be the dominant isoform under most conditions and in most tissues, ProDH2 was specifically upregulated during salt stress, when ProDH1 was repressed. The characterisation of ProDH2 as a functional gene requires a careful re-analysis of mutants with a deletion of ProDH1, which were so far considered to be devoid of ProDH activity. We hypothesise that ProDH2 plays an important role in Pro homeostasis in the vasculature, especially under stress conditions that promote Pro accumulation.
脯氨酸（Pro）积累是原核和真核细胞受到渗透胁迫或脱水时的普遍反应。当细胞从应激中释放出来时，Pro 被 Pro-脱氢酶（ProDH）和吡咯啉-5-羧酸脱氢酶（P5CDH）降解为谷氨酸，这两种酶都是真核生物的线粒体酶。虽然 P5CDH 在拟南芥中是一个单拷贝基因，但在基因组中已经鉴定出两个 ProDH 基因。到目前为止，只有 ProDH1（At3g30775）的功能得到了表征。
我们证明了拟南芥 ProDH2 基因（At5g38710）在血管组织中的特异性表达，以及编码蛋白的酶活性和线粒体定位。ProDH2 的表达水平通常较低，但在衰老叶片和花器官的离区中增加。虽然蔗糖抑制 ProDH2 的表达，但 Pro 和 NaCl 被鉴定为诱导物。内源性 ProDH2 的表达不能克服 ProDH1 突变体对 Pro 的敏感性，但过表达 GFP 标记的 ProDH2 形式使植物能够利用 Pro 作为唯一的氮源进行生长。在两个内含子插入突变体中，一个被鉴定为无功能等位基因，而另一个仍产生天然的 ProDH2 转录本。
拟南芥拥有两种功能齐全的 ProDH，它们具有非冗余的、尽管部分重叠的生理功能。这两种 ProDH 同工酶在表达的时空、发育和环境调控方面存在差异。虽然 ProDH1 似乎是大多数条件和大多数组织中占主导地位的同工酶，但 ProDH2 在 ProDH1 被抑制的盐胁迫下特异性地上调。ProDH2 作为一个功能性基因的特征需要对缺失 ProDH1 的突变体进行仔细的重新分析，这些突变体迄今为止被认为缺乏 ProDH 活性。我们假设 ProDH2 在血管中的 Pro 稳态中发挥重要作用，尤其是在促进 Pro 积累的应激条件下。
