Degola Francesca, De Benedictis Maria, Petraglia Alessandro, Massimi Alberto, Fattorini Laura, Sorbo Sergio, Basile Adriana, Sanità di Toppi Luigi
Department of Life Sciences, University of Parma, I-43124 Parma, Italy Contributed equally to this work and should be considered co-first authors.
'Berenini' Institute, I-43036 Fidenza (PR), Italy.
Plant Cell Physiol. 2014 Nov;55(11):1884-91. doi: 10.1093/pcp/pcu117. Epub 2014 Sep 3.
Lunularia cruciata occupies a very basal position in the phylogenetic tree of liverworts, which in turn have been recognized as a very early clade of land plants. It would therefore seem appropriate to take L. cruciata as the startingpoint for investigating character evolution in plants' metal(loid) response. One of the strongest evolutionary pressures for land colonization by plants has come from potential access to much greater amounts of nutritive ions from surface rocks, compared to water. This might have resulted in the need to precisely regulate trace element homeostasis and to minimize the risk of exposure to toxic concentrations of certain metals, prompting the evolution of a number of response mechanisms, such as synthesis of phytochelatins, metal(loid)-binding thiol-peptides. Accordingly, if the ability to synthesize phytochelatins and the occurrence of an active phytochelatin synthase are traits present in a basal liverwort species, and have been even reinforced in 'modern' tracheophytes, e.g. Arabidopsis thaliana, then such traits would presumably have played an essential role in plant fitness over time. Hence, we demonstrated here that: (i) L. cruciata compartmentalizes cadmium in the vacuoles of the phototosynthetic parenchyma by means of a phytochelatin-mediated detoxification strategy, and possesses a phytochelatin synthase that is activated by cadmium and homeostatic concentrations of iron(II) and zinc; and (ii) A. thaliana phytochelatin synthase displays a higher and broader response to several metal(loid)s [namely: cadmium, iron(II), zinc, copper, mercury, lead, arsenic(III)] than L. cruciata phytochelatin synthase.
十字叶藓在苔类植物的系统发育树中占据非常基部的位置，而苔类植物又被认为是陆地植物中非常早期的一个分支。因此，将十字叶藓作为研究植物金属（类金属）响应中特征进化的起点似乎是合适的。植物陆地定殖的最强进化压力之一来自于与水相比，从地表岩石中获取大量营养离子的潜在可能性。这可能导致需要精确调节微量元素稳态，并将接触某些金属有毒浓度的风险降至最低，从而促使了一些响应机制的进化，例如植物螯合肽的合成、金属（类金属）结合硫醇肽。因此，如果合成植物螯合肽的能力和活性植物螯合肽合酶的存在是一个基部苔类物种所具有的特征，并且在“现代”维管植物（如拟南芥）中得到了进一步强化，那么随着时间的推移，这些特征可能在植物适应性方面发挥了重要作用。因此，我们在此证明：（i）十字叶藓通过植物螯合肽介导的解毒策略将镉隔离在光合薄壁组织的液泡中，并拥有一种被镉以及铁（II）和锌的稳态浓度激活的植物螯合肽合酶；（ii）拟南芥植物螯合肽合酶对几种金属（类金属）[即：镉、铁（II）、锌、铜、汞、铅、砷（III）]的响应比十字叶藓植物螯合肽合酶更高、更广泛。
