Pb-induced cellular defense system in the root meristematic cells of Allium sativum L

Abstract

BackgroundElectron microscopy (EM) techniques enable identification of the main accumulations of lead (Pb) in cells and cellular organelles and observations of changes in cell ultrastructure. Although there is extensive literature relating to studies on the influence of heavy metals on plants, Pb tolerance strategies of plants have not yet been fully explained. Allium sativum L. is a potential plant for absorption and accumulation of heavy metals. In previous investigations the effects of different concentrations (10-5 to 10-3 M) of Pb were investigated in A. sativum, indicating a significant inhibitory effect on shoot and root growth at 10-3 to 10-4 M Pb. In the present study, we used EM and cytochemistry to investigate ultrastructural alterations, identify the synthesis and distribution of cysteine-rich proteins induced by Pb and explain the possible mechanisms of the Pb-induced cellular defense system in A. sativum.ResultsAfter 1 h of Pb treatment, dictyosomes were accompanied by numerous vesicles within cytoplasm. The endoplasm reticulum (ER) with swollen cisternae was arranged along the cell wall after 2 h. Some flattened cisternae were broken up into small closed vesicles and the nuclear envelope was generally more dilated after 4 h. During 24-36 h, phenomena appeared such as high vacuolization of cytoplasm and electron-dense granules in cell walls, vacuoles, cytoplasm and mitochondrial membranes. Other changes included mitochondrial swelling and loss of cristae, and vacuolization of ER and dictyosomes during 48-72 h. In the Pb-treatment groups, silver grains were observed in cell walls and in cytoplasm, suggesting the Gomori-Swift reaction can indirectly evaluate the Pb effects on plant cells.ConclusionsCell walls can immobilize some Pb ions. Cysteine-rich proteins in cell walls were confirmed by the Gomori-Swift reaction. The morphological alterations in plasma membrane, dictyosomes and ER reflect the features of detoxification and tolerance under Pb stress. Vacuoles are ultimately one of main storage sites of Pb. Root meristematic cells of A. sativum exposed to lower Pb have a rapid and effective defense system, but with the increased level of Pb in the cytosol, cells were seriously injured.

Lead Uptake, Toxicity, and Detoxification in Plants

“…For most plant species, the majority of absorbed lead (approximately 95% or more) is accumulated in the roots, and only a small fraction is translocated to aerial plant parts, as has been reported in Vicia faba, Pisum sativum, and Phaseolus vulgaris (Piechalak et al 2002;Małecka et al 2008;Shahid et al 2011), V. unguiculata (Kopittke et al 2007), Nicotiana tabacum, (Gichner et al 2008), Lathyrus sativus (Brunet et al 2009), Zea mays (Gupta et al 2009), Avicennia marina (Yan et al 2010), non-accumulating Sedum alfredii (Gupta et al 2010), and Allium sativum (Jiang and Liu 2010). Although many metals display the translocation restriction phenomenon mentioned above, this phenomenon is not common to all heavy metals.…”

“…There are several reasons why the transport of lead from roots to aerial plant parts is limited. These reasons include immobilization by negatively charged pectins within the cell wall (Islam et al 2007;Kopittke et al 2007;Arias et al 2010), precipitation of insoluble lead salts in intercellular spaces (Kopittke et al 2007;Islam et al 2007;Meyers et al 2008;Małecka et al 2008), accumulation in plasma membranes (Seregin et al 2004;Islam et al 2007;Jiang and Liu 2010), or sequestration in the vacuoles of rhizodermal and cortical cells (Seregin et al 2004;Kopittke et al 2007). …”

“…Plants have developed various methods for responding to toxic metal exposures. They have internal detoxification mechanisms to deal with metal toxicity that includes selective metal uptake, excretion, complexation by specific ligands, and compartmentalization (Gupta et al 2009;Krzesłowska et al 2010;Maestri et al 2010;Sing et al 2010;Jiang and Liu 2010).…”

Nitric Oxide Increases Pb Tolerance by Lowering Pb Uptake and Translocation as well as Phytohormonal Changes in Cowpea (Vigna unguiculata (L.) Walp.)

“…There are several reasons why the transport of lead from roots to aerial plant parts is limited. These reasons include immobilization by negatively charged pectins within the cell wall, precipitation of insoluble lead salts in intercellular spaces (Islam et al 2007;Kopittke et al 2007), accumulation in plasma membranes (Islam et al 2007;Jiang & Liu 2010;Seregin et al 2004), or sequestration in the vacuoles of rhizodermal and cortical cells (Kopittke et al 2007;Seregin et al 2004). However, these reasons are not sufficient to explain the low rate of lead translocation from root to shoot.…”

Relações da anatomia radicular na absorção, no acúmulo e na tolerância ao chumbo em Echinodorus grandiflorus

“…Em condições de exposição ao Pb, a atividade dos meristemas radiculares tende a diminuir em plantas não tolerantes (Pereira et al, 2013a) podendo demonstrar várias injúrias estruturais (Jiang & Liu, 2010). Desta forma, o aumento na proporção de floema no cilindro vascular das raízes de E. grandiflorus pode ser um importante mecanismo de defesa ao Pb, uma vez que permite manter a alocação de fotoassimilados para o crescimento das raízes.…”