Distribution and speciation of Mn in hydrated roots of cowpea at levels inhibiting root growth

Abstract

The phytotoxicity of Mn is important globally due to its increased solubility in acid or waterlogged soils. Short-term (≤24 h) solution culture studies with 150 µM Mn were conducted to investigate the in situ distribution and speciation of Mn in apical tissues of hydrated roots of cowpea [Vigna unguiculata (L.) Walp. cv. Red Caloona] using synchrotron-based techniques. Accumulation of Mn was rapid; exposure to 150 µM Mn for only 5 min resulting in substantial Mn accumulation in the root cap and associated mucigel. The highest tissue concentrations of Mn were in the root cap, with linear combination fitting of the data suggesting that ≥80% of this Mn(II) was associated with citrate. Interestingly, although the primary site of Mn toxicity is typically the shoots, concentrations of Mn in the stele of the root were not noticeably higher than in the surrounding cortical tissues in the short-term (≤24 h). The data provided here from the in situ analyses of hydrated roots exposed to excess Mn are, to our knowledge, the first of this type to be reported for Mn and provide important information regarding plant responses to high Mn in the rooting environment.

Synchrotron-based Techniques Shed Light on Mechanisms of Plant Sensitivity and Tolerance to High Manganese in the Root Environment

“…Analysis using XAS identified most Mn in shoots as Mn(II) complexed with malate or citrate, indicating Mn(II) to be the major form translocated via the xylem. This is consistent with the rapid uptake of Mn by cowpea (Kopittke et al, 2013) and the transport of 54 Mn via the transpiration stream in white lupin (Page et al, 2006). In this study, therefore, responses of species to high Mn were not related to Mn concentration or speciation in roots and stems.…”

“…Few studies, however, have used synchrotronbased techniques to investigate the mechanisms of Mn toxicity and tolerance in agronomic species despite their importance for food production in regions where soils are acidic or intermittently waterlogged. One study on cowpea, with a critical toxicity concentration of only 2 mM Mn (Edwards and Asher, 1982), has shown an accumulation of Mn-citrate in the root cap and associated mucigel within 5 min of exposure to 150 mM Mn (Kopittke et al, 2013).…”

Imaging element distribution and speciation in plant cells

“…Using S-XRF with a fast detector, the As distribution has been imaged in fresh hydrated roots of cowpea (Vigna unguiculata) following short-term exposure to either As(III) or As(V), showing a strong accumulation of As in the region that is likely to be endodermis and/or pericycle cells [36] (Figure 2C). The same approach has been applied to mapping the in situ distribution of Zn, Cu, Ni, Mn, and Se in hydrated tissues [17,[37][38][39]. A recent study combining S-XRF and NanoSIMS has revealed that As is localized in the vacuoles of the companion cells within the phloem of various types of vascular bundles of the rice node, internode, and leaf sheath [14] ( Figure 2D,E).…”

Malate Synthesis and Secretion Mediated by a Manganese-Enhanced Malate Dehydrogenase Confers Superior Manganese Tolerance in Stylosanthes guianensis

“…1, D and E), suggesting that detoxification of internal Mn might be required for S. guianensis to tolerate Mn (Dou et al, 2009;Fernando et al, 2010;Kopittke et al, 2013). For example, with the help of x-ray absorption near-edge spectroscopy and extended x-ray absorption fine-structure spectroscopy techniques, malate has been demonstrated to chelate internal Mn within the leaf tissues in Gossia bidwillii (Fernando et al, 2010).…”

“…For example, with the help of x-ray absorption near-edge spectroscopy and extended x-ray absorption fine-structure spectroscopy techniques, malate has been demonstrated to chelate internal Mn within the leaf tissues in Gossia bidwillii (Fernando et al, 2010). Furthermore, in the roots of cowpea and leaves of Phytolacca acinosa, it has been estimated that 80% and 90% of internal Mn can be bound in complexes with citrate and oxalate, respectively (Xu et al, 2009;Kopittke et al, 2013). Therefore, in this study, changes in the internal concentrations of malate, oxalate, and citrate in response to Mn toxicity were examined in both S. guianensis genotypes.…”