Globally, the majority of Ni hyperaccumulator plants occur on ultramafic soils in tropical regions, and the genus Phyllanthus, from the Phyllanthaceae family, is globally the most represented taxonomical group. Two species from Sabah (Malaysia) are remarkable because Phyllanthus balgooyi can attain >16 wt% of Ni in its phloem exudate, while Phyllanthus rufuschaneyi reaches foliar concentrations of up to 3.5 wt% Ni, which are amongst the most extreme concentrations of Ni in any plant tissue. Synchrotron X-ray fluorescence microscopy, nuclear microbe (micro-PIXE+BS) and (cryo) scanning electron microscopy with energy dispersive spectroscopy were used to spatially resolve the elemental distribution in the plant organs of P. balgooyi and P. rufuschaneyi. The results show that P. balgooyi has extraordinary enrichment of Ni in the (secondary) veins of the leaves, whereas in contrast, in P. rufuschaneyi Ni occurs in interveinal areas. In the roots and stems, Ni is localized mainly in the cortex and phloem but is much lower in the xylem. The findings of this study show that, even within the same genus, the distribution of nickel and other elements, and inferred processes involved with metal hyperaccumulation, can differ substantially between species.
The extraordinary level of accumulation of nickel (Ni) in hyperaccumulator plants is a consequence of specific metal sequestering and transport mechanisms, and knowledge of these processes is critical for advancing an understanding of transition element metabolic regulation in these plants. The Ni biopathways were elucidated in three plant species, Phyllanthus balgooyi, Phyllanthus securinegioides (Phyllanthaceae) and Rinorea bengalensis (Violaceae), that occur in Sabah (Malaysia) on the Island of Borneo. This study showed that Ni is mainly concentrated in the phloem in roots and stems (up to 16.9% Ni in phloem sap in Phyllanthus balgooyi) in all three species. However, the species differ in their leaves - in P. balgooyi the highest Ni concentration is in the phloem, but in P. securinegioides and R. bengalensis in the epidermis and in the spongy mesophyll (R. bengalensis). The chemical speciation of Ni2+ does not substantially differ between the species nor between the plant tissues and transport fluids, and is unambiguously associated with citrate. This study combines ion microbeam (PIXE and RBS) and metabolomics techniques (GC-MS, LC-MS) with synchrotron methods (XAS) to overcome the drawbacks of the individual techniques to quantitatively determine Ni distribution and Ni2+ chemical speciation in hyperaccumulator plants.
Hevea brasiliensis anther calli were genetically transformed using Agrobacterium GV2260 (p35SGUSINT) that harboured the β-glucuronidase (gus) and neomycin phosphotransferase (nptII) genes. β-Glucuronidase protein (GUS) was expressed in the leaves of kanamycin-resistant plants that were regnerated, and the presence of the gene was confirmed by Southern analysis. GUS was also observed to be expressed in the latex and more importantly in the serum fraction. Transverse sections of the leaf petiole from a transformed plant revealed GUS expression to be especially enhanced in the phloem and laticifers. GUS expression was subsequently detected in every one of 194 plants representing three successive vegetative cycles propagated from the original transformant. Transgenic Hevea could thus facilitate the continual production of foreign proteins expressed in the latex.
The Malaysian state of Sabah on the Island of Borneo has recently emerged as a global hotspot of nickel hyperaccumulator plants. This study focuses on the tissue-level distribution of nickel and other physiologically relevant elements in hyperaccumulator plants with distinct phylogenetical affinities. The roots, old stems, young stems and leaves of Flacourtia kinabaluensis (Salicaceae), Actephila alanbakeri (Phyllanthaceae), Psychotria sarmentosa (Rubiaceae) and young stems and leaves of Glochidion brunneum (Phyllanthaceae) were studied using nuclear microprobe (micro-PIXE and micro-BS) analysis. The tissue-level distribution of nickel found in these species has the same overall pattern as in most other hyperaccumulator plants studied previously, with substantial enrichment in the epidermal cells and in the phloem. This study also revealed enrichment of potassium in the spongy and palisade mesophyll of the studied species. Calcium, chlorine, manganese and cobalt were found to be enriched in the phloem and also concentrated in the epidermis and cortex of the studied species. Although hyperaccumulation ostensibly evolved numerous times independently, the basic mechanisms inferred from tissue elemental localization are convergent in these tropical woody species from Borneo Island.
Successful biotrophic plant pathogens can divert host nutrition towards infection sites. Here we describe how the protist Plasmodiophora brassicae establishes a long-term feeding relationship with its host by stimulating phloem differentiation and phloem-specific expression of sugar transporters within developing galls. Development of galls in infected Arabidopsis thaliana plants is accompanied by stimulation of host BREVIS RADIX (BRX), COTYLEDON VASCULAR PATTERN 2 (CVP2) and OCTOPUS (OPS) gene expression leading to an increase in phloem complexity. We characterised how the arrest of this developmental reprogramming influences both the host and the invading pathogen. Furthermore, we found that infection leads to phloem-specific accumulation of SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTERS (SWEET11 and SWEET12) facilitating local distribution of sugars towards the pathogen. Utilising Fourier-transform infrared (FTIR) microspectroscopy to monitor spatial distribution of carbohydrates, we found that infection leads to the formation of a strong physiological sink at the site of infection. High resolution metabolic and structural imaging of sucrose distributions revealed that sweet11 sweet12 double mutants are impaired in sugar transport towards the pathogen, delaying disease progression. This work highlights the importance of precise regulation of sugar partitioning for plant-pathogen interactions and the dependence of P. brassicae's performance on its capacity to induce a phloem sink at the feeding site.