Magnesium (Mg) is essential for many biological processes in plant cells, and its deficiency causes yield reduction in crop systems. Low Mg status reportedly affects photosynthesis, sucrose partitioning and biomass allocation. However, earlier physiological responses to Mg deficiency are scarcely described. Here, we report that Mg deficiency in Arabidopsis thaliana first modified the mineral profile in mature leaves within 1 or 2 days, then affected sucrose partitioning after 4 days, and net photosynthesis and biomass production after 6 days. The short-term Mg deficiency reduced the contents of phosphorus (P), potassium, manganese, zinc and molybdenum in mature but not in expanding (young) leaves. While P content decreased in mature leaves, P transport from roots to mature leaves was not affected, indicating that Mg deficiency triggered retranslocation of the mineral nutrients from mature leaves. A global transcriptome analysis revealed that Mg deficiency triggered the expression of genes involved in defence response in young leaves.
Minerals and photosynthates are essential for many plant processes, but their imaging in live plants is difficult. We have developed a method for their live imaging in Arabidopsis using a real-time radioisotope imaging system. When each radioisotope, 22Na, 28Mg, 32P-phosphate, 35S-sulfate, 42K, 45Ca, 54Mn and 137Cs, was employed as an ion tracer, ion movement from root to shoot over 24 h was clearly observed. The movements of 22Na, 42K, 32P, 35S and 137Cs were fast so that they spread to the tip of stems. In contrast, high accumulation of 28Mg, 45Ca and 54Mn was found in the basal part of the main stem. Based on this time-course analysis, the velocity of ion movement in the main stem was calculated, and found to be fastest for S and K among the ions we tested in this study. Furthermore, application of a heat-girdling treatment allowed determination of individual ion movement via xylem flow alone, excluding phloem flow, within the main stem of 43-day-old Arabidopsis inflorescences. We also successfully developed a new system for visualizing photosynthates using labeled carbon dioxide, 14CO2. Using this system, the switching of source/sink organs and phloem flow direction could be monitored in parts of whole shoots and over time. In roots, 14C photosynthates accumulated intensively in the growing root tip area, 200–800 µm behind the meristem. These results show that this real-time radioisotope imaging system allows visualization of many nuclides over a long time-course and thus constitutes a powerful tool for the analysis of various physiological phenomena.
Wild species in the genus
Vigna
are a great resource of tolerance to various stresses including salinity. We have previously screened the genetic resources of the genus
Vigna
and identified several accessions that have independently evolved salt tolerance. However, many aspects of such tolerance have remained unknown. Thus, we used autoradiography with radioactive sodium (
22
Na
+
) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to visualize and compare Na
+
allocation in
Vigna angularis
(Willd.) Ohwi & H.Ohashi (azuki bean),
Vigna nakashimae
(Ohwi) Ohwi & H.Ohashi,
Vigna riukiuensis
(Ohwi) Ohwi & H.Ohashi,
Vigna luteola
(Jacq.) Benth. and
Vigna marina
(Burm.) Merr.. The results indicated: 1) Tolerant accessions suppress Na
+
accumulation compared to azuki bean. 2)
V. nakashimae
and
V. marina
does so by accumulating higher amount of K
+
, whereas
V. riukiuensis
and
V. luteola
does so by other mechanisms. 3)
V. luteola
avoids salt-shedding by allocating excess Na
+
to newly expanded leaves. As the mechanisms of the tolerant species were different, they could be piled up in a single crop
via
classical breeding or by genetic engineering or genome editing.
The differences in the transport characteristics in planta between potassium (K+) and caesium (Cs+) was investigated using their radionuclides, 42K+ and 137Cs+. A tracer experiment using nutrient solutions supplemented with 42K and 137Cs revealed that the ratio of the root’s K+ uptake rate to its Cs+ uptake rate was 7–11 times higher than the K+ : Cs+ concentration ratio in the solution, and the number was varied depending on the K concentration in the solution and also on the growth condition. After entering through the root tissues, the 42K+ : 137Cs+ ratio in the shoots was 4.28 times higher than the value in the roots. However, the 42K+ : 137Cs+ ratio in each leaf did not differ significantly, indicating that the primary transport of K+ and Cs+ in the shoots are similarly regulated. In contrast, among the radionuclides stored in the roots over 4 h, 30% of the 42K+ was exported from the roots over the following hour, whereas only 8% of 137Cs+ was exported. In addition, within the xylem, K+ was shown to travel slowly, whereas Cs+ passed quickly through the roots into the shoots. In conclusion, our study demonstrated very different transport patterns for the two ions in the root tissues.
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