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内容記述 |
Understanding the mechanisms of salt tolerance and the underlying genetics is important for the development of practical salt-tolerant crops, given the increasing problems of soil salinization and freshwater shortage. Wild species of the genus Vigna are a great resource for tolerance to various stresses, including salinity. In our studies, we used radio-isotope (RI) imaging to reveal the following unique mechanisms of salt tolerance in the genus Vigna 1) Four salt-tolerant species in the genus Vigna have developed individual mechanisms of salt tolerance. 2) V. riukiuensis has the Na-trapping system by a special starch granule that accumulates in the leaves. 3) Diurnal Na excretion in V. marina by high expression of VmSOS1 and diurnal regulation of VmSOS2. As the mechanisms of the tolerant species were different, it would be able to pile up in a single crop via various breeding techniques and would breed the high salt-tolerant crops. This knowledge will be valuable information for future crop development, especially for cropping with saline water and soil. 1 Diversity of Na+ allocation in four salt-tolerant species We surveyed Na accumulation and allocation in four salt-tolerant species (V. nakashimae, V. riukiuensis, V. luteola and V. marina), which identified as highly salt-tolerant plants [1] using radio-sodium (22Na) and autoradiographic techniques. We performed 22Na+ tracer experiment by treating the plants with 100 mM NaCl including 22Na+ for 3 days in hydroponic culture. As a result, we revealed the different mechanisms of Na+ allocation of the salt-tolerant species in the genus Vigna. 1) V. nakashimae suppresses Na+ allocation to the leaf and shoot apex. 2) V. riukiuensis accumulates a higher amount of Na+ in the leaves. 3) As new leaves grow and expand, V. luteola changes the accumulation of excess Na+ from old to new leaf. 4) V. marina evacuates Na+ by the root and suppresses Na+ allocation to the stem and the leaf. As shown in Fig. 1, wild species have acquired various salt-tolerance mechanisms. Identifying one or more of these salt-tolerance-related genes and introducing them into crops will make it possible to increase the salt tolerance of crops and is expected to contribute to the realization of cultivation on saline soils or with saline water [2].2 Na trapping system by unique starch granule in the leaves of V. riukiuensis As described above, V. riukiuensis accumulates a higher amount of Na+ in the leaves. We first observed vacuoles of the leaf cells to see whether there are any structural differences between V. riukiuensis and other species. However, we could not find any characteristic features in the vacuoles (Fig. 2A). Instead, we found an abundance of starch granules in the chloroplast of V. riukiuensis leaves. Then, we referred to a report that common reed accumulates starch granules that bind Na at the shoot base [3], we were intrigued to test whether the starch granules in V. riukiuensis also have ability to bind Na. We shaded some leaves of V. riukiuensis plants for 24 h to have all the starch granules degraded whether starch granules in V. riukiuensis positively affected Na allocation to the leaves. The plant with shaded leaves was treated with 22Na+ hydroponics solution and the autoradiographic image was taken. It showed that the shaded leaves did not accumulate 22Na+, while all other leaves exhibited strong indication of 22Na+ allocation. We performed scanning electron microscope-energy dispersive x-ray spectrometry (SEM-EDX) to locate Na in leaf and could detect Na in the cross sections of V. riukiuensis, especially in chloroplasts. Moreover, Na was not in the middle of starch granules but was enriched around them, suggesting that Na was trapped by starch granules (Fig. 2B).[4]3 Diurnal Regulation of SOS Pathway and Na Excretion in V. marina V. marina is the most salt-tolerant species in the genus Vigna. In our previous study, we revealed V. marina allocates the least amount of Na in leaves, stems and roots compared to other species. However, it was unclear whether V. marina simply suppresses Na+ uptake from the root or actively excretes Na+ that is once loaded to xylem. To observe Na+ dynamics, we performed the positron-emitting tracer imaging system (PETIS) which enables real-time mapping of sodium in living plants. The imaging revealed that ²²Na levels decreased in the root while increasing in the hydroponic culture. Remarkably, we could find that V. marina actively excretes ²²Na from the root during the light period but not the dark period (Fig. 3). Based on the PETIS results, we conducted transcriptional analysis and confirmed that the SOS1 gene, which encodes one of the most well-known Na+/H+ antiporters, was consistently highly expressed in V. marina. We also found that the expression level of VmSOS2, which is required for activating VmSOS1, was upregulated in the light period and downregulated in the dark period. These findings indicate that the Na+ excretion ability of V. marina is associated with the expression level of VmSOS1, and that the presence or absence of diurnal Na+ excretion is determined by the expression pattern of VmSOS2. [5]4 CONCLUSIONS In addition to Na excretion in V. marina, we will identify genes related to salt tolerance, such as Na-binding to starch in V. riukiuensis, prevention of Na accumulation in the leaves of V. nakashimae, and selective Na accumulation in specific leaves of V. luteola. It would probably be possible to develop crop varieties with much higher salt tolerance. |
