Diversity of Na⁺ allocation in salt-tolerant species of the genus <i>Vigna</i>

Abstract

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.

Diurnal Regulation of SOS Pathway and Sodium Excretion Underlying Salinity Tolerance ofVigna marina

“…This hypothesis is also supported by our results that SOS1 is located in the first QTL of salt tolerance (Fig 1B) and that SOS1 transcription was highly correlated not only with salt tolerance (Table 1) but the ability of sodium excretion (Figs 4,5). In addition, the pattern of SOS1 expression also well explains the manner of salt allocation observed in this study (Fig 1A) and our previous study (Noda et al, 2022): The lower expression of SOS1 in the root of V. angularis and V. luteola (river) cannot suppress sodium loading to xylem and allow high allocation to the shoot. In contrast, the higher expression of SOS1 in the root of V. marina , even before salt stress, enables immediate excretion of sodium after initiation of salt stress.…”

“…In addition to our previous study that revealed the 22 Na allocation in V. marina, V. luteola (beach) and V. angularis (Noda et al, 2022), we visualized that in V. luteola (river) in the condition of 100 mM NaCl (Fig 1A). The result of V. luteola (river) showed a typical pattern of glycophytes, which was similar to V. angularis, presenting high 22 Na allocation to leaves and low allocation to roots.…”

Starch-dependent sodium accumulation in the leaves of Vigna riukiuensis.

“…To con rm that V. riukiuensis accumulates higher amount of Na + in the leaves, we evaluate Na allocation in V. angularis, V. nakashimae and V. riukiuensis by autoradiograph and mass spectrometry. The results reproduced our previous observations (Yoshida et al, 2016, Noda et al, 2022, where Na allocation to the leaves were low in V. nakashimae but high in V. riukiuensis (Fig. 1).…”

“…The genus Vigna is a reservoir of diversity and many species are adapted to harsh environments, such as marine beach, desert, limestone karsts and marshes (Tomooka et al, 2014, van Zonneveld et al, 2020. As expected, the species collected from coastal areas presented high tolerance to salt (Iseki et al, 2016) The following studies on sodium allocation have revealed that V. riukiuensis accumulates relatively higher amount of sodium in the leaves, whereas V. nakashimae, a close relative of V. riukiuensis, suppresses sodium accumulation to the leaves (Yoshida et al, 2016, Noda et al, 2022. Thus, we have hypothesized that V. riukiuensis has a mechanism of salt-includer, which isolates sodium into vacuoles to lower sodium concentration in cytoplasm.…”

“…Since we previously revealed that V. riukiuensis allocate relatively higher amount of Na to the leaves, we have considered it has an includer-type mechanism, which sequesters excess Na + into vacuoles (Yoshida et al, 2016, Noda et al, 2022. However, recent studies argue that vacuole is not an ideal organelle for Na + sequestration, as tonoplast is permeable to Na + and thus allows back-leak to cytosol (Shabala et al, 2019).…”