Comparing Kinetics of Xylem Ion Loading and Its Regulation in Halophytes and Glycophytes

Abstract

Although control of xylem ion loading is essential to confer salinity stress tolerance, specific details behind this process remain elusive. In this work, we compared the kinetics of xylem Na+ and K+ loading between two halophytes (Atriplex lentiformis and quinoa) and two glycophyte (pea and beans) species, to understand the mechanistic basis of the above process. Halophyte plants had high initial amounts of Na+ in the leaf, even when grown in the absence of the salt stress. This was matched by 7-fold higher xylem sap Na+ concentration compared with glycophyte plants. Upon salinity exposure, the xylem sap Na+ concentration increased rapidly but transiently in halophytes, while in glycophytes this increase was much delayed. Electrophysiological experiments using the microelectrode ion flux measuring technique showed that glycophyte plants tend to re-absorb Na+ back into the stele, thus reducing xylem Na+ load at the early stages of salinity exposure. The halophyte plants, however, were capable to release Na+ even in the presence of high Na+ concentrations in the xylem. The presence of hydrogen peroxide (H2O2) [mimicking NaCl stress-induced reactive oxygen species (ROS) accumulation in the root] caused a massive Na+ and Ca2+ uptake into the root stele, while triggering a substantial K+ efflux from the cytosol into apoplast in glycophyte but not halophytes species. The peak in H2O2 production was achieved faster in halophytes (30 min vs 4 h) and was attributed to the increased transcript levels of RbohE. Pharmacological data suggested that non-selective cation channels are unlikely to play a major role in ROS-mediated xylem Na+ loading.

Back to the Wild: On a Quest for Donors Toward Salinity Tolerant Rice

“…Thus, we call for a paradigm shift in rice breeding and more efforts toward targeting mechanisms of the tissue tolerance. The possible components may include: (1) efficient internal sequestration of Na + in mesophyll cell vacuoles, with a strong emphasis on control of tonoplast leak channels (Shabala et al, 2019); (2) more efficient control of xylem ion loading to allow rapid osmotic adjustment and turgor maintenance in the shoot (Zarei et al, 2019); (3) efficient cytosolic K + retention in both root and leaf mesophyll cells; and (4) incorporating a possibility of Na + sequestration in external structures such as trichomes . This calls for a better utilization of the potential of wild rice where such traits are already present.…”

The genotype-dependent phenotypic landscape of quinoa in salt tolerance and key growth traits

“…Recently, quinoa has been studied as a model plant to understand the molecular mechanisms of abiotic stress tolerance in plants. In the case of salt tolerance, the roles and functions of inorganic ions for osmotic adjustment, 11 tonoplast channels, 55 epidermal bladder cells, 28 , 56–60 and xylem ion loading 61 have been reported. Recently, the role of peroxisome proliferation in heat and drought stress 62 and heat stress responses in quinoa shoots 63 have been also demonstrated.…”

Virus-Mediated Transient Expression Techniques Enable Functional Genomics Studies and Modulations of Betalain Biosynthesis and Plant Height in Quinoa

“…Indeed, in recent years, quinoa has been used as a model plant to explore the molecular mechanisms underlying abiotic stress tolerance in plants. Salt tolerance has been extensively studied with respect to the roles and functions of epidermal bladder cells (Shabala et al, 2014;Kiani-Pouya et al, 2017, 2019, 2020Zou et al, 2017;Böhm et al, 2018), xylem ion loading (Zarei et al, 2020), tonoplast channels (Bonales-Alatorre et al, 2013), and inorganic ions for osmotic adjustment (Hariadi et al, 2011). Unlike rice and maize (Zea mays L.), which require high-light conditions to keep plants healthy, quinoa plants grow well in the growth chambers normally used for the model plant Arabidopsis.…”