Xylem Ion Loading and Its Implications for Plant Abiotic Stress Tolerance

Abstract

Plant adaptive potential is critically dependent on efficient communication and coordination of resource allocation and signalling between above and below-ground plant parts. Control of xylem ion loading plays an important role in this process. This review focuses on the molecular identity, tissue-specific expression patterns, and transcriptional and posttranslational regulation of transporters mediating xylem loading of Na + , K + , and Clin plants grown under abiotic stress conditions such as drought, salinity, and soil flooding. The data is discussed in the context of breeding crops for stress resilience, which remains one of the highest priorities for dealing with the global food security challenge. This resilience was present in wild ancestors, but has been then lost during domestication of crop species and exacerbated by the selection for higher yielding cultivars over the last 100 years. Thus, the progress in the field requires a major rethink of current paradigms in crop breeding and the targeting of previously unexplored genes and traits. We argue that control of xylem ion loading is one of these traits and represents an unexplored opportunity for genetic improvement of plant germplasms. HKT transporters. From a biophysical point of view, these transporters are highly suited for active xylem Na + loading under conditions that favour passive outward K + movement across the parenchyma cell plasma membrane (e.g. the depolarised membrane potential that occurs under saline conditions; Wegner et al., 2011). Supporting this notion, strong OsHKT2;1 expression was reported in the stellar root tissues of rice (Jabnoune et al., 2009). NSCC. Xylem Na + loading could also be mediated by a passive mechanism that involves non-selective cation channels (NSCC). NSCC show diverse characteristics in terms of regulation and functional expression (Demidchik & Maathuis, 2007). NSCC do not strongly differentiate between cations, particularly between K + and Na + ; K + /Na + permeability ratios range between 0.3 and 3 (see Pottosin & Dobrovinskaya, 2014 for review). The two major known families of NSCC are the cyclic nucleotide gated channels (CNGC; 20 genes in Arabidopsis) and the ionotropic glutamate receptors (GLR; 20 genes in Arabidopsis), some of which show high expression in root tissues (Demidchik, Davenport, & Tester, 2002; Gobert et al., 2006; Lam et al., 1998). The potential importance of CNGC and GLR in generating cytosolic Ca 2+ signals in response to stress conditions has been proposed (Swarbreck, Colaço, & Davies, 2013) and their engagement in Na + transport at the xylem/symplast boundary is also highly plausible. CNGC. CNGC are considered likely candidates for the voltage-independent NSCC (Demidchik, Davenport, & Tester, 2002; Kaplan, Sherman, & Fromm, 2007). Heterologously expressed CNGCs act as cation transporters (Gobert et al., 2006; Li et al., 2005) and electrophysiological studies have shown that both AtCNGC1 and AtCNGC4 can transport K + and Na + (Balague et al., 2003; Leng et al., 2002). Furthermore, CNGC...

Development of an In Vivo Sensor to Monitor the Effects of Vapour Pressure Deficit (VPD) Changes to Improve Water Productivity in Agriculture

“…We have shown how a bioristor can dynamically monitor the physiological changes correlated with the regulation of the VPD and that the sensor response showed an opposite trend with VPD. The results achieved in this research paper supports in vivo the hypothesis that the reduction of the leaf transpiration rate in low VPD conditions leads to the accumulation of ions in the early phases of the plant response, followed by a decrease in ionic concentration due to re-opening of stomata under high VPD conditions [6,37,41,42]. During reversible transitions in VPD conditions we identified a clear and unique trend of the R signal, giving a direct and immediate response of the sensor from the inside of the plant system.…”

“…In low VPD conditions and low transpiration rate, the bioristor always responded with a rapid increase of the R value (2–4 dpi and 9–12 dpi; Figure 2), presumably because of the accumulation of electrolyte (mainly as Na + and K + ) [36,37] in the xylem sap as consequence of stomatal closure and the subsequent reduction of the transpiration stream [38].…”

Mechanisms of Plant Responses and Adaptation to Soil Salinity

“…Both active and passive transport systems are probably involved, but their respective roles may differ depending on the length of time since salinity onset. 249,250 Passive Na + loading is likely to be mediated by non-selective cation channels (NSCC). 251 Consideration of thermodynamics suggests, however, that under most physiologically relevant scenarios, xylem Na + loading should be an active process (see Shabala 250 for supporting arguments).…”

Calmodulin HvCaM1 Negatively Regulates Salt Tolerance via Modulation of HvHKT1s and HvCAMTA4

“…Another important factor that affects the amount of Na 1 transport to the shoot is the rate of xylem Na 1 loading. The xylem Na 1 loading is thermodynamically active (Shabala, 2013) and is also mediated by either SOS1 or CCC transporters operating at the xylem parenchyma interface (Colmenero-Flores et al, 2007;Ishikawa et al, 2018). Here, we showed that transcript levels of SOS genes are significantly down-regulated in RNAi lines (Fig.…”