Trends in Plant Science
OpinionFriend or Foe? Chloride Patterning in Halophytes
Section snippets
Chloride – A Toxin or a Beneficial Osmoticum?
The physiological and molecular mechanisms conferring plant salinity tolerance have been intensively studied over the past four decades. Most investigations have focused on Na+, but the past few years have witnessed a ‘renaissance period’ for Cl− research. This interest is mainly driven by the need to fully understand the role of Cl− as a nutrient and its conflicting role in limiting plant growth in saline soils 1, 2, 3, 4, 5. However, this current attention on the role of Cl− in plant salinity
Should Chloride Be Excluded?
In non-halophytes, the current notion is that Cl− exclusion from the shoot (either from the root epidermis or the xylem) is crucial for salt tolerance 1, 3, 7, 8, 9, 10, 11, 12. These arguments are supported by findings in some salt-sensitive species that high shoot Cl− levels correlate with severe physiological dysfunctions that not only affect the yield but also the quality and edibility of non-halophytic crops 4, 13, 14. Nonetheless, tissue Cl− concentrations in halophytes can exceed 500 mM 15
Energetics of Chloride Transport
The cellular targets for Cl− toxicity remain elusive, but high Cl− concentrations have been found to interfere with root NO3− uptake [31], stomatal regulation, and leaf photosynthetic capacity 11, 32. Thus, using Cl− for osmotic adjustment comes with the risk of overaccumulation and potential toxicity to cellular functions. In most plant cells, the large central vacuole serves as a storage reservoir where, during salt stress, cytotoxic Cl− and Na+ are sequestered away to maintain optimal
Do Chloride Transporters Differ between Halophytes and Non-Halophytes?
The mechanisms of Cl− uptake at the root PM in halophytes remain to be discovered. In non-halophytes, the molecular identity of transporters likely to catalyse Cl− transport at the root PM has recently been revealed (Box 1), and these putative Cl− transport systems could also occur in halophytes. However, the picture is far from complete. More in-depth phylogenetic and functional analyses of these transporter/channel families are required both in halophytes and non-halophytes. Recent evidence
How Is Chloride Transport Regulated?
Compared to their non-halophytic relatives, halophytes have enhanced constitutive expression of specific gene families involved in ion transport (e.g., NHXs, HKTs, SLAHs, and AHAs 57, 58, 59, 60). Nevertheless, salt-induced changes in protein activity are not always associated with changes in transcriptional regulation, emphasising the important role of post-translational regulation 33, 35, 39, 57, 58. Anion transport is controlled by many cytosolic factors 61, 62, and it is plausible that the
Concluding Remarks and Future Perspectives
Regulation of Cl− uptake and translocation in plants is a significant issue, even more so under saline conditions where Cl− toxicity potentially comes into play. We argue here that, under saline conditions, halophytes: (i) require Cl− as an osmoticum for cell turgor and growth, (ii) invest metabolic energy for Cl− uptake by roots, and (iii) regulate Cl− influx rather than relying on efflux to tightly control net root Cl− uptake. The ability of halophytes to limit Cl− influx over the long term
Acknowledgments
N.B. is a recipient of a Marie Curie Fellowship (grant 700001).
Glossary
- Active ion transport
- movement of ions across a membrane against the electrochemical gradient, thus requiring energy. Passive ion transport does not require energy input.
- Calcineurin B-like protein (CBL)–CBL-interacting protein kinase (CIPK) network
- a signalling network that acts in diverse plant stress responses. In plants, different environmental stimuli generate specific Ca2+ signatures. The CBL proteins are one of the plant sensors that perceive these Ca2+ signatures and then interact with and
References (90)
Chloride on the move
Trends Plant Sci.
(2017)Review on the significance of chlorine for crop yield and quality
Plant Sci.
(2018)Chloride stress triggers maturation and negatively affects the postharvest quality of persimmon fruit. Involvement of calyx ethylene production
Plant Physiol. Biochem.
(2016)Modification of the chloride requirement for photosynthetic O2 evolution
FEBS Lett.
(1984)Photosynthetic oxygen evolution in relation to ion contents in the chloroplasts of Suaeda maritima
Plant Sci. Lett.
(1984)Constitutive high-level SOS1 expression and absence of HKT1;1 expression in the salt-accumulating halophyte Salicornia dolichostachya
Plant Sci.
(2015)- et al.
Chloride in soils and its uptake and movement within the plant: a review
Ann. Bot.
(2001) Differential tolerance to combined salinity and O2 deficiency in the halophytic grasses Puccinellia ciliata and Thinopyrum ponticum: the importance of K+ retention in roots
Environ. Exp. Bot.
(2013)- et al.
Futile cycling at the plasma membrane: a hallmark of low-affinity nutrient transport
Trends Plant Sci.
(2006) The role of mitochondrial respiration in salinity tolerance
Trends Plant Sci.
(2011)