Salt tolerant mechanisms in single-cell C₄ species Bienertia sinuspersici and Suaeda aralocaspica (Chenopodiaceae)

Suaeda aralocaspica and Bienertia sinuspersici (Chenopodiaceae), which have unusual mechanisms of C₄ photosynthesis by dimorphic chloroplasts within individual chlorenchyma cells, grow in saline semi-arid desert regions in Central Asia and around the Persian Gulf. Their response to salinity was studied, along with those of the related C₃ Suaeda heterophylla and Kranz (dual cell) type C₄ Suaeda eltonica in subfamily Suaedoideae. Light response curves for CO₂ fixation with salt treatment (200 mM NaCl) indicated these species have high tolerance to salinity. All accumulated glycine betaine (GB) which is known to protect against abiotic stress. Western blots showed that the protein levels of choline monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH), enzymes catalyzing synthesis of GB from choline, increased under salt stress in both single-cell type C₄ species. Their cDNAs for CMO and BADH were isolated, sequenced and phylogenetic analyses showed that CMO for both species, and BADH for B. sinuspersici, are in a clade in subfamily Suaedoideae. Results of in situ immunolocalization experiments with BADH antibody show that both chloroplast types in the single-cell C₄ species and in the Kranz type C₄ S. eltonica function in GB synthesis. Scanning electron microscopy (SEM) and X-ray microanalysis showed B. sinuspersici has very prominent salt glands, which accumulate sodium chloride under salt stress, while S. aralocaspica lacks salt glands. The bases for salt tolerance in these species are discussed considering anatomical and biochemical features.