Bracelet salt glands of the recretohalophyte Limonium bicolor:Distribution,morphology,and induction

Halophytes complete their life cycles in saline environments.The recretohalophyte Limonium bicolor has evolved a specialized salt secretory structure,the salt gland,which excretes Na+to avoid salt damage.Typical L.bicolor salt glands consist of 16 cells with four fluorescent foci and four secretory pores.Here,we describe a special type of salt gland at the base of the L.bicolor leaf petiole named bracelet salt glands due to their beaded-bracelet-like shape of blue auto-fluorescence.Bracelet salt glands contain more than 16 cells and more than four secretory pores.Leaf disc secretion measurements and non-invasive micro-test techniques indicated that bracelet salt glands secrete more salt than normal salt glands,which helps maintain low Na+levels at the leaf blade to protect the leaf.Cytokinin treatment induced bracelet salt gland differentiation,and the developed ones showed no further differentiation when traced with a living fluorescence microscopy imager,even though new salt gland development and leaf expansion were observed.Transcriptome revealed a NAC transcription factor gene that participates in bracelet salt gland development,as confirmed by its genome editing and overexpression in L.bicolor.These findings shed light on bracelet salt gland development and may facilitate the engineering of salt-tolerant crops.

Progress in mechanism of salt excretion in recretohalopytes

The recretohalophyte with specialized saltsecreting structures including salt glands and salt bladders can secrete salt from their bodies and easily adapt themselves to many kinds of salt habitats.Salt glands and salt bladders,arose from dermatogen cells,are excretory organs specially adapted for dealing with ionic homeostasis in the cells of recretohalophytes.The main function of salt glands or salt bladders is to secrete excess ions that invade the plant.The structures of salt glands or salt bladders differ among plant species.In addition to structural differences,salt glands also differ in their secretion abilities.In this review,we mainly focus on recent progress in the mechanism of salt excretion of salt glands and salt bladders,and in particular,emphasize the vesicle-mediated secretion systems from the vacuole to the plasmalemma and the possibly involved membrane-bound translocating proteins for salt secretion of plant gland secretory cell.

The genome of the recretohalophyte Limonium bicolor provides insights into salt gland development and salinity adaptation during terrestrial evolution

Halophytes have evolved specialized strategies to cope with high salinity.The extreme halophyte sea lavender(Limonium bicolor)lacks trichomes but possesses salt glands on its epidermis that can excrete harmful ions,such as sodium,to avoid salt damage.Here,we report a high-quality,2.92-Gb,chromosome-scale L.bicolor genome assembly based on a combination of Illumina short reads,single-molecule,real-time long reads,chromosome conformation capture(Hi-C)data,and Bionano genome maps,greatly enriching the genomic information on recretohalophytes with multicellular salt glands.Although the L.bicolor genome contains genes that show similarity to trichome fate genes from Arabidopsis thaliana,it lacks homologs of the decision fate genes GLABRA3,ENHANCER OF GLABRA3,GLABRA2,TRANSPARENT TESTA GLABRA2,and SIAMESE,providing a molecular explanation for the absence of trichomes in this species.We identified key genes(LbHLH and LbTTG1)controlling salt gland development among classical trichome homologous genes and confirmed their roles by showing that their mutations markedly disrupted salt gland initiation,salt secretion,and salt tolerance,thus offering genetic support for the long-standing hypothesis that salt glands and trichomes may share a common origin.In addition,a whole-genome duplication event occurred in the L.bicolor genome after its divergence from Tartary buckwheat and may have contributed to its adaptation to high salinity.The L.bicolor genome resource and genetic evidence reported in this study provide profound insights into plant salt tolerance mechanisms that may facilitate the engineering of salt-tolerant crops.

The RING zinc finger protein LbRZF1 promotes salt gland development and salt tolerance in Limonium bicolor

The recretohalophyte Limonium bicolor thrives in high-salinity environments because salt glands on the above-ground parts of the plant help to expel excess salt.Here,we characterize a nucleus-localized C3HC4(RING-HC)-type zinc finger protein of L.bicolor named RING ZINC FINGER PROTEIN 1(LbRZF1).LbRZF1 was expressed in salt glands and in response to NaCl treatment.LbRZF1 showed no E3 ubiquitin ligase activity.The phenotypes of overexpression and knockout lines for LbRZF1 indicated that LbRZF1 positively regulated salt gland development and salt tolerance in L.bicolor.lbrzf1 mutants had fewer salt glands and secreted less salt than did the wild-type,whereas LbRZF1-overexpressing lines had opposite phenotypes,in keeping with the overall salt tolerance of these plants.A yeast two-hybrid screen revealed that LbRZF1 interacted with LbCATALASE2(LbCAT2)and the transcription factor LbMYB113,leading to their stabilization.Silencing of LbCAT2 or LbMYB113 decreased salt gland density and salt tolerance.The heterologous expression of LbRZF1 in Arabidopsis thaliana conferred salt tolerance to this non-halophyte.We also identified the transcription factor LbMYB48 as an upstream regulator of LbRZF1 transcription.The study of LbRZF1 in the regulation network of salt gland development also provides a good foundation for transforming crops and improving their salt resistance.