Abstract
Salt stress is a complex trait that poses a grand challenge in developing new crops better adapted to saline environments. Some plants, called recretohalophytes, that have naturally evolved to secrete excess salts through salt glands, offer an underexplored genetic resource for examining how plant development, anatomy, and physiology integrate to prevent excess salt from building up to toxic levels in plant tissue. In this review we examine the structure and evolution of salt glands, salt gland-specific gene expression, and the possibility that all salt glands have originated via evolutionary modifications of trichomes. Salt secretion via salt glands is found in more than 50 species in 14 angiosperm families distributed in caryophyllales, asterids, rosids, and grasses. The salt glands of these distantly related clades can be grouped into four structural classes. Although salt glands appear to have originated independently at least 12 times, they share convergently evolved features that facilitate salt compartmentalization and excretion. We review the structural diversity and evolution of salt glands, major transporters and proteins associated with salt transport and secretion in halophytes, salt gland relevant gene expression regulation, and the prospect for using new genomic and transcriptomic tools in combination with information from model organisms to better understand how salt glands contribute to salt tolerance. Finally, we consider the prospects for using this knowledge to engineer salt glands to increase salt tolerance in model species, and ultimately in crops.
Highlights
Plants face many challenges from the abiotic world, and among the most significant of these is salt stress
Studies on Spartina spp. offer multiple snapshots into the leaf transcriptomics that investigate how salt glands contribute to salt tolerance (Baisakh et al, 2008; Ferreira de Carvalho et al, 2013; Bedre et al, 2016)
The trichomes of Arabidopsis thaliana are one of the most well-studied models for plant development at the cellular level, and it was recently suggested that knowledge from Arabidopsis trichome development could be used to guide the engineering of bladder cell-type salt glands in crop plants (Shabala et al, 2014)
Summary
Plants face many challenges from the abiotic world, and among the most significant of these is salt stress. The Type 2 multicellular salt glands of asterids (Figure 1), which are distributed among five families (Figure 2; Table 1), tend to have one or two stalk cells connecting the secretory cells to the basal collecting cells contrasting the structure of the Tamarix-type salt glands (Shimony et al, 1973; Drennan et al, 1987; Das, 2002).
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