Plant growth under low K + availability or salt stress requires tight control of K + and Na + uptake, long-distance transport, and accumulation. The family of membrane transporters named HKT (for High-Affinity K + Transporters), permeable either to K + and Na + or to Na + only, is thought to play major roles in these functions. Whereas Arabidopsis (Arabidopsis thaliana) possesses a single HKT transporter, involved in Na + transport in vascular tissues, a larger number of HKT transporters are present in rice (Oryza sativa) as well as in other monocots. Here, we report on the expression patterns and functional properties of three rice HKT transporters, OsHKT1;1, OsHKT1;3, and OsHKT2;1. In situ hybridization experiments revealed overlapping but distinctive and complex expression patterns, wider than expected for such a transporter type, including vascular tissues and root periphery but also new locations, such as osmocontractile leaf bulliform cells (involved in leaf folding). Functional analyses in Xenopus laevis oocytes revealed striking diversity. OsHKT1;1 and OsHKT1;3, shown to be permeable to Na + only, are strongly different in terms of affinity for this cation and direction of transport (inward only or reversible). OsHKT2;1 displays diverse permeation modes, Na + -K + symport, Na + uniport, or inhibited states, depending on external Na + and K + concentrations within the physiological concentration range. The whole set of data indicates that HKT transporters fulfill distinctive roles at the whole plant level in rice, each system playing diverse roles in different cell types. Such a large diversity within the HKT transporter family might be central to the regulation of K + and Na + accumulation in monocots.Although it is not clear what levels of Na + are toxic in the plant cell cytosol and actually unacceptable in vivo, the hypothesis that this cation must be excluded from the cytoplasm is widely accepted. The most abundant inorganic cation in the cytosol is K + , in plant as in animal cells. This cation has probably been selected during evolution because it is less chaotropic than Na + (i.e. more compatible with protein structure even at high concentrations; Clarkson and Hanson, 1980). Its selection might also be due to the fact that in primitive cells, which originated in environmental conditions (seawater) where Na + was more abundant than K + , a straightforward process to energize the cell membrane was to accumulate the less abundant cation and to exclude the most abundant one.In the cell, K + plays a role in basic functions, such as regulation of cell membrane polarization, electrical neutralization of anionic groups, and osmoregulation. Concerning the latter function, K + uptake or release is the usual way through which plant cells control their water potential and turgor. Although toxic at high concentrations, Na + can be used as osmoticum and substituted for K and tissue levels have actually been shown to be essential in plant adaptation to salt stress (Greenway and Munns, 1980;Flowers, 1985;Hasegawa...

Plant roots have a large range of functions, including acquisition of water and nutrients, as well as structural support. Dissecting the genetic and molecular mechanisms controlling rice root development is critical for the development of new rice ideotypes that are better adapted to adverse conditions and for the production of sustainably achieved rice yield potential. Most knowledge regarding the gene networks involved in root development has been accumulated in the model dicotyledon plant species Arabidopsis thaliana. Rice, the model monocotyledon species, presents several singularities compared to A. thaliana, including a root architecture characterized by a fibrous root system comprising five types of embryonic and postembryonic roots. The anatomy and morphology of the rice root system, which is typical for a cereal, differs from that of A. thaliana, for instance, by the presence of a lysigenous cortex and additional cell layers compared to the dicotyledon model. Moreover, the structure and functions of the root apical meristem (RAM) of rice are distinct from those of A. thaliana. Recently, several rice root mutants have been identified via forward or reverse genetics, and these will aid in forming hypothesis to characterize either the divergence or conservation of genetic pathways relative to A. thaliana. Furthermore, these mutants will help to identify key genes in rice roots that may be missing in A. thaliana. This review summarizes both classical and recent data concerning the molecular genetics of rice root development, including root anatomy and morphology, RAM structure, RAM patterning, and root mutants.

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