Fundamental parenchyma cells are involved in Na+ and Cl- removal ability in rice leaf sheath.

Salt sensitivity in rice plants is associated with the accumulated amount of Na+ and Cl- in shoots and, more significantly, in photosynthetic tissues. Therefore, salt removal ability at the leaf sheath level is an important mechanism of salt tolerance. In the present study we attempted to determine whether rice leaf sheaths excluded Cl- as well as Na+, and to identify the tissues that were involved in the removal ability of both ions. In two rice genotypes, salt-tolerant FL478 and -sensitive IR29, leaf sheaths excluded Na+ and Cl- under NaCl treatment as estimated using their sheath:blade ratios. The sheath:blade ratio of Na+ but not of Cl-, was increased by NaCl treatment. Under NaCl treatment, Na+ concentration was higher in the basal leaf sheath, whereas Cl- concentration was higher in the middle and tip parts. At the tissue level, fundamental parenchyma cells of leaf sheaths retained the highest amounts of Na and Cl when treated with high amount of NaCl. These results imply that the leaf sheath potentially functions to remove excess Na+ and Cl- from xylem vessels in different locations along the axis, with the fundamental parenchyma cells of leaf sheaths being involved in over-accumulation of both Na+ and Cl-.

Experimental validation of plant peroxisomal targeting prediction algorithms by systematic comparison of in vivo import efficiency and in vitro PTS1 binding affinity.

Most peroxisomal matrix proteins possess a C-terminal targeting signal type 1 (PTS1). Accurate prediction of functional PTS1 sequences and their relative strength by computational methods is essential for determination of peroxisomal proteomes in silico but has proved challenging due to high levels of sequence variability of non-canonical targeting signals, particularly in higher plants, and low levels of availability of experimentally validated non-canonical examples. In this study, in silico predictions were compared with in vivo targeting analyses and in vitro thermodynamic binding of mutated variants within the context of one model targeting sequence. There was broad agreement between the methods for entire PTS1 domains and position-specific single amino acid residues, including residues upstream of the PTS1 tripeptide. The hierarchy Leu>Met>Ile>Val at the C-terminal position was determined for all methods but both experimental approaches suggest that Tyr is underweighted in the prediction algorithm due to the absence of this residue in the positive training dataset. A combination of methods better defines the score range that discriminates a functional PTS1. In vitro binding to the PEX5 receptor could discriminate among strong targeting signals while in vivo targeting assays were more sensitive, allowing detection of weak functional import signals that were below the limit of detection in the binding assay. Together, the data provide a comprehensive assessment of the factors driving PTS1 efficacy and provide a framework for the more quantitative assessment of the protein import pathway in higher plants.