SummaryAtHKT1 is a sodium (Na þ ) transporter that functions in mediating tolerance to salt stress. To investigate the membrane targeting of AtHKT1 and its expression at the translational level, antibodies were generated against peptides corresponding to the first pore of AtHKT1. Immunoelectron microscopy studies using anti-AtHKT1 antibodies demonstrate that AtHKT1 is targeted to the plasma membrane in xylem parenchyma cells in leaves. AtHKT1 expression in xylem parenchyma cells was also confirmed by AtHKT1 promoter-GUS reporter gene analyses. Interestingly, AtHKT1 disruption alleles caused large increases in the Na þ content of the xylem sap and conversely reduced the Na þ content of the phloem sap. The athkt1 mutant alleles had a smaller and inverse influence on the potassium (K þ ) content compared with the Na þ content of the xylem, suggesting that K þ transport may be indirectly affected. The expression of AtHKT1 was modulated not only by the concentrations of Na þ and K þ but also by the osmolality of non-ionic compounds. These findings show that AtHKT1 selectively unloads sodium directly from xylem vessels to xylem parenchyma cells. AtHKT1 mediates osmolality balance between xylem vessels and xylem parenchyma cells under saline conditions. Thus AtHKT1 reduces the sodium content in xylem vessels and leaves, thereby playing a central role in protecting plant leaves from salinity stress.

SummaryCytokinesis is a crucial event in the cell cycle of all living cells. In fungal cells, it requires co-ordinated contraction of an actomyosin ring and synthesis of both plasmatic membrane and a septum structure that will constitute the new cell wall end. Schizosaccharomyces pombe contains four essential putative

The chemical composition of the cell wall of Sz. pombe is known as b-1,3-glucan, b-1,6-glucan, a-1,3-glucan and a-galactomannan; however, the three-dimensional interactions of those macromolecules have not yet been clarified. Transmission electron microscopy reveals a three-layered structure: the outer layer is electron-dense, the adjacent layer is less dense, and the third layer bordering the cell membrane is dense. In intact cells of Sz. pombe, the high-resolution scanning electron microscope reveals a surface completely filled with a-galactomannan particles. To better understand the organization of the cell wall and to complement our previous studies, we set out to locate the three different types of b-glucan by immuno-electron microscopy. Our results suggest that the less dense layer of the cell wall contains mainly b-1,6-branched b-1,3-glucan. Occasionally a line of gold particles can be seen, labelling fine filaments radiating from the cell membrane to the a-galactomannan layer, suggesting that some of the radial filaments contain b-1,6-branched b-1,3-glucan. b-1,6-glucan is preferentially located underneath the a-galactomannan layer. Linear b-1,3-glucan is exclusively located in the primary septum of dividing cells. b-1,6-glucan only labels the secondary septum and does not co-localize with linear b-1,3-glucan, while b-1,6-branched b-1,3-glucan is present in both septa. Linear b-1,3-glucan is present from early stages of septum formation and persists until the septum is completely formed; then just before cell division the label disappears. From these results we suggest that linear b-1,3-glucan is involved in septum formation and perhaps the separation of the two daughter cells. In addition, we frequently found b-1,6-glucan label on the Golgi apparatus, on small vesicles and underneath the cell membrane. These results give fresh evidence for the hypothesis that b-1,6-glucan is synthesized in the endoplasmic reticulum-Golgi system and exported to the cell membrane.

Autophagy is a conserved bulk protein degradation process that is proposed to play a role in events that arise when organisms are forced to radically change their fate, including nutritional starvation, differentiation and development. In our present study, we have identified fission yeast autophagy as a bulk protein degradation process induced by the deprivation of environmental nitrogen, the effects of which are known to trigger sexual differentiation as an adaptive response. Autophagy-defective mutants were found to be sterile in the absence of environmental nitrogen, but could complete sexual differentiation when nitrogen was supplied, suggesting that the major function of autophagy is to provide a nitrogen source. In addition, the environmental nitrogen levels act as an autophagy "on/off " switch, whereas components essential for sexual differentiation were dispensable for this regulation. We propose that fission yeast autophagy functions to supply nitrogen and is activated when cells cannot access exogenous nitrogen, thus ensuring that they can adapt and subsequently propagate.

Cell separation in Schizosaccharomyces pombe is achieved by the concerted action of the Eng1 endo-␤-1,3-glucanase and the Agn1 endo-␣-1,3-glucanase, which are transported to the septum and localize to a ringlike structure that surrounds the septum. The requirements for the correct localization of both hydrolases as a ring were analyzed using green fluorescent protein fusion proteins. Targeting to the septum required a functional exocyst, because both proteins failed to localize correctly in sec8-1 or exo70⌬ mutants, suggesting that Agn1 and Eng1 might be two of the cargo proteins present in the vesicles that accumulate in exocyst mutants. Septins and Mid2 were also required for correct formation of a ring. In their absence, Eng1 and Agn1 were found in a disklike structure that spanned the septum, rather than in a ring. Even though septin and mid2⌬ mutants have a cell separation defect, the septum and the distribution of linear ␤-1,3-glucans were normal in these cells, suggesting that mislocalization of Eng1 and Agn1 might be the reason underlying the failure to separate efficiently. Thus, one of the functions of the septin ring would be to act as a positional marker for the localization of hydrolytic proteins to the medial region.