Plant cells are not just green yeast.

Are plant cells just yeast cells with chloroplasts? Should plant cell biologists who don’t work on chloroplasts just switch to yeast to help solve the basic questions that are common to all eukaryotes? The recent completion of the sequencing of the entire yeast (Saccharomyces cerevisiae) genome and the availability of hundreds of well-characterized mutants certainly make this a tempting proposition. At the same time, these tools provide unprecedented opportunities for plant biologists. We can exploit these resources, but there is already enough evidence to interpret the results with caution, because plant cells are not just green yeast. The isolation of plant genes by screening cDNA expression libraries for complementation of yeast mutant phenotypes is a valuable technique for identifying gene products that have a specific biochemical activity or are involved in a particular pathway. In addition, the reverse process, testing a previously identified plant gene with sequence similarity to a yeast gene for complementation of the corresponding yeast mutant, has allowed a function to be assigned to the gene product in cases where discovering function in the plant would have been extremely difficult. However, recent data from our laboratory and from other investigators illustrate the need to carefully study protein localization and function in plants rather that relying solely on the results obtained with yeast, as this may be misleading. Plant proteins may not always be localized correctly when expressed in yeast, particularly when overexpressed from a multicopy plasmid, and the specificity of a number of proteins may be dependent on subcellular compartmentation. Care must therefore be taken when analyzing data from yeast expression, and the information obtained with yeast needs to be confirmed as much as possible in the plant. Traditional biochemical approaches in general were unsuccessful in the isolation of genes encoding transporters and channels, whereas yeast complementation enabled a wide variety of different genes to be isolated. For example, the screening of Arabidopsis cDNA libraries for the complementation of two different yeast amino acid transport mutants led to the isolation of the same gene encoding an amino acid permease (NAT2/AAP1; Frommer et al., 1993; Hsu et al., 1993). The Arabidopsis gene is unrelated in sequence to the yeast gene, but clearly the protein has a related transport activity. Similarly, screening for complementation of a yeast mutant defective in K uptake led to the isolation of three different Arabidopsis genes encoding putative K transporters: AKT1 (Sentenac et al., 1992), KAT1 (which is similar in sequence to AKT1 but not allelic; Anderson et al., 1992), and HKT1 (which is unrelated to AKT1/KAT1; Schachtman and Schroeder, 1994). The yeast cells also provide a convenient system for uptake studies using these and other transporters, and have been useful for investigating their transport mechanism and specificity. However, the activity of the transporters in plants may be modulated by their interactions with other proteins that are absent in yeast, and the expression of the genes may be developmentally or environmentally regulated, which may contribute to their specific functions. In addition, the transporters are in general present at the plasma membrane in yeast, and it is assumed that the same is true in plants. This needs to be addressed for each protein individually, as the plant cell has a more complex endomembrane system that presumably also contains a variety of transport activities, and some of these could be mistargeted to the plasma membrane upon expression in yeast. One example of a transporter that appears to be localized differently in yeast and plants has been described recently (Apse et al., 1999; Gaxiola et al., 1999). AtNHX1 from Arabidopsis is a member of a family of intracellular Na/H exchangers and was identified based on sequence similarity to yeast Nhx1. In yeast, Nhx1 is found on a prevacuolar compartment (PVC) and is involved in salt tolerance by mediating Na sequestration in the PVC. The AtNHX1 cDNA is able to complement some of the phenotypes of the yeast nhx1 mutant (Gaxiola et al., 1999), and overexpression of AtNHX1 in Arabidopsis conferred salt tolerance on the transgenic plants (Apse et al., 1999). However, subcellular localization of the protein in plants indicated that it is in fact a tonoplast protein (Apse et al., 1999), and its ability to complement the yeast mutant may reflect its mislocalization to the PVC upon heterologous expression. * Corresponding author; e-mail [email protected]; fax 517–353–9168. Plant Physiology, April 2000, Vol. 122, pp. 999–1001, www.plantphysiol.org © 2000 American Society of Plant Physiologists