Terrestrial plants have two to four times more ATP-binding cassette (ABC) transporter genes than other organisms, including their ancestral microalgae. Recent studies found that plants harboring mutations in these transporters exhibit dramatic phenotypes, many of which are related to developmental processes and functions necessary for life on dry land. These results suggest that ABC transporters multiplied during evolution and assumed novel functions that allowed plants to adapt to terrestrial environmental conditions. Examining the literature on plant ABC transporters from this viewpoint led us to propose that diverse ABC transporters enabled many unique and essential aspects of a terrestrial plant's lifestyle, by transporting various compounds across specific membranes of the plant.

SummaryStomata are the tiny valves on the plant surface that mediate gas exchange between the plant and its environment. Stomatal opening needs to be tightly regulated to facilitate CO 2 uptake and prevent excess water loss. Plant Rho-type (ROP) GTPase 2 (ROP2) is a molecular component of the system that negatively regulates light-induced stomatal opening. Previously, ROP-interactive Cdc42-and Rac-interactive binding motif-containing protein 7 (RIC7) was suggested to function downstream of ROP2. However, the underlying molecular mechanism remains unknown.To understand the mechanism by which RIC7 regulates light-induced stomatal opening, we analyzed the stomatal responses of ric7 mutant Arabidopsis plants and identified the target protein of RIC7 using a yeast two-hybrid screen.Light-induced stomatal opening was promoted by ric7 knockout, whereas it was inhibited by RIC7 overexpression, indicating that RIC7 negatively regulates stomatal opening in Arabidopsis. RIC7 interacted with exocyst subunit Exo70 family protein B1 (Exo70B1), a component of the vesicle trafficking machinery. RIC7 and Exo70B1 localized to the plasma membrane region under light or constitutively active ROP2 conditions. The knockout mutant of Exo70B1 and ric7/exo70b1 exhibited retarded light-induced stomatal opening.Our results suggest that ROP2 and RIC7 suppress excess stomatal opening by inhibiting Exo70B1, which most likely participates in the vesicle trafficking required for light-induced stomatal opening.

Concern about global warming has prompted an intense interest in developing economical methods of producing biofuels. Microalgae provide a promising platform for biofuel production, because they accumulate high levels of lipids, and do not compete with food or feed sources. However, current methods of producing algal oil involve subjecting the microalgae to stress conditions, such as nitrogen deprivation, and are prohibitively expensive. Here, we report that the fungicide fenpropimorph rapidly causes high levels of neutral lipids to accumulate in Chlamydomonas reinhardtii cells. When treated with fenpropimorph (10 μg mL-1) for 1 h, Chlamydomonas cells accumulated at least fourfold the amount of triacylglycerols (TAGs) present in the untreated control cells. Furthermore, the quantity of TAGs present after 1 h of fenpropimorph treatment was over twofold higher than that formed after 9 days of nitrogen starvation in medium with no acetate supplement. Biochemical analysis of lipids revealed that the accumulated TAGs were derived mainly from chloroplast polar membrane lipids. Such a conversion of chloroplast polar lipids to TAGs is desirable for biodiesel production, because polar lipids are usually removed during the biodiesel production process. Thus, our data exemplified that a cost and time effective method of producing TAGs is possible using fenpropimorph or similar drugs.

ROP GTPases function as molecular switches in diverse cellular processes. Previously, we showed that ROP2 GTPase is activated upon light irradiation, and thereby negatively regulates light-induced stomatal opening. Here we studied the role of ROP2 during stomatal closure. The expression of a constitutively active form of ROP2 (CArop2) in Arabidopsis thaliana and Vicia faba resulted in slower and reduced stomatal closure in response to abscisic acid (ABA) and CO2. In contrast, the expression of a dominant-negative form of ROP2 (DN-rop2) and the knockout mutation of ROP2 (rop2 KO) promoted ABAinduced stomatal closure in Arabidopsis. As early as 10 min after ABA treatment, ROP2 was inactivated and translocated to the cytoplasm of the stomatal guard cells. To elucidate the mechanism by which active ROP2 suppresses stomatal closure, we monitored endocytotic membrane trafficking, which is regulated by Rho GTPases in animal cells. We found that the endocytosis of plasma membrane (PM), as tracked by FM4-64, was lower in CA-rop2-expressing guard cells than in those of wild-type plants, which suggests that active ROP2 suppresses the endocytotic internalization of PM, a process required for stomatal closure. Together, our results suggest that ROP2 is inactivated by ABA, and that this inactivation is required for the timely stomatal closure.

ABCG subfamily proteins are highly enriched in terrestrial plants. Many of these proteins secrete secondary metabolites that repel or inhibit pathogens. To establish why the ABCG subfamily proteins proliferated extensively during evolution, we constructed phylogenetic trees from a broad range of eukaryotic organisms. ABCG proteins were massively duplicated in land plants and in oomycetes, a group of agronomically important plant pathogens, which prompted us to hypothesize that plant and pathogen ABCGs coevolved. Supporting this hypothesis, full-size ABCGs in host plants (Arabidopsis thaliana and Glycine max) and their pathogens (Hyaloperonospora arabidopsidis and Phytophthora sojae, respectively) had similar divergence times and patterns. Furthermore, generalist pathogens with broad ranges of host plants have diversified more ABCGs than their specialist counterparts. The hypothesis was further tested using an example pair of ABCGs that first diverged during multiplication in a host plant and its pathogen: AtABCG31 of A. thaliana and HpaP802307 of H. arabidopsidis. AtABCG31 expression was activated following infection with H. arabidopsidis, and disrupting AtABCG31 led to increased susceptibility to H. arabidopsidis. Together, our results suggest that ABCG genes in plants and their oomycete pathogens coevolved in an arms race, to extrude secondary metabolites involved in the plant's defense response against pathogens.