SUMMARYHeavy metals such as cadmium (Cd) and mercury (Hg) are toxic pollutants that are detrimental to living organisms. Plants employ a two-step mechanism to detoxify toxic ions. First, phytochelatins bind to the toxic ion, and then the metal-phytochelatin complex is sequestered in the vacuole. Two ABCC-type transporters, AtABCC1 and AtABCC2, that play a key role in arsenic detoxification, have recently been identified in Arabidopsis thaliana. However, it is unclear whether these transporters are also implicated in phytochelatindependent detoxification of other heavy metals such as Cd(II) and Hg(II). Here, we show that atabcc1 single or atabcc1 atabcc2 double knockout mutants exhibit a hypersensitive phenotype in the presence of Cd(II) and Hg(II). Microscopic analysis using a Cd-sensitive probe revealed that Cd is mostly located in the cytosol of protoplasts of the double mutant, whereas it occurs mainly in the vacuole of wild-type cells. This suggests that the two ABCC transporters are important for vacuolar sequestration of Cd. Heterologous expression of the transporters in Saccharomyces cerevisiae confirmed their role in heavy metal tolerance. Over-expression of AtABCC1 in Arabidopsis resulted in enhanced Cd(II) tolerance and accumulation. Together, these results demonstrate that AtABCC1 and AtABCC2 are important vacuolar transporters that confer tolerance to cadmium and mercury, in addition to their role in arsenic detoxification. These transporters provide useful tools for genetic engineering of plants with enhanced metal tolerance and accumulation, which are desirable characteristics for phytoremediation.

Cytokinins are phytohormones that induce cytokinesis and are essential for diverse developmental and physiological processes in plants. Cytokinins of the trans-zeatin type are mainly synthesized in root vasculature and transported to the shoot, where they regulate shoot growth. However, the mechanism of long-distance transport of cytokinin was hitherto unknown. Here, we report that the Arabidopsis ATP-binding cassette (ABC) transporter subfamily G14 (AtABCG14) is mainly expressed in roots and plays a major role in delivering cytokinins to the shoot. Loss of AtABCG14 expression resulted in severe shoot growth retardation, which was rescued by exogenous trans-zeatin application. Cytokinin content was decreased in the shoots of atabcg14 plants and increased in the roots, with consistent changes in the expression of cytokinin-responsive genes. Grafting of atabcg14 scions onto wild-type rootstocks restored shoot growth, whereas wild-type scions grafted onto atabcg14 rootstocks exhibited shoot growth retardation similar to that of atabcg14. Cytokinin concentrations in the xylem are reduced by ∼90% in the atabcg14 mutant. These results indicate that AtABCG14 is crucial for the translocation of cytokinin to the shoot. Our results provide molecular evidence for the long-distance transport of cytokinin and show that this transport is necessary for normal shoot development.ABC transporter | tZ-type cytokinin translocation | long-distance signal I n plants, roots and shoots communicate to synchronize and optimize growth in response to environmental changes. The autotrophic shoot undergoes photosynthesis, and the products of photosynthesis are used as an energy source for root growth. The roots absorb water and nutrients from the soil and deliver these to the shoots. Thus, the growth of these two structures is coordinated; a change in root growth alters the growth of the shoot, and vice versa. The coordination requires communication mediated by signal molecules that move between the aboveground and belowground structures. The dominant signals transmitted between roots and shoots are (i) cytokinin, which is translocated both from roots to shoots and from shoots to roots (1, 2), and (ii) auxin, which moves from shoots to roots. Whereas the long-distance transport of auxin has been thoroughly investigated (3,4), that of cytokinin is largely unexplored at the molecular level (1, 2, 5).Cytokinins are a group of phytohormones that promote cytokinesis. They are involved in a myriad of developmental and physiological processes, including the maintenance of shoot and cambial meristem activities, which determine shoot size and structure (6). Several compounds that exhibit cytokinin activity have been characterized, including trans-zeatin (tZ), N 6 -(Δ 2 -isopentenyl) adenine (iP), and cis-zeatin (cZ). These are active at the site of synthesis (7), as well as in distant tissues, which they reach via long-distance transport. Recent studies revealed that tZ-type cytokinins are translocated from roots to shoots via xylem to regulate shoot...

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.

Arsenic (As) is a chronic poison that causes severe skin lesions and cancer. Rice (Oryza sativa L.) is a major dietary source of As; therefore, reducing As accumulation in the rice grain and thereby diminishing the amount of As that enters the food chain is of critical importance. Here, we report that a member of the Oryza sativa C-type ATP-binding cassette (ABC) transporter (OsABCC) family, OsABCC1, is involved in the detoxification and reduction of As in rice grains. We found that OsABCC1 was expressed in many organs, including the roots, leaves, nodes, peduncle, and rachis. Expression was not affected when plants were exposed to low levels of As but was up-regulated in response to high levels of As. In both the basal nodes and upper nodes, which are connected to the panicle, OsABCC1 was localized to the phloem region of vascular bundles. Furthermore, OsABCC1 was localized to the tonoplast and conferred phytochelatin-dependent As resistance in yeast. Knockout of OsABCC1 in rice resulted in decreased tolerance to As, but did not affect cadmium toxicity. At the reproductive growth stage, the As content was higher in the nodes and in other tissues of wild-type rice than in those of OsABCC1 knockout mutants, but was significantly lower in the grain. Taken together, our results indicate that OsABCC1 limits As transport to the grains by sequestering As in the vacuoles of the phloem companion cells of the nodes in rice.A rsenic (As) is a highly toxic metalloid that is classified as a nonthreshold class-1 carcinogen (1, 2). Long-term exposure to As in humans causes a number of diseases, including hyperpigmentation, keratosis, and skin and internal cancers (3). Due to As contamination of drinking water and soil from both anthropogenic and geogenic sources, millions of people worldwide suffer from As toxicity. This problem is particularly serious in countries in South and Southeast Asia, such as India and Bangladesh, where groundwater, which is used both as a drinking water supply and for irrigating rice, contains high concentrations of As (4). Therefore, reducing the As concentration in drinking water and foods is a critical goal for promoting human health.Rice (Oryza sativa L.), a staple food of half of the world's human population, is a major dietary source of As (5, 6). A recent cohort study in West Bengal, India showed that high concentrations of As in rice are associated with elevated genotoxic effects in humans (7). Rice accumulates As in the shoots and grains more efficiently than do other cereal crops such as wheat (Triticum aestivum) and barley (Hordeum vulgare) (8, 9). This higher efficiency has been attributed to the increased bioavailability of As under flooded conditions (such as those found in paddy fields) and the efficient As uptake system in rice (10-12). In the anaerobic paddy field, As is mainly present in the form of arsenite, which is taken up by two silicon (Si) transporters-namely, Lsi1 (low silicon 1), a Si influx transporter, and Lsi2 (low silicon 2), a Si efflux transporter (11). These transpo...

Plant hormones (phytohormones) integrate endogenous and exogenous signals thus synchronizing plant growth with environmental and developmental changes. Similar to animals, phytohormones have distinct source and target tissues, hence controlled transport and focused targeting are required for their functions. Many evidences accumulated in the last years about the regulation of long-distance and directional transport of phytohormones. ATP-binding cassette (ABC) transporters turned out to play major roles in routing phytohormones not only in the plant body but also towards the outer environment. The ABCG-type proteins ABCG25 and ABCG40 are high affinity abscisic acid (ABA) transporters. ABCG14 is highly co-expressed with cytokinin biosynthesis and is the major root-to-shoot cytokinin transporter. Pleiotropic drug resistance1 (PDR1) from Petunia hybrida transports strigolactones (SLs) from the root tip to the plant shoot but also outside to the rhizosphere, where SLs are the main attractants to mycorrhizal fungi. Last but not least, ABCG36 and ABCG37 possibly play a dual role in coumarine and IBA transport.