Plant ABC Transporters Enable Many Unique Aspects of a Terrestrial Plant＇s Lifestyle

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.

Cytochrome b5 Reductase 1 Triggers Serial Reactions that Lead to Iron Uptake in Plants

Rhizosphere acidification is essential for iron （Fe） uptake into plant roots. Plasma membrane （PM） H＊-ATPases play key roles in rhizosphere acidification. However, it is not fully understood how PM H＋-ATPase activity is regulated to enhance root Fe uptake under Fe-deficient conditions. Here, we present evidence that cytochrome b5 reductase 1 （CBR1） increases the levels of unsaturated fatty acids, which stimulate PM H＋-ATPase activity and thus lead to rhizosphere acidification. CBRl-overexpressing （CBRI-OX） Arabidopsis thaliana plants had higher levels of unsaturated fatty acids （18：2 and 18：3）, higher PM H＊-ATPase activity, and lower rhizosphere pH than wild-type plants. By contrast, cbrl loss-of-function mutant plants showed lower levels of unsaturated fatty acids and lower PM H＊-ATPase activity but higher rhizosphere pH. Reduced PM H＊-ATPase activity in cbrl could be restored in vitro by addition of unsatu- rated fatty acids. Transcript levels of CBR1, fatty acids desaturase 2 （FAD2）, and fatty acids desaturase 3 （FAD3） were increased under Fe-deficient conditions. We propose that CBR1 has a crucial role in increasing the levels of unsaturated fatty acids, which activate the PM H＊-ATPase and thus reduce rhizosphere pH. This reaction cascade ultimately promotes root Fe uptake.

Two birds with one stone: CEPR2 phosphorylates dual targets in ABA signaling

Abscisic add(ABA)is a major phytohormone that governs plant development and their responses to many stressors(Chen et al.,2020).After its biosynthesis in specific tissues and cells,ABA is transported throughout the plant and perceived by dedicated receptors.The PYR/PYLVRCAR(pyrabactin resistance1/PYR1-like/regulatory components of A巳A receptor)proteins are such ABA receptors and are localized inside the cell.Due to their location,and since ABA is often biosynthesized far from its target cells,ABA transporters are needed to move the phytohormone什om the cells that synthesize A巳A to the target cells.