Early stages of legume-rhizobia symbiosis are controlled by ABCG-mediated transport of active cytokinins.

Growing evidence has highlighted the essential role of plant hormones, notably, cytokinins (CKs), in nitrogen-fixing symbiosis, both at early and late nodulation stages1,2. Despite numerous studies showing the central role of CK in nodulation, the importance of CK transport in the symbiosis is unknown. Here, we show the role of ABCG56, a full-size ATP-binding cassette (ABC) transporter in the early stages of the nodulation. MtABCG56 is expressed in roots and nodules and its messenger RNA levels increase upon treatment with symbiotic bacteria, isolated Nod factor and CKs, accumulating within the epidermis and root cortex. MtABCG56 exports bioactive CKs in an ATP-dependent manner over the plasma membrane and its disruption results in an impairment of nodulation. Our data indicate that ABCG-mediated cytokinin transport is important for proper establishment of N-fixing nodules.

Role of ABCG transporters in modulation of symbiotic interactions / Rola transporterów ABCG w modulacji oddzialywan symbiotycznych.

ABC proteins, which include ABCG transporters, form one of the largest and most evolutionarily conserved protein families found in all systematic groups. Their function is associated with the active transport of several structurally and functionally unrelated compounds across cell membranes. All members of this protein family have a characteristic domain organization, which quantity and orientation determine their division and classification into subfamilies. ABCGs are recognized as being crucial for plant development as well as interactions with the environment. However, researchers have only just begun to discover the role of ABCG transporters as important modulators of symbioses.

The role of cytokinins in Legume-Rhizobium symbiosis / Rola cytokinin w modulacji oddzialywan symbiotycznych roslin bobowatych i bakterii wiazacych azot atmosferyczny.

Legume plants have a unique ability to form associations with nitrogen-fixing rhizobial species. An exchange of specific signaling molecules between host and microsymbiont constitutes the first step required both for bacterial infection and the formation of new, root-derived organs, called nodules. Since these two events occur in different root tissues, namely in the rhizodermis and in the underlying cortical cells, their strict regulation and coordination have to exist. An essential role of plant hormones, especially cytokinins, in the modulation of nitrogen-fixing symbiosis has been widely postulated. Activation of the cytokinin signaling pathway in the root cortex, by an unknown signal, is thought to be a key event of the infection process. As a consequence bioactive cytokinins are biosynthesized in the root susceptible zone, and they are a part of a positive feedback loop to trigger cortical cell division and sustain nodule organogenesis. Understanding the genetic mechanisms underlying events that occur in the inner layers of the root is one of the key challenges emerging in the study of symbiotic processes.

Medicago truncatula ABCG10 is a transporter of 4-coumarate and liquiritigenin in the medicarpin biosynthetic pathway.

The ABCG10 protein of the model legume Medicago truncatula is required for efficient de novo production of the phenylpropanoid-derived phytoalexin medicarpin. Silencing the expression of MtABCG10 results, inter alia, in a lower accumulation of medicarpin and its precursors. In this study, we demonstrate that the impairment of medicarpin biosynthesis can be partially averted by the exogenous application of 4-coumarate, an early precursor of the core phenylpropanoid pathway, and the deoxyisoflavonoid formononetin. Experiments conducted using HPLC/MS in a heterologous system as well as in vitro transport assays with labelled substrate revealed that MtABCG10 is responsible for the membrane translocation of 4-coumarate and liquiritigenin, molecules representing key branching points in the phenylpropanoid pathway. The identification of transporters participating in the distribution of precursors is an important step in understanding phenylpropanoid biosynthesis.

Membrane transporters and drought resistance - a complex issue.

Land plants have evolved complex adaptation strategies to survive changes in water status in the environment. Understanding the molecular nature of such adaptive changes allows the development of rapid innovations to improve crop performance. Plant membrane transport systems play a significant role when adjusting to water scarcity. Here we put proteins participating in transmembrane allocations of various molecules in the context of stomatal, cuticular, and root responses, representing a part of the drought resistance strategy. Their role in the transport of signaling molecules, ions or osmolytes is summarized and the challenge of the forthcoming research, resulting from the recent discoveries, is highlighted.