The major goal of this Workshop is to address the fundamental properties of plant-microbe symbioses which are essential to improve the developmental and adaptive potential of the plant. This goal is very topical since in the last years a range of molecular mechanisms similar for beneficial and pathogenic plant-microbe interactions were revealed suggesting common evolutionary roots and potentially conserved approaches for their regulation.

Nitrogen-fixing symbioses provide us with excellent opportunities to demonstrate that beneficial plant-microbe interactions are characterized by a more complicated genetic control than pathogenic interactions. In legume-rhizobia symbioses, gene-for-geneâsystems are restricted to the early developmental stages of partner recognition, signal exchange and root infection by rhizobia. The late stages in which novel cellular and tissue structures develop where metabolic integration of both partners occurs, involve formation of the persistent inter-organism gene networks. These networks enable the plant to build-up special symbiotic compartments such as infection threads, fixation threads, and symbiosomes in which the microbial cells are hosted. In these compartments the genes of the microsymbiont that encode the host-beneficial properties are regulated by special plant signals.

The programme of this Workshop will concentrate on the co-operative aspects of nitrogen-fixing symbioses which develop, function and evolve as integrated plant-bacterial genetic systems. Using the model of legume-rhizobia symbiosis, we will illustrate that the genetic integrity of symbioses is expressed at the organismic level (bidirectional signal exchange results in novel cellular/tissue structures and metabolic functions) and at the ecological level (interplay of selective pressures in both partners results in their co-evolution). A potential to genetically improve the agronomic efficiency of integrated symbiotic gene networks will be discussed.

1. The development of intra-cellular niches, symbiosomes, in which the rhizobia cells are hosted and their beneficial functions are regulated.
2. Activation of nif genes in bacteria hosted within by the plant tissues and cells.
3. Regulation of the total number of nodules by local and systemic mechanisms.
4. Evolution of mutualistic symbioses as genetically integrated population systems.
5. The regulatory potential of plants to establish multi-component symbiotic communities.

Each oral presentation includes a 5 min discussion

14.00 - 15.35

Session 1

14.00 - 14.25

Erik Limpens (The Netherlands): Symbiosome development, a matter of membrane identiy

14.25 - 14.50

Peter M. Gresshoff (Australia): Systemic and local regulation of legume nodulation; receptor kinases, novel signals, targets and activated states

14.50 - 15.15

Kiwamu Minamisawa (Japan): Ethylene-mediated interactions between plants and bacteria: Rhizobitoxine production and ACC deaminase

15.15 - 15.40

Alexey Yu. Borisov (Russia): Legume genes controlling the fate of microbes inside beneficial plant-microbe

15.40 - 15.55

Coffee break

15.55 - 17.05

Session 2

15.55 - 16.20

Nikolai A. Provorov (Russia): Host-directed evolution of beneficial microbes

16.20 - 16.35

Svetlana Yurgel: Regulation of Sinorhizonium meliloti nitrogen stress response in free-living cells and in symbiosis with alfalfa

16.35 - 16.50

Alessio Mengoni: Sinorhizobium meliloti populations in soil, nodules and plant tissues: what is their ecological meaning?

16.50 - 17.05

Natalya Savelieva: The evolution of symbiotic pathway