Many taxonomically diverse plant species are attacked by Erwinia chrysanthemi, a member of the causal agents of soft-rotting diseases. Symptom development is due to the collective action of pectin-degrading enzymes secreted by the bacterium through a type II secretion system (T2SS). Using Arabidopsis thaliana as a susceptible host, we show that plants respond to E. chrysanthemi 3937 by expressing cell-wall reactions, production of an oxidative burst, and activation of salicylic acid (SA) and jasmonic acid (JA) or ethylene (ET) signaling pathways. We found that the oxidative burst is mainly generated via the expression of the AtrbohD gene, constitutes a barrier of resistance to bacterial attack, and acts independently of the SA-mediated response. To determine the importance of T2SS-secreted proteins in elicitation of these defenses, we used a T2SS deficient mutant and purified enzymatic preparations of representative members of strain 3937 pectate lyase activity. The T2SS-secreted proteins were responsible only partially for the activation of SA and JA or ET signaling pathways observed after infection with the wild-type bacterium and were not involved in the expression of other identified defense reactions. Our study shows the differential role played by pectate lyases isoenzymes in this process and highlights the complexity of the host immune network, which is finely controlled by the bacterium.

Late embryogenesis-abundant (LEA) proteins are one of the components involved in desiccation tolerance (DT) by maintaining cellular structures in the dry state. Among them, MtPM25, a member of the group 5 is specifically associated with DT in Medicago truncatula seeds. Its function is unknown and its classification as a LEA protein remains elusive. Here, evidence is provided that MtPM25 is a hydrophobic, intrinsically disordered protein that shares the characteristics of canonical LEA proteins. Screening protective activities by testing various substrates against freezing, heating and drying indicates that MtPM25 is unable to protect membranes but able to prevent aggregation of proteins during stress. Prevention of aggregation was also found for the water soluble proteome of desiccationsensitive radicles. This inhibition was significantly higher than that of MtEM6, one of the most hydrophilic LEA protein associated with DT. Moreover, when added after the stress treatment, MtPM25 is able to rapidly dissolve aggregates in a non-specific manner. Sorption isotherms show that when it is unstructured, MtPM25 absorbs up to threefold more water than MtEM6. MtPM25 is likely to act as a protective molecule during drying and plays an additional role as a repair mechanism compared with other LEA proteins.

The nutrient contribution of lichens as litterfall in forests is discussed for a number of different ecosystems and it is hypothesized that lichens are important in capturing nutrients from wet deposition, occult precipitation, sedimentation, impaction and gaseous uptake. Most nutrients captured by these processes represent new nutrient inputs that would otherwise not be intercepted by the ecosystem. Part of these nutrients will be incorporated into lichen biomass and only become available upon death and decomposition, but a portion will be leached by precipitation and become deposited on the soil surface. Although quantifying nutrient sources, fluxes and pool sizes is a potentially complex task, we describe a simplified approach for determining whether lichens significantly affect the mineral cycling of a forest. Preliminary results for an oak woodland in California document that epiphytic lichens may reduce throughfall and alter throughfall chemistry.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. American Bryological and Lichenological Society is collaborating with JSTOR to digitize, preserve and extend access to The Bryologist.Abstract. The fruticose lichen Ramalina menziesii varies morphologically across its range in central, coastal California: The thallus is thin and filamentous near the ocean, net-like and coarser inland. We used a nondestructive sampling technique to estimate seasonal and annual linear growth of the inland form transplanted within an inland site and to a coastal site. Growth at the inland site was concentrated in thefall and winter months when precipitation occurred; growth at the coastal site was nearly constant through the first year of the study. Ramalina menziesiifrom the inland site grew significantly faster at the coast (38.7%/yr.) than inland (20.1%/yr.). Lichen from both sites was transplanted both within and between sites and biomass increase was measured destructively after 1 yr. Both morphs grew equally within the same site, and more rapidly at the coastal site. Annual growth rates were lower when estimated as biomass: Approximately 14%/yr. inland and 24%/yr. at the coast. Some genetic differences appeared to exist between morphological types, but their growth responses to the environment were plastic. Ramalina menziesii Tayl. is a fruticose lichen, characterized by a reticulate thallus, that dominates the epiphytic community of central coastal California, often forming a dense, pendulous canopy. Its thallus morphology ranges from relatively coarse nets in sunny, inland areas to thin filaments in foggy, coastal regions (Larson 1983; Larson et al. 1985;Rundel 1974). We examined absolute and relative growth rates of long-term within-site and reciprocal transplants from a coastal and an inland site by two methods--length and biomass increase. The results are compared with photosynthetic estimates of annual turnover (Matthes-Sears & Nash 1986) and are used to suggest potential adaptive significance of the two distinct morphological types studied.Growth in lichens can be expressed as an increase in linear dimension or biomass. In either case, growth is generally a function of the original size of the thallus (Hale 1974; Hawksworth & Hill 1984) and is best expressed in relative terms, as percent per unit time (Farrar 1974; Woolhouse 1968).Direct measurement of growth in many lichens requires long-term study because growth rates are very slow (Armstrong 1975;Hale 1973Hale , 1974. However some lichens have rapid, readily detectable growth (Rhoades 1977; Stone 1986). In a model of R. menziesii monthly gross carbon fixation, Matthes-Sears and Nash (1986) estimated that gross carbon gain at an inland site was 215% per year.