Heme sensing and trafficking in fungi.

Fungal pathogens cause life-threatening diseases in humans, and the increasing prevalence of these diseases emphasizes the need for new targets for therapeutic intervention. Nutrient acquisition during infection is a promising target, and recent studies highlight the contributions of endomembrane trafficking, mitochondria, and vacuoles in the sensing and acquisition of heme by fungi. These studies have been facilitated by genetically encoded biosensors and other tools to quantitate heme in subcellular compartments and to investigate the dynamics of trafficking in living cells. In particular, the applications of biosensors in fungi have been extended beyond the detection of metabolites, cofactors, pH, and redox status to include the detection of heme. Here, we focus on studies that make use of biosensors to examine mechanisms of heme uptake and degradation, with guidance from the model fungus Saccharomyces cerevisiae and an emphasis on the pathogenic fungi Candida albicans and Cryptococcus neoformans that threaten human health. These studies emphasize a role for endocytosis in heme uptake, and highlight membrane contact sites involving mitochondria, the endoplasmic reticulum and vacuoles as mediators of intracellular iron and heme trafficking.

Organic acids and glucose prime late-stage fungal biotrophy in maize.

Many plant-associated fungi are obligate biotrophs that depend on living hosts to proliferate. However, little is known about the molecular basis of the biotrophic lifestyle, despite the impact of fungi on the environment and food security. In this work, we show that combinations of organic acids and glucose trigger phenotypes that are associated with the late stage of biotrophy for the maize pathogen Ustilago maydis. These phenotypes include the expression of a set of effectors normally observed only during biotrophic development, as well as the formation of melanin associated with sporulation in plant tumors. U. maydis and other hemibiotrophic fungi also respond to a combination of carbon sources with enhanced proliferation. Thus, the response to combinations of nutrients from the host may be a conserved feature of fungal biotrophy.

Long-term survival of Chlamydomonas reinhardtii during conditional senescence.

Chlamydomonas reinhardtii undergoes conditional senescence when grown in batch culture due to nutrient limitation. Here, we explored plastid and photo-physiological adaptations in Chlamydomonas reinhardtii during a long-term ageing experiment by methodically sampling them over 22 weeks. Following exponential growth, Chlamydomonas entered an extended declining growth phase where cells continued to divide, although at a lower rate. Ultimately, this ongoing division was fueled by the recycling of macromolecules that was obvious in the rapidly declining protein and chlorophyll content in the cell during this phase. This process was sufficient to maintain a high level of cell viability as the culture entered stationary phase. Beyond that the cell viability starts to plummet. During the turnover of macromolecules after exponential growth that saw RuBisCO levels drop, the LHCII antenna was relatively stable. This, along with the upregulation of the light stress-related proteins (LHCSR), contributes to an efficient energy dissipation mechanism to protect the ageing cells from photooxidative stress during the senescence process. Ultimately, viability dropped to about 7% at 22 weeks in a batch culture. We anticipate that this research will help further understand the various acclimation strategies carried out by Chlamydomonas to maximize survival under conditional senescence.

Chloroplasts and Plant Immunity: Where Are the Fungal Effectors?

Chloroplasts play a central role in plant immunity through the synthesis of secondary metabolites and defense compounds, as well as phytohormones, such as jasmonic acid and salicylic acid. Additionally, chloroplast metabolism results in the production of reactive oxygen species and nitric oxide as defense molecules. The impact of viral and bacterial infections on plastids and chloroplasts has been well documented. In particular, bacterial pathogens are known to introduce effectors specifically into chloroplasts, and many viral proteins interact with chloroplast proteins to influence viral replication and movement, and plant defense. By contrast, clear examples are just now emerging for chloroplast-targeted effectors from fungal and oomycete pathogens. In this review, we first present a brief overview of chloroplast contributions to plant defense and then discuss examples of connections between fungal interactions with plants and chloroplast function. We then briefly consider well-characterized bacterial effectors that target chloroplasts as a prelude to discussing the evidence for fungal effectors that impact chloroplast activities.