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Arbuscular mycorrhizal symbiosis is an ancient interaction between plants and fungi of the phylum Glomeromycota. In exchange for photosynthetically fixed carbon, the fungus provides the plant host with greater access to soil nutrients via an extensive network of root-external hyphae. Here, to determine the impact of the symbiosis on the host ionome, the concentration of 19 elements was determined in the roots and leaves of a panel of 30 maize varieties, grown under phosphorus-limiting conditions, with or without inoculation with the fungus Funneliformis mosseae. Although the most recognized benefit of the symbiosis to the host plant is greater access to soil phosphorus, the concentration of a number of other elements responded significantly to inoculation across the panel as a whole. In addition, variety-specific effects indicated the importance of plant genotype to the response. Clusters of elements were identified that varied in a co-ordinated manner across genotypes, and that were maintained between non-inoculated and inoculated plants.

One of the striking features of multicellular plants is their developmental plasticity in response to environmental change. For example, flowering plants can transform the vegetative meristem into a reproductive meristem for bearing seeds in a timely manner. This event is controlled by multiple environmental conditions (Cho et al. 2017). Another example is the root system architecture, where lateral root meristems develop from pericycle cells, enabling the formation of sophisticated root systems for foraging nutrients and water (Shahzad and Amtmann 2017). Thus intrinsic programs, which are otherwise cryptic, are involved in directing this mode of development in response to environmental stimuli that act as cues. This process is largely achieved through the division and differentiation of stem cells, which are limited in population. Intriguingly, however, there are phenomena that convert the fates of already differentiated cells; de-differentiation through cellular reprogramming, which, regardless of subcellular, transcriptional or epigenetic modification, appears pivotal for this transition. The plasticity of plants is not fully understood at present, but probably resides in the developmentally versatile potential of plant cells. In this special issue, we focus our attention on two aspects of cellular reprogramming: plant interactions with other organisms, and plant regeneration.

Magnaporthe oryzae causes rice blast, the most serious foliar fungal disease of cultivated rice (Oryza sativa). During hemibiotrophic leaf infection, the pathogen simultaneously combines biotrophic and necrotrophic growth. Here, we provide cytological and molecular evidence that, in contrast to leaf tissue infection, the fungus adopts a uniquely biotrophic infection strategy in roots for a prolonged period and spreads without causing a loss of host cell viability. Consistent with a biotrophic lifestyle, intracellularly growing hyphae of M. oryzae are surrounded by a plant-derived membrane. Global, temporal gene expression analysis used to monitor rice responses to progressive root infection revealed a rapid but transient induction of basal defense-related gene transcripts, indicating perception of the pathogen by the rice root. Early defense gene induction was followed by suppression at the onset of intracellular fungal growth, consistent with the biotrophic nature of root invasion. By contrast, during foliar infection, the vast majority of these transcripts continued to accumulate or increased in abundance. Furthermore, induction of necrotrophy-associated genes during early tissue penetration, previously observed in infected leaves, was not seen in roots. Collectively, our results not only report a global characterization of transcriptional root responses to a biotrophic fungal pathogen but also provide initial evidence for tissue-adapted fungal infection strategies.