In this paper, we describe a mechanism for the transfer of nutrients from symbiotic microbes (bacteria and fungi) to host plant roots that we term the ‘rhizophagy cycle.’ In the rhizophagy cycle, microbes alternate between a root intracellular endophytic phase and a free-living soil phase. Microbes acquire soil nutrients in the free-living soil phase; nutrients are extracted through exposure to host-produced reactive oxygen in the intracellular endophytic phase. We conducted experiments on several seed-vectored microbes in several host species. We found that initially the symbiotic microbes grow on the rhizoplane in the exudate zone adjacent the root meristem. Microbes enter root tip meristem cells—locating within the periplasmic spaces between cell wall and plasma membrane. In the periplasmic spaces of root cells, microbes convert to wall-less protoplast forms. As root cells mature, microbes continue to be subjected to reactive oxygen (superoxide) produced by NADPH oxidases (NOX) on the root cell plasma membranes. Reactive oxygen degrades some of the intracellular microbes, also likely inducing electrolyte leakage from microbes—effectively extracting nutrients from microbes. Surviving bacteria in root epidermal cells trigger root hair elongation and as hairs elongate bacteria exit at the hair tips, reforming cell walls and cell shapes as microbes emerge into the rhizosphere where they may obtain additional nutrients. Precisely what nutrients are transferred through rhizophagy or how important this process is for nutrient acquisition is still unknown.
In this research, we conducted histochemical, inhibitor and other experiments to evaluate the chemical interactions between intracellular bacteria and plant cells. As a result of these experiments, we hypothesize two chemical interactions between bacteria and plant cells. The first chemical interaction between endophyte and plant is initiated by microbe-produced ethylene that triggers plant cells to grow, release nutrients and produce superoxide. The superoxide combines with ethylene to form products hydrogen peroxide and carbon dioxide. In the second interaction between microbe and plant the microbe responds to plant-produced superoxide by secretion of nitric oxide to neutralize superoxide. Nitric oxide and superoxide combine to form peroxynitrite that is catalyzed by carbon dioxide to form nitrate. The two chemical interactions underlie hypothesized nutrient exchanges in which plant cells provide intracellular bacteria with fixed carbon, and bacteria provide plant cells with fixed nitrogen. As a consequence of these two interactions between endophytes and plants, plants grow and acquire nutrients from endophytes, and plants acquire enhanced oxidative stress tolerance, becoming more tolerant to abiotic and biotic stresses.
We tested the ability
of 14 strains of Trichoderma to emit
volatile compounds that decreased or stopped the growth
of Phytophthora infestans. Volatile
organic compounds (VOCs) emitted from Trichoderma strains designated T41 and T45 inhibited the mycelial growth of P. infestans grown on a laboratory medium by 80 and
81.4%, respectively, and on potato tubers by 93.1 and 94.1%, respectively.
Using the DNA sequence analysis of the translation elongation factor
region, both Trichoderma strains were
identified as Trichoderma atroviride. VOCs emitted by the strains were analyzed, and 39 compounds were
identified. The most abundant compounds were 3-methyl-1-butanol, 6-pentyl-2-pyrone,
2-methyl-1-propanol, and acetoin. Electron microscopy of the hyphae
treated with T. atroviride VOCs revealed
serious morphological and ultrastructural damages, including cell
deformation, collapse, and degradation of cytoplasmic organelles.
To our knowledge, this is the first report describing the ability
of Trichoderma VOCs to suppress the
growth of the late blight potato pathogen.
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