Virulent Rhodococcus fascians maintains cotyledons as a sink tissue

Both pathogenic and epiphytic organisms obtain nutrients from host plants. Dhandapani et al. show the differential impact of an epiphytic (avirulent) strain and a pathogenic strain of the cytokinin-producing bacterium, Rhodococcus fascians on source/sink relationships within pea cotyledons during and following germination.

Response of Pisum sativum following seed inoculation with Rhodococcus fascians.
Response of Pisum sativum following seed inoculation with Rhodococcus fascians. (1) Growth stages and morphology of P. sativum inoculated with R. fascians avirulent strain 589 (Avir), virulent strain 602 (Vir) and medium as mock inoculation (Con) grown in sterile agar containers at 5, 15, 25 and 45 days post-inoculation (dpi). (2) Light micrographs of sections of P. sativum seed coat and cotyledon following inoculation with R. fascians at 4 h post-inoculation (hpi) (A–C), 2 dpi (D–L), 5 dpi (J–L), 15 dpi (M–O) and 35 dpi (P–R). Sc, seed coat; Is, intercellular spaces in the seed coat; Se, sub-epidermal layer of the seed coat, and PL, parenchyma layer of the seed coat. Magnification for control (×40; 50 μm scale bar); magnification for R. fascians-inoculated tissues (×100; 20 μm scale bar). (3) Morphological differences between P. sativum cotyledons inoculated with R. fascians avirulent strain 589 (Avir), virulent strain 602 (Vir) and mock inoculated (Con) at 15 and 25 dpi. (4) Total chlorophyll content in cotyledons of P. sativum imbibed for 4 h with R. fascians avirulent strain 589 (Avirulent), virulent strain 602 (Virulent) and medium as mock inoculation (Control) grown in sterile agar containers until 35 dpi. The error bars are ± 1 s.d. of two biological replicates and four technical replicates.

Both strains commandeered resources from the cotyledon, but only the pathogenic strain markedly impacted development. The authors suggest that the interaction between cytokinins, cell-wall invertases (CWINVs) and SWEET genes may lead to the loss of apical dominance and the appearance of multiple shoots that characterise infection by virulent R. fascians.

This paper is part of the Annals of Botany Special Issue on Plant Immunity. It will be free access till June 2017 and after April 2018.

Reference

Dhandapani, P., Song, J., Novak, O., & Jameson, P. E. (2016). Infection by Rhodococcus fascians maintains cotyledons as a sink tissue for the pathogen. Annals of Botany, mcw202. https://doi.org/10.1093/aob/mcw202