Tomato and tobacco leaves infected with HRG strain (as a temperature sensitive strain of TMV) showed mosaic mottling, blistering, severe distortion and abnormalities. Light and electron microscopic examination of infected leaves revealed the undulation and wrinkling of the upper and lower epidermis of tomato leaves. The upper epidermis contained multicellular glandular hairs with multicellular head. Reduction in the number of stomata especially in the lower epidermis was observed. Generally, the mesophyll is not clearly differentiated into palisade and spongy tissues. In some parts of the blade, the mesophyll composed of loosely palisade and spongy tissues with wide intercellular spaces. In other parts, palisade cells were compact, short and appear rich in chloroplasts. There was an increase in the number and size of chloroplasts. Parenchyma cells in the upper midrib protrusion showed more crystals. Phloem and xylem elements were reduced in number and were arranged irregularly. The lignification of xylem vessels was increased. Xylem parenchyma, phloem parenchyma and the surrounding cells appear deformed. Xylem vessels and sometimes the surrounding cells degenerated. Tobacco infected leaves showed the accumulation of different shapes and types of inclusions in the cytoplasm of both palisade and spongy cells in the form of short rod-shaped virus particles, prismatic and massive aggregate of long rods, X-bodies or X-material and P-protein in the form of bands or groups. The virus particles were usually surrounded by endoplasmic reticulum. Chloroplast alterations in the form of membrane disappearance, fewer grana, abnormal starch grains and the increase in the number of osmiophilic globules. Most of the chloroplasts show strange haphazard scattering of thylakoid system and maceration of grana and intergrana lamellae. Cell wall thickening were frequently observed. Mitochondrial alterations in the form of mitochondrial disruption, lacking or absence of cristae, swelling and malformed mitochondria. The number of microbodies was increased due to virus infection.
Key words: Holmes Rib Grass (HRG), Histopathology, Electron microscopy
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