The physiological and biochemical effects on Napier grass plants following Napier grass stunt phytoplasma infection

Napier grass stunt (NGS) phytoplasma, a phloem limited bacterium, infects Napier grass leading to severe yield losses in East Africa. The infected plants are strongly inhibited in growth and biomass production. In this study, phytoplasma-induced morphological changes of the vascular system and physiological changes were analysed and compared with uninfected plants. The study showed that the phytoplasmas are more abundant in source leaves and range from 103 bacteria/µg total DNA in infected roots to 106 in mature Napier grass leaves. Using microscopical, biochemical and physiological tools, we demonstrated that the ultrastructure of the phloem and sieve elements is severely altered in the infected plants, which results in the reduction of the both mass flow and the translocation of photoassimilates in the infected leaves. The reduced transport rate inhibits the photochemistry of photosystems II in the infected plants, which is accompanied by loss of chloroplastic pigments in response to the phytoplasma infection stress eventually resulting in yellowing of diseased plants. The phytoplasma infection stress also causes imbalances in the levels of defense-related antioxidants, glutathione and ascorbic acid, reactive oxygen species (ROS) and in particular hydrogen peroxide. This study shows that the infection of NGS phytoplasma in the phloem of Napier grass has an impact on the primary metabolism and activates a ROS dependent defense response.