Impact Of Phytoplasma Infection On Phytohormone Levels, Vascular Tissue Morphology And Phloem Mass Flow In Napier Grass

ABSTRACT

The Napier grass stunt (NGS) phytoplasma is a phloem limited bacterium that is

transmitted by insect vector, Maiestas banda (Hemiptera: Cicadellidae). The

phytoplasma causes NGS disease in Napier grass resulting into huge forage yield losses

hence impacting negatively the economy of smallholder dairy farmers in East Africa.

Phytoplasma infections causes massive changes in phloem physiology including the sieve

elements which may lead to reduced rate of phloem mass flow that interfere with

translocation of food in host plants. Hormonal imbalance also occurs in infected plants

and is a defense mechanism used by the host plants to counteract the pathogen attack.

Therefore, the structures of vascular tissues were examined using fluorescent microscopy

and phytoplasmas were visualized by confocal microscopy using 4',6-diamidino-2-

phenylindole (DAPI) dye while the rate of phloem mass flow determined using 5,6

carboxyfluorescein diacetate (CFDA) dye in healthy and phytoplasma infected Napier

grass plants. The cells of phytoplasma were detected and quantified by real-time qPCR

while the levels of phytohormones were investigated using liquid chromatography

tandem-mass spectrometry (LC-MS/MS) method. The areas of all vascular tissues in

phytoplasma-infected Napier grass leaves were significantly reduced as follows; vascular

bundles (34.8 %), xylem vessels (42.6 %), phloem (27.2 %) and sieve elements (53.5 %).

The individual and aggregates of phytoplasma cells were detected on the membranes of

the sieve elements that demonstrated high yields aan average of 2.29×105 cells/μg of total

DNA extracted in the plant leaves by qPCR results. The rates of phloem mass flow were

significantly reduced by 40.7 % in the phytoplasma-infected plants. With the exception of

SA and ABA, all measured phytohormones were affected by phytoplasma infection. The

concentrations of cis-12-oxo-phytodienoic acid, jasmonic acid and jasmonic acid

isoleucine were significantly reduced 3, 2 and 8 folds, respectively. This study sheds light on the significance and mechanisms of phytoplasma infection