Department of Plant Pathology, College of Agricultural and Environmental Sciences, University of Georgia, Griffin Campus, Griffin 30223
Using Murashige and Skoog (MS) as a basal medium, the effects of varying levels and combinations of plant growth regulators required for shoot tip and root proliferation in healthy and aster yellows phytoplasma (AYP)- and Spiroplasma citri-infected periwinkle (Catharanthus roseus) shoots were studied. Number of shoots and fresh and dry mass of healthy and AYP-infected shoots increased when benzyladenine (BA) concentrations were increased from 0.5 to 4 mg/liter. The maximum number of shoots for both healthy and AYP-infected plants was obtained when grown in MS medium supplemented with BA at 4 mg/liter and indole-3-acetic acid (IAA) at 0.5 mg/liter. S. citri-infected shoots proliferated the most when grown in MS medium containing BA at 2 mg/liter and IAA at 0.5 mg/liter. The best medium for root production in healthy periwinkle shoots contained α-naphthaleneacetic acid (NAA) at 0.5 mg/liter, whereas the best medium for AYP-infected shoots contained indole-3-butyric acid (IBA) at 2.5 mg/liter, both in combination with kinetin at 0.1 mg/liter. S. citri-infected shoots had the best root growth when grown in medium supplemented with IBA at 5.0 mg/liter and kinetin at 0.1 mg/liter. The concentration of cytokinin and auxin needed for maximum shoot proliferation differed between AYP- and S. citri-infected shoot tips, strongly indicating that the two mollicutes may cause different changes in endogenous cytokinin and auxin levels. The concentrations of NAA and IBA needed for root growth of S. citri-infected shoots were two- to fivefold higher than the concentrations needed for healthy and AYP-infected shoots, clearly demonstrating that S. citri infection caused a shortage of auxins that resulted in retardation of secondary root growth. Chlorophyll content was markedly reduced in periwinkles infected with AYP or S. citri compared with chlorophyll in healthy periwinkles. AYP caused a decrease in carotenoid in leaves 6 weeks after graft-inoculation, but carotenoid content was unchanged in S. citri-infected leaves throughout the test period. Anthocyanin content in periwinkles infected with AYP decreased significantly by 4 weeks postinoculation, whereas anthocyanin content in periwinkles infected with S. citri increased. Anthocyanin content in leaf tissues, however, was reduced as a result of AYP and S. citri infection. Pigment changes induced by AYP and S. citri, whether similar or different compared with those of healthy periwinkle shoots, provide important information for interpreting pathogenesis when linked with plant growth regulators.