Document Type



Master of Science


Biology, Biotechnology

Date of Defense


Graduate Advisor

Lisa M. Schechter, PhD.


Dr Lisa Schechter

Dr Bethany Zolman

Dr Wendy Olivas


Pseudomonas syringae is a bacterial plant pathogen that infects a large variety of agricultural crops. Bacteria colonize leaf surfaces and enter plant mesophyll tissue through wounds or stomata. Once inside, P. syringae can alter plant cell signaling pathways and suppress plant defense responses enabling it to grow in the intercellular space in the mesophyll. P. syringae possesses at least two types of virulence factors that suppress plant defense responses: i) small phytotoxin molecules, and ii) effector proteins that are translocated through specialized secretion systems. Gram-negative bacteria possess at least six types of secretion systems. The P. syringae type II and type III secretion systems (T2SS and T3SS) are both involved in secreting proteins that are important for P. syringae pathogenesis. Functions of the other secretion systems have not been explored. This study investigates the role of the newly discovered type VI secretion system (T6SS) in P. syringae interaction with plants. The results show that T6SS genes are expressed in three sequenced strains of P. syringae, P. syringae pv. tomato DC3000 (Pst DC3000), P. syringae pv. phaseolicola 1448a (Psp 1448a) and P. syringae pv. syringae B728a (Pss B728a). The T6SSs of Psp 1448a and Pss B728a were also able to secrete the Hcp protein into culture supernatants, showing that they are active. In planta virulence and growth studies revealed that the T6SS may not be essential for Psp 1448a and Pss B728a to cause disease in host plants. However, the T6SS may be involved in regulating biofilm formation, since a mutant Psp 1448a T6SS formed denser biofilm than the wild-type bacteria. These results suggest that the T6SS may secrete factors important for controlling bacterial aggregation on leaves.

OCLC Number