Investigation of genetic basis for nonhost disease resistance in Solanum americanum against Pseudomonas syringae pv. tomato DC3000

Abstract

Solanaceae family crops including potato, tomato, peppers, and eggplants are economically important. However, they are suffering from disease caused by various plant bacterial pathogens leading to economic loss. Plants have innate immune responses against these pathogens by recognizing specific effector proteins injected by bacteria. Plants induce Effector-triggered immunity (ETI) such as growth restriction and programmed cell death called Hypersensitive Response (HR). Solanum americanum has potential to become model Solanaceae plants to study immunity of Solanaceae crops against pathogens. Because S. americanum is genetically diverse and diploid. Also, it has short life cycles and easily transformed using Agrobacterium tumefaciens. Therefore, S. americanum can be the useful source to study immune responses of Solanaceae crops in the laboratory. To study immune responses against bacterial pathogens, virulent bacterial pathogens are needed. However, there are no virulent model strains that cause diseases in S. americanum. For example, Pseudomonas syringae pv. tomato DC3000 is well-studied model bacterial pathogen to study ETI because the genome of Pst DC3000 was analyzed. Pst DC3000 secretes effector proteins and S. americanum recognizes some specific effectors inducing immune responses. At first, Pst DC3000 mutant lacking 36 effectors (D36E), Pst DC3000 mutant 29 effectors (D29E), and Pst DC3000 lacking 18 effectors (D18E) were infiltrated. Pst DC3000 D18E triggered HR, but D29E and D36E did not trigger immune responses. To identify avirulent effectors triggering cell death in S. americanum, 11 different effectors between Pst DC3000 D29E and D18E were infiltrated. Among 11 effectors, AvrPto, HopE1, AvrPtoB, HopAD1, and HopAM1 triggered cell death in S. americanum accession SP2273. We created Pst DC3000 effector knockout mutants lacking avirulent effectors. Pst DC3000 mutants grew at least ten folds higher than the wild-type Pst DC3000 however, they still triggered HR in S. americanum SP2273. To eliminate the cell death response, additional effectors are checked. A single effector which was deleted in D18E was infiltrated in S. americanum SP2273. Among 18 effectors, HopC1, HopAA1-1, HopM1, and AvrE1 are found to trigger cell death in S. americanum SP2273. To make virulent Pst DC3000 derivative, additional avirulent effectors will be deleted. Virulent Pst DC3000 mutant strain may help to study the interaction between Solanaceae crops and bacterial avirulent effectors.