Label-free quantitative proteomics reveals differentially regulated proteins in the latex of sticky diseased Carica papaya L. plants

Abstract

Papaya meleira virus (PMeV) is so far the only described laticifer-infecting virus, the causal agent of papaya (Carica papaya L.) sticky disease. The effects of PMeV on the laticifers’ regulatory network were addressed here through the proteomic analysis of papaya latex. Using both 1-DE- and 1D-LC-ESI-MS/MS, 160 unique papaya latex proteins were identified, representing 122 new proteins in the latex of this plant. Quantitative analysis by normalized spectral counting revealed 10 down-regulated proteins in the latex of diseased plants, 9 cysteine proteases (chymopapain) and 1 latex serine proteinase inhibitor. A repression of papaya latex proteolytic activity during PMeV infection was hypothesized. This was further confirmed by enzymatic assays that showed a reduction of cysteine-protease-associated proteolytic activity in the diseased papaya latex. These findings are discussed in the context of plant responses against pathogens and may greatly contribute to understand the roles of laticifers in plant stress responses.

Label-free quantitative proteomic analysis of pre-flowering PMeV-infected Carica papaya L.

“…endochitinase and PR-4 [22], while latex samples show lower abundance of chymopapain cysteine proteases and a latex serine proteinase inhibitor [6]. Together, this data indicates the existence of systemic acquired resistance (SAR) response in PSD symptomatic plants.…”

“…The GO-based grouping of the identified proteins showed they were largely associated with metabolism, stress responses and cellular organization. C. papaya proteins known to be differentially accumulated in sticky-diseased papaya leaf [22] and latex [6] samples, for instance tubulin beta, HSP70 and latex serine protease inhibitor, were among the identified proteins, suggesting the obtained dataset likely includes other proteins relevant to the pathosystem.…”

“…To our knowledge, previously published studies have used gel (1-and 2-DE and DIGE)-based separations of C. papaya proteins followed by peptide mass spectrometry analysis. In summary, the total number of identified proteins per sample type is 76 from somatic embryos [31], 54 from papaya fruit pulp [32,33], 71 from leaves [22], 186 from latex [6,34] and 1,581 proteins from isolated chromoplasts [35]. In this study, field-grown pre-flowering C. papaya plants' leaf samples were submitted to protein extraction and in-solution digestion, and the resulting peptides were analyzed using an LC-MS/MS based approach.…”

“…PMeV-infected laticifers [5], along with reduced cysteine-protease abundance and activity [6]. In parallel, the accumulation of H 2 O 2 in the phloem [5] and the increased activity of ROS-detoxifying enzymes peroxidase and superoxide dismutase in sticky-diseased C.…”

“…Some proteins in this group are already known to be involved with C. papaya x PMeV interaction such as 26s proteasome regulatory subunit, calreticulin and acidic endochitinase (Supplemental Table S3) [6,22]. The group represents a main source of other proteins potentially involved with plant responses to viruses.…”