Ectopic expression of S-RNase of Petunia inflata in pollen results in its sequestration and non-cytotoxic function.
The specificity of S-RNase-based self-incompatibility (SI) is controlled by two S-locus genes, the pistil S-RNase gene and the pollen S-locus-F-box gene. S-RNase is synthesized in the transmitting cell; its signal peptide is cleaved off during secretion into the transmitting tract; and the mature "S-RNase", the subject of this study, is taken up by growing pollen tubes via an as-yet unknown mechanism. Upon uptake, S-RNase is sequestered in a vacuolar compartment in both non-self (compatible) and self (incompatible) pollen tubes, and the subsequent disruption of this compartment in incompatible pollen tubes correlates with the onset of the SI response. How the S-RNase-containing compartment is specifically disrupted in incompatible pollen tubes, however, is unknown. Here, we circumvented the uptake step of S-RNase by directly expressing S(2)-RNase, S(3)-RNase and non-glycosylated S(3)-RNase of Petunia inflata, with green fluorescent protein (GFP) fused at the C-terminus of each protein, in self (incompatible) and non-self (compatible) pollen of transgenic plants. We found that none of these ectopically expressed S-RNases affected the viability or the SI behavior of their self or non-self-pollen/pollen tubes. Based on GFP fluorescence of in vitro-germinated pollen tubes, all were sequestered in both self and non-self-pollen tubes. Moreover, the S-RNase-containing compartment was dynamic in living pollen tubes, with movement dependent on the actin-myosin-based molecular motor system. All these results suggest that glycosylation is not required for sequestration of S-RNase expressed in pollen tubes, and that the cytosol of pollen is the site of the cytotoxic action of S-RNase in SI.
Meng
Xiaoying
X
Intercollege Graduate Degree Program in Plant Biology, The Pennsylvania State University, University Park, PA 16802, USA.
Hua
Zhihua
Z
Wang
Ning
N
Fields
Allison M
AM
Dowd
Peter E
PE
Kao
Teh-hui
TH
D015870:Gene Expression; D006031:Glycosylation; D032306:Petunia / Q000201:enzymology* / Q000235:genetics / Q000502:physiology; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism*; D011058:Pollen / Q000201:enzymology / Q000235:genetics / Q000502:physiology; D054817:Pollination; D021381:Protein Transport; D012260:Ribonucleases / Q000235:genetics* / Q000378:metabolism*
D010940:Plant Proteins; D012260:Ribonucleases; C022542:ribonuclease S