NaStEP: A Proteinase Inhibitor Essential to Self-Incompatibility and a Positive Regulator of HT-B Stability inNicotiana alataPollen Tubes    

Abstract

In Solanaceae, the self-incompatibility S-RNase and S-locus F-box interactions define self-pollen recognition and rejection in an S-specific manner. This interaction triggers a cascade of events involving other gene products unlinked to the S-locus that are crucial to the self-incompatibility response. To date, two essential pistil-modifier genes, 120K and High Top-Band (HT-B), have been identified in Nicotiana species. However, biochemistry and genetics indicate that additional modifier genes are required. We recently reported a Kunitz-type proteinase inhibitor, named NaStEP (for Nicotiana alata Stigma-Expressed Protein), that is highly expressed in the stigmas of self-incompatible Nicotiana species. Here, we report the proteinase inhibitor activity of NaStEP. NaStEP is taken up by both compatible and incompatible pollen tubes, but its suppression in Nicotiana spp. transgenic plants disrupts S-specific pollen rejection; therefore, NaStEP is a novel pistil-modifier gene. Furthermore, HT-B levels within the pollen tubes are reduced when NaStEP-suppressed pistils are pollinated with either compatible or incompatible pollen. In wild-type self-incompatible N. alata, in contrast, HT-B degradation occurs preferentially in compatible pollinations. Taken together, these data show that the presence of NaStEP is required for the stability of HT-B inside pollen tubes during the rejection response, but the underlying mechanism is currently unknown.

Distinct Self-recognition in the <i>Prunus</i> S-RNase-based Gametophytic Self-incompatibility System

“…10). This model is supported by observations of pollinated pistils lacking the stylar-part SI modifier, such as HT-B, 120K, and the unidentified 4936-factor (Goldraij et al, 2006;Jiménez-Durán et al, 2013;McClure et al, 2011). Down-regulation or mutation of the stylar-part modifiers results in continued vacuolar compartmentalization, which causes SC.…”

“…The accumulation of HT-B in vacuoles continued in incompatible pollen tubes, while HT-B abundance decreased in the vacuoles of compatible pollen tubes (Goldraij et al, 2006). Another stylar-part SI modifier, StEP, which is a Kunitz-type proteinase inhibitor produced in the stigma and taken up by pollen tubes, is involved in SI by stabilizing HT-B in incompatible pollen tubes (Jiménez-Durán et al, 2013). Vacuolar breakdown is believed to be induced by the inhibition of HT-B degradation, which is supposed to be controlled by self-recognition mediated by S-RNases and F-box proteins in the pollen tube cytosol.…”

Molecular mechanisms of postmating prezygotic reproductive isolation uncovered by transcriptome analysis

“…; Jiménez‐Durán et al . ).The interleaved gene Solyc07g007250 is also a metallocarboxypeptidase inhibitor showing high expression in both lyc and pen . A syntenic cluster of all three genes is also found in S. pennellii reference genome (as Sopen07g003300 , Sopen07g00331 0 and Sopen07g00332 0).…”

Pollen-Pistil Interactions and Their Role in Mate Selection

“…The 120-kD glycoprotein (120K), another SI modifier, is an abundant S-RNase-binding protein located in the N. alata transmitting tract (Lind et al, 1994(Lind et al, , 1996Cruz-Garcia et al, 2005) that also is required for S-specific pollen rejection . More recently, NaStEP (N. alta Stigma Expressed Protein) was identified as a proteinase inhibitor that is required for SI and also affects HT protein stability (Busot et al, 2008;Jiménez-Durán et al, 2013). Further insights into the functions of HT, 120K, and NaStEP are needed.…”