Apple S‐RNase triggers inhibition of tRNA aminoacylation by interacting with a soluble inorganic pyrophosphatase in growing self‐pollen tubes in vitro

Abstract

Apple exhibits S-RNase-based self-incompatibility (SI), in which S-RNase plays a central role in rejecting self-pollen. It has been proposed that the arrest of pollen growth in SI of Solanaceae plants is a consequence of the degradation of pollen rRNA by S-RNase; however, the underlying mechanism in Rosaceae is still unclear. Here, we used S -RNase as a bait to screen an apple pollen cDNA library and characterized an apple soluble inorganic pyrophosphatase (MdPPa) that physically interacted with S-RNases. When treated with self S-RNases, apple pollen tubes showed a marked growth inhibition, as well as a decrease in endogenous soluble pyrophosphatase activity and elevated levels of inorganic pyrophosphate (PPi). In addition, S-RNase was found to bind to two variable regions of MdPPa, resulting in a noncompetitive inhibition of its activity. Silencing of MdPPa expression led to a reduction in pollen tube growth. Interestingly, tRNA aminoacylation was inhibited in self S-RNase-treated or MdPPa-silenced pollen tubes, resulting in the accumulation of uncharged tRNA. Furthermore, we provide evidence showing that this disturbance of tRNA aminoacylation is independent of RNase activity. We propose an alternative mechanism differing from RNA degradation to explain the cytotoxicity of the S-RNase apple SI process.

Cytoskeleton, Transglutaminase and Gametophytic Self-Incompatibility in the Malinae (Rosaceae)

“…The identified soluble inorganic pyrophosphatase (MdPPa) was highly expressed in germinated pollen and interacted with S-RNases, both in vivo and in vitro assays, suggesting that it plays a role in the apple SI response. MdPPa was identical in several apple cultivars and its activity decreased in SI-treated pollen tubes, suggesting that MdPPa is involved in the SI response [11].…”

“…While it is widely accepted that S-RNases degrade pollen RNA during SI, thereby preventing fertilization, it is likely that degradation of the target RNA is not the only mechanism by which self-pollen is rejected, and recently, an alternative mechanism differing from RNA degradation was proposed to explain the cytotoxicity of the S-RNase apple SI process [11]. In the apple SI response, S-RNase was shown to noncompetitively inhibit the specific pyrophosphatase activity of a soluble inorganic pyrophosphatase (MdPPa), leading to inorganic pyrophosphate accumulation, inhibition of tRNA aminoacylation and finally to PCD [11]. RNA degradation and MdPPa inhibition are not the solely mechanisms by which pollen tube growth is inhibited during SI.…”

A gamma‐thionin protein from apple, MdD1, is required for defence against S‐RNase‐induced inhibition of pollen tube prior to self/non‐self recognition

“…Plant reproduction requires reciprocal recognition and communication events between the pollen and the stigma/style cells (Boavida et al ., ; Higashiyama, ; Suzuki, ; Thomas and Noni, ). While many flowering plants exhibit intraspecific reproductive barrier that is widely found to prevent self‐fertilization in order to promote cross‐breeding, this process is called self‐incompatibility (SI) (Chen et al ., ; Franklin‐Tong, ; Li et al ., ; Nettancourt, ; Takayama and Isogai, ). To date, a number of genes involved in SI system between pollen and pistil have been identified, among these many plant species from the Rosaceae, Plantaginaceae and Solanaceae family exhibit S ‐RNase‐based gametophytic self‐incompatibility (GSI), which is the most commonly occurring and has been centrally studied (Anderson et al ., ; Broothaerts et al ., ; De et al ., ; Roalson and Mccubbin, ; Sassa et al ., , ).…”

“…Recent studies in Rosaceae mainly focus on the downstream signalling cascade likely occurs in SI in order to terminate incompatible pollen tubes growth, such as changes of calcium concentration, actin cytoskeleton depolymerization, burst out of reactive oxygen and tRNA aminoacylation in pollen tubes (Di et al ., ; Li et al ., ; Wang and Zhang, ; Wang et al ., ). Indeed, in general, when an extracellular protein enters a cell, a series of processes will be triggered, including defence response and signal transduction (Kwak et al ., ; Sawamoto et al ., ; Wan et al ., ).…”

Temperature-Dependent Compatible and Incompatible Pollen-Style Interactions in Citrus clementina Hort. ex Tan. Show Different Transglutaminase Features and Polyamine Pattern

“…In the latter, the stylar S locus encodes for ribonuclease glycoproteins (S-RNases), which are taken up by the pollen tube. In compatible pollen, S-RNases are degraded, while they remain active in the incompatible one, causing the degradation of pollen RNA and leading to the block of pollen tube growth and to programmed cell death (PCD) as demonstrated in Papaveraceae ( Thomas et al., 2006 ; Wilkins et al., 2014 ) and, more recently, in the Malineae ( Wang et al., 2010 ; Li et al., 2018 ).…”