Recent findings of the tree fruit self-incompatibility studies

Studying self-incompatibility (SI) in various plant genera represents a continuously intensifying field within plant science. This is also reflected by the fact that more than 50 scientific reports are published yearly dealing with this type of research. This review intends to highlight the latest achievements regarding the molecular bases of SI and also the results arose from the most recent studies in each fruit tree species. We demostrate the most essential contributions in details that accumulated during the last two years in order to be able to provide updated description of the state of the art. Self-incompatibility prevents the production of “self” seed and consequent inbreeding by providing a recognition and rejection system for “self,” or genetically identical pollen. Studies of gametophytic SI (GSI) species at a molecular level have identified two completely different S-genes in the Solanaceae, Rosaceae and Scrophulariaceae with S-RNase as the pistil S-component and an F-box protein (SFB) as the pollen S-component. Based on studies with loss-of-function SFB mutant sweet cherry cultivars, an updated model for the self-incompatibility response was described by Sonneveld et al. (2005), supposing the presence of a general inactivation mechanism with SFB proteins to prevent self S-RNases from being degraded, and not recruit non-self S-RNases for degradation. However, this has also several unclear details regarding the precise role of SFB in protecting self S-RNases from degradation. The intensive research on solanaceous species revealed that non-S-locus factors (the so-called modifier genes) could also be required for the SI response. Juárez-Díaz et al. (2006a) reported that a new thioredoxin h from Nicotiana alata (NaTrxh) was secreted similarly to animal. With fractionation studies as well as transiently expressing a NaTrxh-green fluorescent fusion protein in Nicotiana benthamiana and Arabidopsis thaliana leaves it was demonstrated that NaTrxh was a secretion protein such as SRNases and NaTTS (N. alata transmitting tissue-specific protein). In vitro reduction assays identified that S-RNase was one of the several potential substrates of the NaTrxh in the extracellular matrix (ECM). Furthermore, NaTrxh was proved by affinity chromatography to specifically interact with S-RNase. Because NaTrxh is localized in the ECM of the stylar transmitting tract and it shows specific interaction with S-RNase to reduce it in vitro, Juárez-Díaz et al. (2006a) suggested that this thioredoxin h might be involved either in general pollen–pistil interaction processes or particularly in S-RNase-based self-incompatibility. It is confirmed by the observation that NaTrxh is six-fold higher expressed in SI Nicotiana alata than in SC Nicotiana plumbaginifolia (Juárez-Díaz et al., 2006b). Based on this information, Halász et al. (unpublished) could determine stylar RNase activity in extracts supplied with antioxidants in a Prunus fruit tree, apricot. Goldraij et al. (2006) proposed an alternative model for the self-incompatibility reaction in Nicotiana. This model dictates that compatibility is due to S-RNase compartmentalization, rather than its degradation. S-RNase, HT-B, and 120 kDa glycoprotein (see review by Cruz-Garcia et al., 2003) are secreted into the transmitting tract. They are taken up by endocytosis and transported to the vacuole (Figure 1). An unidentified pollen protein (PP) is hypothesized to degrade HT-B in compatible pollen tubes. Regardless of the Recent findings of the tree fruit self-incompatibility studies