Abstract
In clementine, failure of fertilization can result in parthenocarpic fruit development, which has several advantages, such as seedless fruit, longer shelf-life, and greater consumer appeal. Recently, S-RNases have been identified in Citrus grandis, thus revealing that the self-incompatibility (SI) reaction relies on the S-RNase gametophytic mechanism. The fundamental role of environmental factors, mostly temperature, in determining the numbers of pollen tubes reaching the ovary is also well established in Citrus. In the present work, temperature-dependent pollen–pistil interactions in C. clementina were analyzed, focusing on several morphological aspects, as well as on polyamine (PA) content and the activity and distribution of transglutaminase (TGase), both reported to be involved in the SI response in pear and in pummelo. Results clearly indicate that temperature contributed to a different activation of the SI response, which occurs at optimal temperature of 25°C but was by-passed at 15°C. TGase activity was stimulated during the SI response, and it localized differently in the compatible and incompatible interaction: in compatible pollinated styles, TGase localized inside the style canal, while it was detected all around it in incompatible crosses. TGase localization and activity were congruent with the levels of soluble and insoluble conjugated PAs and with morphological evidences, which highlighted cell wall modification occurring as a result of SI.
Highlights
In order to avoid self-fertilization, plants have adopted different strategies, among which, the asynchronous development of male and female organs, their specific localization within the flower or in the tree crown, and genetics-based strategies (Del Duca et al, 2019), the latter defined as “self-incompatibility” (SI)
In incompatible crosses at 25°C the stigmatic region showed only few pollen tubes growing inside it (Figure 1A); some pollen tubes could be observed in underlying regions at level of style (Figure 1B) and were characterized by the accumulation of callose at the apex
Previous reports indicated that in C. clementina temperature has a strong effect on flower and ovary development, pollen germination, and pollen tube growth (Distefano et al, 2018)
Summary
In order to avoid self-fertilization, plants have adopted different strategies, among which, the asynchronous development of male and female organs, their specific localization within the flower or in the tree crown, and genetics-based strategies (Del Duca et al, 2019), the latter defined as “self-incompatibility” (SI). Based on the pollen grain genome involved in the SI response, it can be gametophytic SI (GSI) or sporophytic SI (SSI) (Fujii et al, 2016). Even though the final result is the rejection of pollen, in GSI, growth of self-pollen tubes is initially allowed but blocked further along the style, while pollen rejection occurs on the stigma in SSI, and pollen tube growth is not allowed at all, and in some cases not even pollen grain rehydration (Mcclure and Franklin-Tong, 2006). Two GSI systems have been well characterized, one in Papaveraceae and the other in various families, including Solanaceae, Plantaginaceae, and Rosaceae. In the latter, the stylar S locus encodes for ribonuclease glycoproteins (S-RNases), which are taken up by the pollen tube. 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)
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