Characterization of Auxin Metabolism in the Aborted-Seeded Litchi Cultivar Salathiel
Seed size is an important fruit quality trait for litchi (Litchi chinensis) fruit production. Litchi seeds can be classified into three categories (normal, small, and aborted). Aborted seed formation can be defined as stenospermocarpy because aborted-seeded fruit are generally harvested after fertilization. Some cultivars, such as Salathiel, bear a relatively high proportion of aborted-seeded fruit regardless of environmental and cultivation conditions. In these cultivars, genetic factors are believed to be associated with the increased aborted-seeded fruit production. However, little is known about the factors affecting the formation of aborted seeds in Salathiel litchi fruit. In other horticultural fruit crops, such as tomato (Solanum lycopersicum), auxin signaling is critical for parthenocarpy. The purpose of this study was to characterize the physiological and morphological properties of maternal reproductive organs in Salathiel female flowers, with a specific focus on the associated auxin metabolism. Microscopic examinations of the internal structures of the Salathiel embryo revealed that Salathiel reproductive organ differentiation is similar to that of normal-seeded cultivars. However, the obturator was significantly smaller in aborted-seeded cultivars than in normal-seeded cultivars, suggesting that developmental differences in maternal reproductive organs may affect the production of aborted seeds in aborted-seeded cultivars. Our investigation of indole acetic acid (IAA) contents revealed that IAA levels were significantly higher in Salathiel than in the other cultivars. Moreover, the IAA metabolite contents also differed significantly between Salathiel and the normal-seeded and small-seeded cultivars. Expression analyses of the genes related to auxin biosynthesis, catabolism, transport, and signaling indicated that the IAA influx-related and efflux-related gene expression levels were respectively significantly higher and lower in Salathiel than in the other cultivars, which was consistent with the increased IAA accumulation in the Salathiel ovary. The possible involvement of high IAA contents in the aborted-seeded fruit production in Salathiel is discussed herein.