Abstract
Most flowering plants show porogamy in which the pollen tubes reach the egg apparatus through the micropyle. However, several species show chalazogamy, an unusual pollen tube growth, in which the pollen tubes reach the embryo sac through the chalaza. While ovary signals for pollen tube growth and guidance have been extensively studied in porogamous species, few studies have addressed the process in chalazogamous species such as mango (Mangifera indica L.), one of the five most important fruit crops worldwide in terms of production. In this study, we characterize pollen–pistil interaction in mango, paying special attention to three key players known to be involved in the directional pollen tube growth of porogamous species such as starch, arabinogalactan proteins (AGPs), and γ-aminobutyric acid (GABA). Starch grains were observed in the style and in the ponticulus at anthesis, but their number decreased 1 day after anthesis. AGPs, revealed by JIM8 and JIM13 antibodies, were homogenously observed in the style and ovary, but were more conspicuous in the nucellus around the egg apparatus. GABA, revealed by anti-GABA antibodies, was specifically observed in the transmitting tissue, including the ponticulus. Moreover, GABA was shown to stimulate in vitro mango pollen tube elongation. The results support the heterotrophic growth of mango pollen tubes in the style at the expense of starch, similarly to the observations in porogamous species. However, unlike porogamous species, the micropyle of mango does not show high levels of GABA and starch, although they were observed in the ponticulus and could play a role in supporting the unusual pollen tube growth in chalazogamous species.
Highlights
IntroductionAfter pollen germination, the pollen tubes face a long journey before the two male sperms meet the female gametophyte or embryo sac, which at maturity usually contains seven cells (two synergids, the egg cell, the central cell, and three antipodal cells), embedded in the sporophytic tissues of the ovule
In flowering plants, after pollen germination, the pollen tubes face a long journey before the two male sperms meet the female gametophyte or embryo sac, which at maturity usually contains seven cells, embedded in the sporophytic tissues of the ovule
The presence of starch grains defines the pathway for pollen tube growth, and this has been barely studied in chalazogamous species
Summary
After pollen germination, the pollen tubes face a long journey before the two male sperms meet the female gametophyte or embryo sac, which at maturity usually contains seven cells (two synergids, the egg cell, the central cell, and three antipodal cells), embedded in the sporophytic tissues of the ovule. Pollen grains first land on the stigma where they germinate and pollen tube growth takes place along the style, until reaching the ovarian cavity, the locule, at the base of Pollen Tube Guidance in Mango the style. At this point, most flowering plants show porogamy in which pollen tubes reach the egg apparatus through the micropyle in order to achieve the double fertilization process typical of angiosperms (Maheshwari, 1950; Lora et al, 2016). The presence of starch grains defines the pathway for pollen tube growth, and this has been barely studied in chalazogamous species
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