Developmental Morphology, Physiology, and Molecular Basis of the Pentagram Fruit of Averrhoa carambola

<i>Averrhoa carambola</i>, a key tropical and subtropical economic tree in the Oxalidaceae family, is distinguished by its unique pentagram-shaped fruit. This study investigates the developmental processes shaping the polarity of <i>A. carambola</i> fruit and their underlying hormonal and genetic mechanisms. By analyzing the Y1, Y2, and Y3 developmental stages-defined by the fruit diameters of 3-4 mm, 4-6 mm, and 6-12 mm, respectively-we observed that both cell number and cell size contribute to fruit development. Our findings suggest that the characteristic pentagram shape is established before flowering and is maintained throughout development. A hormonal analysis revealed that indole-3-acetic acid (IAA) and abscisic acid (ABA) show differential distribution between the convex and concave regions of the fruit across the developmental stages, with IAA playing a crucial role in polar auxin transport and shaping fruit morphology. A transcriptomic analysis identified several key genes, including <i>AcaGH3.8</i>, <i>AcaIAA20</i>, <i>AcaYAB2</i>, <i>AcaXTH6</i>, <i>AcaYAB3</i>, and <i>AcaEXP13</i>, which potentially regulate fruit polarity and growth. This study advances our comprehension of the molecular mechanisms governing fruit shape, offering insights for improving fruit quality through targeted breeding strategies.