We often come across many colorful and diverse flowers, which are the most beautiful products of nature and are also the most commonly used decorative plants for expressing emotions and admiration. From the outside to the inside, each flower typically contains only four whorls of floral organs: sepals, petals, stamens, and pistils. However, it is the variations in the color and shape of these four whorls of floral organs, as well as their different combinations, that create the diverse and colorful flowers in nature. So, how are these beautiful flowers formed? Until the 1980s, with the rapid development of modern molecular biology and molecular genetics in plant science research, breakthroughs were made in the study of the molecular mechanisms controlling plant flower development.
In the study of abnormal mutants of flower development in model plants such as Arabidopsis and Antirrhinum, three types of homeotic mutants affecting the changes in the attributes of floral organs were found, each of which affects the development of two adjacent whorls of floral organs. The first type of mutants mainly affects the development of the first and second whorls of floral organs, with the sepals in the first whorl transforming into carpels and the petals in the second whorl transforming into stamens; the second type of mutants mainly affects the development of the second and third whorls of floral organs, with the petals in the second whorl transforming into sepals and the stamens in the third whorl transforming into carpels; the third type of mutants mainly affects the development of the third and fourth whorls of floral organs, with the stamens in the third whorl transforming into petals and the carpels in the fourth whorl transforming into a new mutant flower, and the organs of the new flower continue to differentiate inwardly and cannot be terminated normally, forming an abnormal structure of flowers within flowers.
These three types of mutants suggest the existence of three functional genes A, B, and C, which respectively control the development of adjacent two whorls of floral organs. Each whorl of floral organs is controlled by one or two functional genes. On this basis, the ABC model controlling floral organ development was summarized, which successfully explained the molecular mechanism of floral organ attribute determination. The A functional gene mainly controls the development of the first and second whorls of floral organs. When the A functional gene is mutated, the sepals in the first whorl (sepals) transform into carpels (pistils), and the petals in the second whorl transform into stamens; the B functional gene mainly acts on the development of the second and third whorls of floral organs. After the B functional gene is deleted, the petals in the second whorl transform into sepals, and the stamens in the third whorl transform into carpels; the C functional gene participates in the regulation of the development of the third and fourth whorls of floral organs. When the C functional gene is deleted, the stamens in the third whorl transform into petals, and the carpels in the fourth whorl transform into sepals.
Therefore, in the first whorl of floral organs, the A functional gene acts alone and determines the attribute of the sepals; in the second whorl of floral organs, the A and B functional genes act together and determine the attribute of the petals; in the third whorl of floral organs, the B and C functional genes act together and determine the attribute of the stamens; and in the fourth whorl of floral organs, the C functional gene acts alone and determines the attribute of the carpels while terminating the continued differentiation of floral organs. Further analysis shows that the action areas of the A and C functional genes are antagonistic: when the A functional gene is deleted, the C functional gene is translocated to the first and second whorls of floral organs to function; and when the C functional gene is deleted, the A functional gene is translocated to the third and fourth whorls of floral organs to function. When both the A and C functional genes are deleted, the first and fourth whorls of floral organs are similar to stem leaves, and the second and third whorls of floral organs are similar to chimeras of petals and stamens, indicating that the B functional gene can exist independently of the A and C functional genes.
Subsequently, a new E functional gene was identified, which can coordinate with the A, B, and C functional genes to control the development of the four whorls of floral organs, and the ABC model was modified to the ABCE "quartet model". At the same time, when the A, B, C, and E functional genes are constitutively expressed, the leaves can be transformed into floral organs.