Apple MADS Genes are Involved in Parthenocarpy and Floral Organ Formation

Abstract

Parthenocarpic apple cultivars have been recognized with simultaneous homeotic floral organ mutations. The mutations included replacements of petals to sepals and stamens to carpels, which were same as the class B mutations of floral organs from Arabidopsis and snapdragon. For apple, the parthenocarpy and class B mutations were tightly inherited and MdPISTILLATA (MdPI) gene deficiency caused homeotic mutations. However, the relationship between the suppression of MdPI and parthenocarpy was unclear. Transgenic apples with suppressed MdPI expression using an anti-sense or co-suppression method were found to have the same homeotic floral organ mutations as parthenocarpic cultivars. Further, the transformants with co-suppression showed parthenocarpy and overexpression of MdPI in fruits prevented normal fruit growth. Other apple MADS genes were analyzed for parthenocarpy. MdMADS13 is classified as another class B gene, which plays a role in floral organ formation together with MdPI. In addition, MdMADS1/8 and MdMADS9 are classified as class E genes, which are could function like SEP1 and 2 genes from Arabidopsis. The MdMADS1/8 and MdMADS9 gene-suppressed apple transformants showed strong inhibition of fruit enlargement, supporting the idea that MdMADS1/8 and MdMADS9 contribute to hypanthium development. It is possible that the MdPI, MdMADS13, MdMADS1/8, and MdMADS9 proteins formed a heterotetramer as a transcriptional regulator and were involved in fruit development. Other plant species such as tomato and grape also showed the respective class B genes affected fruit development. The suppression of tomato class B genes led to class B floral organ mutations and parthenocarpy, and a grape mutant with class B gene expression ectopically inhibited fruit flesh development. Both class B genes seemed to prevent fruit development similar to the apple MdPI. This suggests that the class B genes play not only a role in forming the identities of petals and stamens, but also a pivotal role in fruit development.