Rice transcription factors OsMADS2 and OsMADS4 regulate floret organ development: Deciphering their gene targets, traits and functions related to their unequal genetic redundancy

Abstract

Organs in modern dicot flowers are positioned in concentric rings (whorls). The outermost whorl has green protective sepals, internal to which are showy petals, and the reproductive stamen and carpel whorls. Florets of rice, cereals and grasses evolved certain morphological and functionally distinct features in their non-reproductive organs. Striking among them are the highly modified petal analogs; called lodicules, and the large bract-like outermost organs named palea and lemma. The analogy of these modified rice floret organs to sepals and petals is debated. The two lodicules of rice florets are small (limited growth in Proximal-Distal axis), thick (extensive growth in Dorsal-Ventral axis) and are asymmetrically positioned to occupy only one half of the second whorl. They perform an important mechanical role in the partial opening of the flower for stamen emergence and subsequent closing. Their asymmetric position, small fleshy structure with many parenchymatous cell layers and their regulated physiology for swelling and collapse are critical for these functions. Understanding the developmental mechanisms of these organs that underlie their function is of direct interest to evo-devo plant biologists and breeding programs aimed at crop yield improvement. Given these implications, for a deeper understanding of plant development and potential future uses in crop breeding, we define the objectives of this study and report our key findings Objective 1: Uncovering the gene targets of rice class B PISTILLATA-like (PI-like) factors that regulate lodicule and stamen development Objective 2: Characterizing an OsMADS2 target gene; AP2/EREBP86 encoding AINTEGUMENTA-like/PLETHORA (AIL/PLT) family transcription factor.

Overall, this study expands our knowledge on traits and molecular mechanisms controlled by rice class B PI-like factors: OsMADS2 and OsMADS4, and provides new insights on their functional divergence that greatly extend our understanding of lodicule and stamen development. This study sheds light on some molecular mechanisms triggered by AP2/EREBP86 that can initiate and maintain shoot meristem fate which have the potential to improve somatic embryogenesis.