A peptide pair coordinates regular ovule initiation patterns with seed number and fruit size

Abstract

Ovule development in Arabidopsis thaliana involves pattern formation which ensures that ovules are regularly arranged in the pistils to reduce competition for nutrients and space. Mechanisms underlying pattern formation in plants, such as phyllotaxis, flower morphogenesis or lateral root initiation, have been extensively studied, and genes controlling the initiation of ovules have been identified. However, how a regular spacing of ovules is achieved is not known. Using natural variation analysis combined with quantitative trait locus analysis, we found that the spacing of ovules in the developing fruits is controlled by two secreted peptides, EPFL2 and EPFL9 (also known as Stomagen), and their receptors from the ERECTA (ER) family that act from the carpel wall and the placental tissue. We found that a signalling pathway controlled by EPFL9 acting from the carpel wall through the LRR-receptor kinases ER, ERL1 and ERL2 promotes fruit growth. Regular spacing of ovules depends on EPFL2 expression in the carpel wall and in the inter-ovule spaces, where it acts through ERL1 and ERL2. Loss of EPFL2 signalling results in shorter fruits and irregular spacing of ovules or even ovule twinning. The EPFL2 expression pattern between ovules is under negative-feedback regulation by auxin, which accumulates in the arising ovule primordia. We propose that the auxin-EPFL2 signalling module evolved to control the initiation and regular, equidistant spacing of ovule primordia, which serves to minimise competition between developing seeds. Together, EPFL2 and EPFL9 coordinate ovule patterning and thereby seed number with fruit growth through a set of shared receptors.

The annotation and analysis of complex 3D plant organs using 3DCoordX

“…In the case of the cone-shaped ovule primordium a small, near point-size Bezier ring is positioned at the distal tip of the primordium. Positioning the Bezier ring at the distal tip correlates with a biologically relevant maximum of the phytohormone auxin at the tip as inferred from the expression of the auxin response reporter pDR5::GFP [39], the spatial signal of the auxin sensor R2D2 [40,41], and the finding that polar auxin transport mediated by PINFORMED1 (PIN1) is required for ovule primordium formation [42,43]. In case of the mature funiculus that is close in shape to a curved cylinder, a larger Bezier ring is placed at its proximal end.…”

“…Third, subsequent assignment of a proximal-distal distance value to each cell is constricted by these two prepatterns. For the placement of reference Bezier rings we took cues from the localization of important developmental regulators, such as the presence of an auxin maximum at the distal tip of the ovule primordium or the CUC3 expression in the chalaza 21 [39][40][41]52]. As a result of the approach each cell is annotated in 3D with respect to the radial and proximal-distal dimensions as well as to either an anterior-posterior or mediallateral axis.…”