Gene networks controlling petal organogenesis

Abstract

One of the biggest unanswered questions in developmental biology is how growth is controlled. Petals are an excellent organ system for investigating growth control in plants: petals are dispensable, have a simple structure, and are largely refractory to environmental perturbations that can alter their size and shape. In recent studies, a number of genes controlling petal growth have been identified. The overall picture of how such genes function in petal organogenesis is beginning to be elucidated. This review will focus on studies using petals as a model system to explore the underlying gene networks that control organ initiation, growth, and final organ morphology.

Floral Organogenesis: When Knowing Your ABCs Is Not Enough

“…During petal initiation, RBE controls the microRNA miR164-dependent pathway (Huang et al, 2012), which mediates boundary formation in-between organ primordia via regulation of CUP-SHAPED COTYLEDON genes (Sieber et al, 2007). During this initial phase of petal development, RBE was found to repress the expression of certain TCP transcription factor-coding genes (Huang and Irish, 2015), which are thought to control the transition from cell division to cell expansion and differentiation during organ growth (Huang and Irish, 2016). This repression is lost or reduced as petal development progresses (Huang and Irish, 2015), leading to the promotion of growth and differentiation, thus contributing to the formation of mature petals.…”

Seasonal Regulation of Petal Number

“…For example, most plants in the Brassicaceae family have four-petal flowers, like Arabidopsis. The production of four petals in Arabidopsis requires genes that control meristem size, lateral organ outgrowth, and polarity, but also genes that confer petal identity and establish boundaries that demarcate the position of petals on the floral meristem (Irish, 2008;Huang and Irish, 2016). A critical step in this process is the suppression of growth in the regions between sepals, which creates space for auxin-induced petal initiation on the floral meristem (Huang et al, 2012;Lampugnani et al, 2013).…”

Implications of region‐specific gene expression for development of the partially fused petunia corolla

“…We found that, up to and including the 4–5 mm stage, lobes dominate growth, after which the UCT growth rate overtakes that of lobes (Figure l). Such variable growth trajectories across the distinct corolla regions can be explained by differential regulation of cell division and elongation, with the expectation that division occurs from a combination of primordial, intercalary and marginal meristem activities, and that elongation takes over from cell division late in development to determine overall petal size and shape (Huang and Irish, ; Smyth, ). To better characterize dominant regions of cell division across P. axillaris petal development, we hybridized sectioned floral tissues with the cell proliferation marker Histone4 ( PaxH4) that was not found to be differentially expressed between any petal dissection at the 4–5 mm stage (Figures and S2).…”

“…In early development, growth rates are highest, occurring via cell divisions throughout the petal. As petals then begin to establish their differentially shaped distal blade and proximal claw, cell divisions become more anisotropic, and finally slow, giving way to a basipetal wave of cell expansion (Sauret‐Gueto et al ., ; Huang and Irish, , ). Although the proximal (tube) region of petunia petals is relatively wider than that of A. thaliana , stemming from a continuous ring primordium, the distal lobes like the A. thaliana blades become wider than the tube, resulting in their overlap at mid‐stages of development, and their trumpet‐like flair at maturity.…”

“…Following establishment of the four free A. thaliana petals, auxin continues to induce cell division, particularly in the distal ‘blade’ region, by upregulating the expression of genes such as AINTEGUMENTA ( ANT ) and JAG , both of which positively affect the transcription of cell cycle genes such as cyclins (Sauret‐Gueto et al ., ). At later stages, around the time of greatest petal growth, de‐repression of class II TCP genes marks the transition from cell proliferation to cell expansion, with genes such as AUXIN RESPONSE FACTOR8 ( ARF8 ) and BIGPETALp ( BPEp ) limiting the extent of such growth (Varaud et al ., ; reviewed in Huang and Irish, ). Concomitant with the cell expansion stage, cell differentiation along the proximal–distal and adaxial–abaxial axes occurs through region‐specific expression of developmental gene pathways.…”