Floral organ size control

Abstract

Growth of lateral organs is a complex mechanism that starts with formation of lateral primordia.Basal developmental programs like polarity, organ identity and environmental cues influence the final organ size achieved via coordinated cell division and expansion. recent evidence shows that the precise balance between these two processes, known as compensation mechanisms, seems to be influenced by the identity of the organ. Furthermore, studies of mutants affected in floral organ size suggest the existence of developmental compartments within different floral whorls that show distinct compensation behaviors.

Global Aspect of Lateral Organ Growth

Formation of lateral organs starts with the recruitment of meristematic cells that acquire a primordial fate. Newly formed primordia start extending as a result of an organ polarity program.¹ A meristem signal is required to establish polarity within a developing organ.² How much an organ grows and what shape it achieves implicates two additional basal modulators, the organ identity imposed on the growing primordium and the level of interactions with the environment in terms of size and/or differentiation.

Flowers of angiosperms like Arabidopsis thaliana, Antirrhinum majus or Petunia sp. have in common the formation of concentric whorls of organs that include sepals, petals, stamens and carpels. Its precise arrangement is due to combinatorial genetic functions that give raise to the four identities found in the flower.^3–5 The formation of whorls requires the activation of the so-called organ identity genes that in Antirrhinum correspond mainly to DEFICIENS, GLOBOSA and PLENA.⁶ Direct protein-protein interactions between these MADS-box genes give rise to heterodimers^7,8 and ternary complexes^9–11 that drive target gene expression required for floral morphogenesis. There is evidence that genes involved in polarity interact with early genes of SAM maintenance¹ and also with organ identity genes^12,13 affecting both organ growth and development.

Besides genetic developmental control, plants are able to change their morphogenesis adapting to environmental condition and modifying growth, flowering and sometimes survival.¹⁴

Understanding Control of Floral Organ Size

Recent work has focused on understanding how floral size is controlled.¹⁵ Two issues have been addressed; one is the existence of genes that control floral size in a specific way. Indeed, QTL affecting floral size have been found in Arabidopsis,¹⁶ Petunia¹⁷ and tomato.¹⁸ The other issue is the genetic dissection of floral organ size control and its relation with floral organ identity genes.

Lateral organ growth can be divided in two phases: one, which includes cell mass increment coupled to cell proliferation and a second one, that takes place once cells exit the proliferative period and growth is mainly due to cell expansion.^19,20 Final organ cell number depends on the number of cells founding primordia, active dividing cells and cell proliferation rate and period.^19,21,22 Genetic analysis in Arabidopsis revealed several independent routes controlling plant organ size via changes in the cell division interval. For instance the auxin signaling through ARGOS and AINTEGUMENTA (ANT) extend the cell proliferation phase.^22,23 KLUH, which promotes growth through a non-cell autonomous signaling, seems to maintain cell division until primordia reach a determinate size.²⁴ JAGGED (JAG) and NUBBIN (NUB) act together to promote cell proliferation in marginal regions of lateral organs. Genes with opposite effects include BIG BROTHER (BB)²⁵ and DA1,²⁶ which restrict proliferation.

Cell expansion can induce organ size changes through various ways, including vacuolation, ploidy level, biosynthesis of cell wall components and cytoskeleton.^27,28 For instance the bHLH transcription factor BIGPETALp (BPEp), activated by floral identity genes, was shown to regulate petal size via cell expansion.²⁹

Organ Level Coordination between Cell Division and Expansion

In several higher organisms, normal organ size has been observed in mutants with aberrant or deficient cell division due to increased cell expansion.³⁰ This phenomenon is known as compensation mechanism and it adds another level of control on organ size by monitoring and coordinating cell division and expansion.³¹ The current hypothesis suggests that it could be a common procedure to control the size of determinate organs.³² Recently published data suggest that reduction in cell expansion, by a decrease in endoreduplication, could also trigger compensation via increased cell proliferation to attain a normal leaf size.³³

There are several studies in leaves showing reduction in cell number, which conduces to an increment in cell expansion. Mutation in TANGLED gene in corn shows aberrant cell division but no morphological defects,³⁴ mutations in SHORT INTEGUMENT 2 gene also reduce cell number in integument but display almost normal morphology.³⁵ In Arabidopsis, null mutations in ANGUSTIFOLIA3 restrict the proliferation period with a partial compensatory increment in cell expansion and narrower leaves.³⁶ Furthermore, several other mutations that induce compensation have been described as struwelpeter, swell-map, G-protein α-subunit1 and deformed roots and leaves.^37–40 A compensation mechanisms triggered by variations in cell division⁴¹ has been identified in floral size mutants of Antirrhinum. The mutant compacta ähnlich (coan) shows a reduction in floral organ size in all floral whorls and this change is mainly due to a reduction in cell division (see below).⁴² Although the mutants described in Arabidopsis and Antirrhinum show compensation mechanisms, lateral organs tend to be smaller than wild type.

The opposite compensation phenomenon has been documented in Arabidopsis integuments overexpressing KNAT1, with additional cell cycle rounds and reduced cell expansion, and also, in the mutant swellmap^38;43 and in the Antirrhinum mutant formosa (fo) with increased floral organ size due to higher cell number but a reduction in cell area.⁴¹

Organ Identity Coming into Play

Several lines of evidence suggest that floral organ identity plays a key role as a regulator of organ size. Weak hypomorphic alleles of DEFICIENS (DEF)⁴⁴ in Antirrhinum, or single mutants of PhDEF and PhGLO1 in petunia⁴⁵ where a full loss of organ identity is only achieved in the double mutant,⁴⁵ share a reduced petal growth as phenotype. These observations suggest that final organ size requires high expression of B function genes. Direct evidence has been obtained in temperature sensitive alleles of DEF that show decreased organ size under non-permissive temperatures.⁴⁶

Several mutants affecting organ size do it in an organ-identity dependent fashion. Ectopic expression of ANT causes increased cell division in all floral whorls,²² and also shows enhanced cell expansion in petals, stamens and pistils but not in sepals.⁴⁷ In tomato, the OVATE gene represses growth in fruits, but additional gene copy numbers expand the size of all lateral organs with a stronger influence in sepals and stamens than in petals and carpels.⁴⁸ Transgenic tobacco expressing dominant negative constructs of Cdc2 kinase display reduction in cell number in all lateral organs, that triggers compensation in leaves but not in flowers.⁴⁹ Mutations in the Antirrhinum FO gene increase cell division in the three inner floral whorls and displays compensation with a reduction in cell expansion in petals and pistils but not in stamens.⁴¹ In contrast the mutant coan has reduced cell division in petals, stamens and pistils but compensation is triggered in petals via increased cell expansion. These differences in organ size control depending on identity is also supported by a double mutant between fo, affecting floral organ size, an a flower identity mutant, plena (ple), in a C function gene. The ple fo double mutant only shows size differences in petals of second whorl which have normal identity (Fig. 1A and B). Inner whorls with altered identity don't show any significant size variation between ple single mutant and fo ple. However fo effect over petal size in ple genetic background is stronger than in single mutant with an increment in dorsal petal expansion that doubles the one observed in single fo mutant. Cell compensation is not observed in fo ple petals anymore, showing an increment in conical cell expansion in contrast to the reduction observed in single fo mutant (Fig. 1C and D). This would suggest an implication of PLE in organ size control in a non-autonomous way. Several previous works in C function (AGAMOUS) mutation in Arabidopsis suggest its non-autonomous role in specifying the pattern of cell division in different layers of the second whorl,⁵⁰ intercellular communication between different whorls⁵⁰ and in petal development.⁵¹ Moreover, the gene BPEp mainly expressed in petals, has been described to be regulated by identity genes including AGAMOUS.²⁹

Figure 1.

Pictures showing flowers of ple single mutant and fo ple double mutant (left and right respectively) (A) lateral view and (B) longitudinal section. White bars represent 1 cm. SEM images of petal conical cells (C) ple mutant and (D) fo ple double mutant. Black bars represent 100 µm.

Developmental Compartments Show Distinct Compensation Behaviors

Petals are highly specialized organs comprised of distinct regions like the tube or the petal blade. The latter has two cell types distinguished by shape and function since conical cells are involved in light scattering and scent production.^52,53 In coan cell behavior is complex as cell size is only affected in petal regions with epidermal conical cells while flat cells adjacent to the petal tube do not show this compensation process.⁴² The opposite was observed in another floral size mutant of Antirrhinum, Nitida (Ni), where the reduction in floral size in petals also induces decreased expansion of flat cells while no variation was observed in conical petal cells.⁴²

The evidences described confirm compensation could be triggered both in leaves and flowers but seems to depend on organ identity and even on different developmental compartments inside organs. This suggests the existence of a global control of organ size integrating cell division, expansion and development, but there is no mechanism clarifying communication between those programs yet. Hopefully forthcoming studies would reveal the intricate network controlling final organ size.