Control of plant architecture – A TALE of two families

Principle investigator: Marcel Proveniers

Evelien van Eck-Stouten, Marcel Proveniers and Nicole Rodenburg

A striking feature of plants is the huge variety of plant forms that can be found in nature. This enormous diversity is due to variation in the shape, size, proportion and relative position of the different organs in the aerial part of the plant. Evolutionary changes in the three-dimensional organization, or architecture, of plant shoots have played a central role in the morphological diversification of plant species. Moreover, plant architecture is a determining factor in the agronomic performance of crop plants. Plant members of the TALE (three-amino acid loop extension)-superclass of homeodomain (HD) transcription factor (TF) proteins play essential roles in the regulation of various aspects of plant architecture, including shoot apical meristem (SAM) maintenance, leaf size, leaf shape, leaf number, phyllotaxy, floral transition, plant height, internode patterning, and floral specification. In plants, the TALE HD protein class comprises two subfamilies: the BELL (BEL1-like) class and the Knotted1-like homeobox (KNOX) class. BELL proteins associate with KNOX proteins to form heterodimers to compose functional complexes that regulate plant development (Figure 1). A common feature of KNOX-BELL interactions is that the KNOX protein partner often interacts with a subset of BELL proteins and vice versa. Different combinations have both unique and overlapping targets. Our work focuses on unravelling BELL-KNOX regulatory networks (TF protein-protein interactions and their respective downstream targets) involved in SAM function and phase transition.

Functional analysis of the Arabidopsis family of BELL-class TALE homeobox genes, revealed that three of its members, ATH1, PNY and PNF have both overlapping and antagonistic functions in these processes. All three proteins interact with the KNOX TALE HD protein STM, known to control SAM initiation and maintenance. These interactions are a prerequisite for proper cellular localization and, thus, functioning of the corresponding TALE HD heterodimeric complexes. Meristem phase identity (bolting and floral transition) is also controlled by ATH1, PNY and PNF (Figure 2), in this case through complex interactions with three additional KNOX proteins, KNAT1, KNAT2 and KNAT6, both at the level of protein-protein interactions and at a gene expression level. We are currently investigating how these proteins affect this crucial developmental phase transition (targets) and at what level(s) they interact with established floral pathways. 

Figure 1. BELL-KNOX heterodimerization controls plant TALE homeobox nuclear localization through masking of Nuclear Export Signals (NES). A proposed model for the regulation of sub-cellular localization of plant TALE HD proteins, based on the model for the regulation of subcellular localization of animal TALE HD proteins as proposed by Kilstrup-Nielsen et al. (2003). In plants, BELL proteins are actively exported from the nucleus, a process requiring NES sequences located within their conserved BELL domain that are recognized by the nuclear export receptor AtCRM1 (dark grey oval). BELL proteins form stable dimers with KNOX proteins through interaction of the BELL and KNOX domains. The BELL-KNOX binding surface coincides with the region required for nuclear export, thereby shielding it from recognition by AtCRM1. The newly formed complex translocates into the nucleus owing to a yet unidentified NLS located within BELL proteins. Black rectangles represent the homeodomains (HD). Light gray boxes represent conserved amino-terminal regions within BELL and KNOX proteins. Black horizontal lines indicate protein-protein contacts.

Figure 2. Model for the function of BELL genes in meristem maintenance and phase identity determination. Plant post-embryonic development can be roughly divided in two distinct SAM identity phases, the vegetative (left panel) and generative (right panel). Preventing differentiation of meristem cells throughout both phases is a key process in SAM maintenance. During the vegetative phase, ATH1 and PNY are both present and redundantly act as partners of the class I KNOX protein STM to prevent SAM differentiation (Rutjens et al., 2009). However, prior to the transition to the generative phase, ATH1 expression is downregulated, whereas PNY expression is maintained at the same level. Simultaneously, expression of a third BELL gene, PNF, is upregulated. Most likely, during generative development PNF takes over the function of ATH1 as a redundantly acting partner with PNY in the process of meristem maintenance. A second function for ATH1 and PNY that has emerged from our experiments is the control of vegetative SAM phase identity. Major contributors to the generative phase identity of the SAM are the FMI genes AP1 and LFY. During vegetative development, FMI gene levels are kept low by the floral repressive activity of FLC. Besides controlling vegetative meristem maintenance, ATH1 and PNY also act as partially redundant, positive regulators of FLC expression. Both ATH1 and PNY are necessary for proper FLC induction (left panel). Prior to the floral transition, ATH1 levels gradually decline, accompanied by a similar decrease of FLC levels (Proveniers et al., 2007). Since PNY alone is incapable of inducing proper FLC expression, the floral transition is initiated. Together with PNF, PNY controls inflorescence and floral development by regulation of, at least, AP1 and LFY expression (Kanrar et al., 2008) (right panel).