Asymmetric leaf development and blade expansion in Arabidopsis are mediated by KANADI and YABBY activities

Asymmetric development of plant lateral organs is initiated by a partitioning of organ primordia into distinct domains along their adaxial/abaxial axis. Two primary determinants of abaxial cell fate are members of the KANADI and YABBY gene families. Progressive loss of KANADI activity in loss-of-function mutants results in progressive transformation of abaxial cell types into adaxial ones and a correlated loss of lamina formation. Novel, localized planes of blade expansion occur in some kanadi loss-of-function genotypes and these ectopic lamina outgrowths are YABBY dependent. We propose that the initial asymmetric leaf development is regulated primarily by mutual antagonism between KANADI and PHB-like genes, which is translated into polar YABBY expression. Subsequently, polar YABBY expression contributes both to abaxial cell fate and to abaxial/adaxial juxtaposition-mediated lamina expansion.     

The YABBY gene family and abaxial cell fate

The establishment of abaxial-adaxial polarity in lateral organs involves factors intrinsic to the primordia and interactions with the apical meristem from which they are derived. Recently, a small plant-specific family of genes, the YABBY gene family, has been proposed to specify abaxial cell fate. Each asymmetric above-ground lateral organ expresses at least one member of the family in a polar manner, and loss- and gain-of-function studies indicate that they are sufficient to specify abaxial cell fate and that they act in both distinct and redundant manners.     

The proteolytic function of the Arabidopsis 26S proteasome is required for specifying leaf adaxial identity

Polarity formation is central to leaf morphogenesis, and several key genes that function in adaxial-abaxial polarity establishment have been identified and characterized extensively. We previously reported that Arabidopsis thaliana ASYMMERTIC LEAVES1 (AS1) and AS2 are important in promoting leaf adaxial fates. We obtained an as2 enhancer mutant, asymmetric leaves enhancer3 (ae3), which demonstrated pleiotropic plant phenotypes, including a defective adaxial identity in some leaves. The ae3 as2 double mutant displayed severely abaxialized leaves, which were accompanied by elevated levels of leaf abaxial promoting genes FILAMENTOUS FLOWER, YABBY3, KANADI1 (KAN1), and KAN2 and a reduced level of the adaxial promoting gene REVOLUTA. We identified AE3, which encodes a putative 26S proteasome subunit RPN8a. Furthermore, double mutant combinations of as2 with other 26S subunit mutations, including rpt2a, rpt4a, rpt5a, rpn1a, rpn9a, pad1, and pbe1, all displayed comparable phenotypes with those of ae3 as2, albeit with varying phenotypic severity. Since these mutated genes encode subunits that are located in different parts of the 26S proteasome, it is possible that the proteolytic function of the 26S holoenzyme is involved in leaf polarity formation. Together, our findings reveal that posttranslational regulation is essential in proper leaf patterning.     

Members of the YABBY gene family specify abaxial cell fate in Arabidopsis.

Lateral organs produced by shoot apical and flower meristems exhibit a fundamental abaxial-adaxial asymmetry. We describe three members of the YABBY gene family, FILAMENTOUS FLOWER, YABBY2 and YABBY3, isolated on the basis of homology to CRABS CLAW. Each of these genes is expressed in a polar manner in all lateral organ primordia produced from the apical and flower meristems. The expression of these genes is precisely correlated with abaxial cell fate in mutants in which abaxial cell fates are found ectopically, reduced or eliminated. Ectopic expression of either FILAMENTOUS FLOWER or YABBY3 is sufficient to specify the development of ectopic abaxial tissues in lateral organs. Conversely, loss of polar expression of these two genes results in a loss of polar differentiation of tissues in lateral organs. Taken together, these observations indicate that members of this gene family are responsible for the specification of abaxial cell fate in lateral organs of Arabidopsis. Furthermore, ectopic expression studies suggest that ubiquitous abaxial cell fate and maintenance of a functional apical meristem are incompatible.     

BLADE-ON-PETIOLE 1 and 2 control Arabidopsis lateral organ fate through regulation of LOB domain and adaxial-abaxial polarity genes

We report a novel function for BLADE-ON-PETIOLE1 (BOP1) and BOP2 in regulating Arabidopsis thaliana lateral organ cell fate and polarity, through the analysis of loss-of-function mutants and transgenic plants that ectopically express BOP1 or BOP2. 35S:BOP1 and 35S:BOP2 plants exhibit a very short and compact stature, hyponastic leaves, and downward-orienting siliques. We show that the LATERAL ORGAN BOUNDARIES (LOB) domain genes ASYMMETRIC LEAVES2 (AS2) and LOB are upregulated in 35S:BOP and downregulated in bop mutant plants. Ectopic expression of BOP1 or BOP2 also results in repression of class I knox gene expression. We further demonstrate a role for BOP1 and BOP2 in establishing the adaxial-abaxial polarity axis in the leaf petiole, where they regulate PHB and FIL expression and overlap in function with AS1 and AS2. Interestingly, during this study, we found that KANADI1 (KAN1) and KAN2 act to promote adaxial organ identity in addition to their well-known role in promoting abaxial organ identity. Our data indicate that BOP1 and BOP2 act in cells adjacent to the lateral organ boundary to repress genes that confer meristem cell fate and induce genes that promote lateral organ fate and polarity, thereby restricting the developmental potential of the organ-forming cells and facilitating their differentiation.     

Establishment of polarity in angiosperm lateral organs

In seed plants, lateral organs such as leaves and floral organs are formed from the flanks of apical meristems. Therefore, they have an inherent positional relationship: organ primordia have an adaxial side next to the meristem, and an abaxial one away from the meristem. Surgical and genetic studies suggest that a morphogenetic gradient, which originates in the meristem, converts the inherent polarity into a functional one. Once an adaxial-abaxial axis of polarity is established within organ primordia, it provides cues for proper lamina growth and asymmetrical development. Several key participants in this process have been identified, and analyses of these genes support and refine our views of axis formation in plants. The complex relationships between and within various members of these plant-specific gene families (class III HD-ZIPs, YABBYs and KANADIs) might account for a substantial part of the morphological variation in lateral organs of seed plants.