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The developmental basis for the generation of divergent leaf forms is largely unknown. Here we investigate this problem by studying processes that distinguish development of two related species: Arabidopsis thaliana, which has simple leaves, and Cardamine hirsuta, which has dissected leaves with individual leaflets. Using genetics, expression studies and cell lineage tracing, we show that lateral leaflet formation in C. hirsuta requires the establishment of growth foci that form after leaf initiation. These growth foci are recruited at the leaf margin in response to activity maxima of auxin, a hormone that polarizes growth in diverse developmental contexts. Class I KNOTTED1-like homeobox (KNOX) proteins also promote leaflet initiation in C. hirsuta, and here we provide evidence that this action of KNOX proteins is contingent on the ability to organize auxin maxima via the PINFORMED1 (PIN1) auxin efflux transporter. Thus, differential deployment of a fundamental mechanism polarizing cellular growth contributed to the diversification of leaf form during evolution. [ABSTRACT FROM AUTHOR]

A key question in biology is how differences in gene function or regulation produce new morphologies during evolution. Here we investigate the genetic basis for differences in leaf form between two closely related plant species, Arabidopsis thaliana and Cardamine hirsuta. We report that in C. hirsuta, class I KNOTTED1-like homeobox (KNOX) proteins are required in the leaf to delay cellular differentiation and produce a dissected leaf form, in contrast to A. thaliana, in which KNOX exclusion from leaves results in a simple leaf form. These differences in KNOX expression arise through changes in the activity of upstream gene regulatory sequences. The function of ASYMMETRIC LEAVES1/ROUGHSHEATH2/PHANTASTICA (ARP) proteins to repress KNOX expression is conserved between the two species, but in C. hirsuta the ARP-KNOX regulatory module controls new developmental processes in the leaf. Thus, evolutionary tinkering with KNOX regulation, constrained by ARP function, may have produced diverse leaf forms by modulating growth and differentiation patterns in developing leaf primordia. [ABSTRACT FROM AUTHOR]

Leaves of flowering plants are determinate organs produced by pluripotent structures termed shoot apical meristems. Once specified, leaves differentiate an adaxial (upper) side specialized for light capture, and an abaxial (lower) side specialized for gas exchange. A functional relationship between meristem activity and the differentiation of adaxial leaf fate has been recognized for over fifty years, but the molecular basis of this interaction is unclear. In Arabidopsis thaliana, activity of the class I KNOX (KNOTTED1-like homeobox) genes SHOOTMERISTEMLESS (STM) and BREVIPEDICELLUS (BP) is required for meristem function but excluded from leaves, whereas members of the HD-Zip III (class III homeodomain leucine zipper) protein family function to promote both meristem activity and adaxial leaf fate. Here we show that the zinc-finger protein SERRATE acts in a microRNA (miRNA) gene-silencing pathway to regulate expression of the HD-Zip III gene PHABULOSA (PHB) while also limiting the competence of shoot tissue to respond to KNOX expression. Thus, SERRATE acts to coordinately regulate meristem activity and leaf axial patterning. [ABSTRACT FROM AUTHOR]

A key problem in developmental biology is understanding the origin of morphological innovations. Comparative studies in plants with different leaf morphologies indicate that the developmental pathway defined by KNOTTED1-type homeodomain proteins could be involved in generating different leaf forms. The differential expression of regulatory proteins has emerged as an important factor in driving morphological innovations in the plant kingdom - an idea that is well supported by quantitative trait locus analyses. [ABSTRACT FROM AUTHOR]

Quist and Chapela's conclusion that the transgenes they claim to have detected in native maize in Oaxaca, Mexico, are predominantly reassorted and inserted into a “diversity of genomic contexts” seems to be based on an artefact arising from the inverse polymerase chain reaction (i-PCR) they used to amplify sequences flanking 35S transgenes from cauliflower mosaic virus (CaMV). [ABSTRACT FROM AUTHOR]