KNOX genes encode transcription factors (TFs), several of which act non-cell-autonomously. KNOX genes evolved in algae, and two classes, class I KNOX and class II KNOX genes, were already present in charophytes. In tracheophytes, class I KNOX genes are expressed in shoot apical meristems (SAMs) and thought to inhibit cell differentiation, whereas class II KNOX genes are expressed in mature organs regulating differentiation. In this review, we summarize the data available on gene families and expression patterns of class I and class II KNOX genes in embryophytes. The expression patterns of class I KNOX genes should be seen in the context of SAM structure and of leaf primordium development where the inhibition of cell differentiation needs to be lifted. Although the SAMs of angiosperms and gnetophytes almost always belong to the duplex type, several other types are distributed in gymnosperms, ferns, lycopods and bryophytes. KNOX gene families remained small (maximally five genes) in the representatives of bryophytes, lycopods and ferns examined thus far; however, they expanded to some extent in gymnosperms and, independently and much more strongly, in angiosperms. The growing sophistication of mechanisms to repress and re-induce class KNOX I expression played a major role in the evolution of leaf shape.

In plants, organogenesis and specification of cell layers and tissues rely on precise symplastic delivery of regulatory molecules via plasmodesmata. Accordingly, abundance and aperture of plasmodesmata at individual cell boundaries should be controlled by the plant. Recently, studies in Arabidopsis established reactive oxygen species as major regulators of plasmodesmata formation and gating. We show that in a barley mutant deficient in the synthesis of chlorophyll b, the numbers of plasmodesmata in leaves and in the shoot apical meristem are significantly higher than in the corresponding wild type, probably due to redox imbalance in the mutant. The resulting disturbance of symplasmic transport is likely to be the reason for the observed delayed floral transition in these mutants. In plants, plasmodesmata (PD) are indispensable regulators of cell-to-cell communications. 1 The function of meristems as well as the specification of cell layers and tissues during organogenesis rely on the symplasmic transport of regulatory molecules such as miRNAs, transcription factors or their transcripts.2 For instance, PD serve for cell-to-cell transport of the homeodomain-containing meristem regulators WUS and KNOX in shoot apical meristems (SAMs), and of floral homeotic MADS-box proteins in inflorescence meristems.3 Leaves are determinate organs the main function of which lies in the production and release of photosynthates to other plant organs. However, leaves are also the source of many signals which spread over the symplasmic route. For instance, Flowering locus T (FT) is synthesized in phloem companion cells in leaves and moves systemically to SAMs where it initiates floral transition upon unloading via PD. 4 Most likely, movement and activity of non-cell-autonomous transcription factors are controlled by developmental changes of the numbers and functional state of PD. 5,3 At the same time, we know relatively little not only about the fine structure of PD and mechanisms of protein translocation via PD, 6,7 but also about how plants control numbers and states of PD at individual cell boundaries. One of the factors involved in these processes could be the plant hormone cytokinine.8 Recent studies provided a major breakthrough in the elucidation of mechanisms regulating PD formation in plant cells. They revealed that the production of reactive oxygen species (ROS) by cellular organelles can differentially influence the formation of PD as well as their aperture. 9,10,11,12 Mutations in a gene encoding the plastid-localized thioredoxin m led to an increase in ROS and a decrease in PD conductivity in Arabidopsis roots; the mutation could be mimicked by application of methyl viologen.9 An opposite effect, namely an increase in PD conductivity, was observed in the ise1 mutant which carries a defect in a mitochondrial RNA helicase; plants defective for ISE1 also exhibited increased ROS production. 13The formation of secondary plasmodesmata was enhanced in the ise1 mutant and also in a mutant lacking the plastid RNA helicase...

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Transfer of developmental regulators, such as miRNA and transcription factors, through plasmodesmata represents one of the key mechanisms regulating morphogenesis in angiosperms. This mechanism has been termed non-cell-autonomous regulation. At present it is not known whether this process is involved in the morphogenesis of plants belonging to the evolutionarily ancient taxa. Importantly, structure and symplastic organization of apical meristems in the representatives of such taxa significantly differ from those in flowering plants. The non-cell-autonomous transcription factors encoded by the KNOX genes which regulate functions of the shoot apical meristem may become a promising model to study this issue. Refs 102. Figs 3.