Genetic Interaction between the AS1–AS2 and RDR6–SGS3–AGO7 Pathways for Leaf Morphogenesis

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Abstract
In higher plants, class I KNOTTED1 -like homeobox ( KNOX ) gene suppression and leaf polarity establishment are two processes crucial for leaf morphogenesis. The Arabidopsis genes, ASYMMETRIC LEAVES1 and 2 ( AS1 and AS2 ), are required for repressing the class I KNOX genes and promoting leaf adaxial cell fates. In addition, the RNA-DEPENDENT RNA POLYMERASE6 ( RDR6 ) gene acts synergistically with AS1 and AS2 to specify the adaxial polarity and repress the KNOX genes in leaves. It is known that RDR6 is one of the key components in plant post-transcriptional gene silencing (PTGS), and is likely to function with other silencing components in a genetic pathway in regulating leaf patterning. Here we report phenotypic analyses of double mutants combining as1 or as2 with other mutations relating to different RNA silencing pathways. We show that plants carrying rdr6, suppressor of gene silencing3 ( sgs3 ) or zippy ( zip , also called ago7 ) in combination with as1 or as2 demonstrate severe morphological defects, and the double mutant plants are generally similar to one another. Detailed phenotypic and molecular analyses reveal that leaves of rdr6 as2 ( 1 ), sgs3 as2 ( 1 ) and zip as2 ( 1 ) all show an abnormal adaxial identity, and contain high levels of microRNA165/166 and FILAMENTOUS FLOWER ( FIL ) transcripts. These results suggest that RDR6, SGS3 and AGO7 act in the same pathway, which genetically interacts with the AS1 – AS2 pathway for leaf development. The RDR6 – SGS3 – AGO7 pathway was previously identified as regulating the plant vegetative phase change. Our results reveal a new function of the pathway, which is also required for normal leaf morphogenesis.