Modified MERI5 expression alters cell expansion in transgenic Arabidopsis plants

MERI5 is one of a series of genes that are preferentially, but not exclusively, expressed in the shoot apical meristem [1]. MERI5 shares homology to xyloglucan endo-transglycosylases that are believed to function in cell expansion. In order to investigate the role of MERI5 in cell epansion and in the shoot apex, transgenic plants designed to over-express the MERI5 cDNA were generated. Transgenic plants were obtained that had both an increase and decrease in the levels of MERI5 mRNA. The reduction in MERI5 mRNA is likely a result of co-suppression. Plants with the same or a reduced amount of MERI5 mRNA had no discernible developmental alterations. In contrast, plants with increased levels of MERI5 transcript had a number of developmental alterations in the shoot and root apices consistent with precocious elongation and/or abnormal expansion. The aberrant expansion at times produced abnormalities in cell proliferation. We suggest several ways that MERI5 expression could directly or indirectly produce these results.