The male gametophytic mutant tepitzin1 indicates a requirement of the homeobox gene WOX5 for pollen tube growth in Arabidopsis

Abstract

The WUSCHEL HOMEOBOX (WOX) gene family of Arabidopsis thaliana transcription factors is composed of at least 15 members encoding homeodomain proteins. We have characterized tepitzin1 (tpz1), a mutant harboring a transposon insertion in the 5′ regulatory region of WOX5. Heterozygous tpz1 plants do not have defects in vegetative growth or female reproductive development but exhibit aberrant pollen transmission, indicating that this mutation acts at the gametophytic level. WOX5 is abundantly expressed in mature pollen grains and elongating pollen tubes. Expression of WOX5 in heterozygous tpz1/+ anthers corresponds to 66% of WOX5 expression in wild-type anthers, confirming that WOX5 activity is affected in the mutant. WOX5-deficient pollen grains did not show developmental abnormalities and germinated at a frequency similar to wild-type pollen; however, in vitro and in vivo germination of pollen from tpz1 heterozygous individuals produced pollen tubes showing delayed germination and reduced length, confirming that the male gametophytic expression of WOX5 is necessary for normal pollen tube elongation. The reduction in average pollen tube length produced by the absence of WOX5 activity is less severe when heterozygous tpz1 individuals are in a homozygous quartet1 background, indicating that the absence of QRT1 activity partially compensates WOX5-deficient pollen tube elongation. Taken together, these results indicate that WOX5 activity is required for normal pollen tube growth in Arabidopsis. Our findings suggest that WOX5 identifies a new genetic pathway integrating homeobox transcription factors and auxin-mediated signals with the progamic phase of plant reproduction.