Lay summary
The plant cell wall is a multifunctional, complex structure that mustprovide stability and at the same time allow cell growth in a controlledmanner. While our understanding of the biosynthesis of cell wallcomponents is increasing, the knowledge on the processes that control cellwall growth and -architecture is still scarce.

Root hairs are thin extensions of root epidermal cells formed bytip-growth. As root hairs are isolated structures that are not surroundedby other tissues, they are an excellent system to study cell growth anddevelopment. Previously, LRX1, a gene encoding an extracellular proteinwith a LRR-domain fused to a structural extensin domain, was identifiedand shown to be essential for root-hair cell-wall development (Baumbergeret al., 2001; Genes & Dev. 15: 1128-1139). Following experiments withLRX2, a paralog of LRX1, have revealed that LRX1 and LRX2 are functionallysimilar and synergistically interact during root hair development.lrx1/lrx2 double mutants show an enhanced lrx1 phenotype and appear mainlyhairless (Baumberger et al., 2003; Plant J. 35: 71-81).

A genetic screen for suppressors of the lrx1 mutant phenotype wasinitiated and several rol (repressors of lrx1) mutants that display awild-type phenotype in the lrx1/lrx1 mutant background were identified ofwhich two (rol2D and rol11D) are dominant and three (rol1, rol4, and rol5)are recessive. We are currently analyzing genetically and microscopicallyfive rol mutants and are establishing the genome-wide gene expressionprofile of the different lines by microarray experiments. The ultimategoal of this genetic screen is to understand the mechanistics of therepressor mutations and the possible role of their gene products inLRX1-dependent cell wall development. To this end, we plan to clone rol11Dand rol4, which displays a weak phenotype (slightly shorter root hairsthan wild-type) that is epistatic to lrx1 but is also visible in theLRX1/LRX1 wild-type background. The ROL11 and ROL4 genes and theirproducts will be characterized in detail.

In parallel, we intend to functionally characterize the structuralextensin domain of LRX1 using the complementation of the lrx1 phenotype asthe parameter of extensin function. Initial experiments have alreadyrevealed very interesting results and will be followed up. As acomplementary approach, we want to biochemically isolate the interactionpartner of LRX1 by immuno-pull down experiments. Finally, the insertionmutant collection of the LRX gene family (Baumberger et al., 2003; PlantPhysiol. 131: 1313-1326) will be extended and an RNAi approach topotentially silence several homologous LRX genes will be continued. Theseexperiments should asses the function of other LRX genes in plantdevelopment.

The proposed experiments are designed to and aim at the isolation ofcomponents involved in LRX1-dependent cell wall development and shouldallow an insight into the function of this protein and possibly LRXproteins in general. This will potentially increase our understanding ofthe processes that drive and control cell wall growth and development.