| The lifelong potential of plant organogenesis is preserved by stem cell populations embedded in groups of dividing cells known as meristems. How these cells maintain their undifferentiated status and distinguish themselves from their offspring is a fundamental question of plant development. To better understand the molecular mechanisms underlying stem cell function in the Arabidopsis root tip, I have isolated and examined a number of transcriptional regulators with enriched expression in these cells. One of these, the MADS box gene AGAMOUS-LIKE42 (AGL42), was identified by enhancer trap. Subsequent genetic analysis revealed that mutations and RNA knockdown do not result in a phenotype in wild type or genetic backgrounds with compromised meristem function. In addition, gain of function approaches involving ectopic overexpression of unmodified or constitutive activating or repressive forms of AGL42 showed no differences from wild type roots. Expression of AGL42 was found to be independent of the stem cell maintenance genes SCARECROW and SHORT-ROOT, and was not responsive to the phytohormone auxin. Using AGL42 cis regulatory sequences, I was able to engineer a GFP reporter that serves as a robust marker of root stem cells. To purify the stem cell population, marked cells were isolated by protoplasting and fluorescence-activated cell sorting. Microarray analysis of the transcriptional content of these cells was compared to the profiles of other root tissues, resulting in a gene set representing transcripts enriched in the stem cells. Using this set, a number of novel transcription factors were identified as candidates for reverse genetic analysis. Absence of a phenotype in corresponding mutants suggest a significant role for redundancy. I outline a methodology that uses spatial information from the root transcriptional map to direct further genetic studies in the root. The set of enriched transcripts also suggests that control of phytohormone biosynthesis, protein turnover, epigenetic mechanisms and signaling operate as regulatory mechanisms in these cells. In addition, a number of genes with characterized functions in the shoot and flower are suggested to contribute to root meristem biology. |
