but some subspecies or populations are tetraploid. Various subspecies A. arenosa have n=8 but can be either 2n (diploid) or 4n (tetraploid). A. suecica is n=13 (5+8) and is an amphidiploid species originated through hybridization between A. thaliana and diploid A. arenosa. A. neglecta is n=8, as are the various subspecies of A. halleri. As of 2005, A. cebennensis, A. croatica and A. pedemontana have not been investigated cytologically. Quorum sensing in Arabidopsis Quorum sensing (QS) is a mechanism in which large groups of bacterial populations can communicate and regulate gene-expression. Arabidopsis thaliana can perceive and respond to such signalings, the chief molecule controlling QS in bacteria is N-acyl homoserine lactones (AHL). Many times Quorum sensing in bacteria will take place during viral infection of the plant to increase virulence gene expression, as well as bacteria that are in symbiotic relationship with the plant may still use QS to communicate with each other. Plants such as Arabodopsis thaliana have specialized receptors on the plasma membrane that allow them to hear (AHL) signals; however, the exact mechanism of perception is unclear. In response to QS plants can mimic (AHL) signals with halogenated furanone which can block (AHL) signals and mimic them in bacteria as well; the exact mechanism is still being researched. Furthermore, the AHL signals themselves are able to result in responses from the plant such as increased growth and or increased resistance mechanisms while there seems to also be a connection between (AHL) carbon length and plant response. The most influential molecules in quorum sensing are N-acyl homoserine lactones (AHLs). There are many types of AHLs, one of which is called "short chain N-hexanoyl-DL-HSL" (C6-HSL). It has been shown that when A. thaliana roots are exposed to C6-HSL, root length is significantly promoted by 1.2 fold, 14 days after inoculation. However, some other AHLs such as long chain homoserine lactones don't have this effect on root growth. Some AHL's such as (C6-HSL) do play a role in root growth regulation physiology. In fact, the exposure to this type of AHL actually leads to a decrease in root growth. Contact to C6-HSL with the roots of A. thaliana results in specific transcriptional changes that lead to increased growth in root cells. Genes that regulate cell growth by producing different levels of growth hormone, specifically auxin, are upregulated by this AHL. IAA induces gene expression of H+-ATPases, and aids in transporting