Polar Auxin Transport Finely Tunes Cell Growth Differed in Root Zones

Auxin is the most important hormone participating in multifacted functions in regulation of growth and development of higher plants. From Acid Growth Theory, auxin stimulates cell elongation via acitivation of PM-H+-ATPases which pumps out of protons to acidify apoplast which initiates cell wall loosening by expansins, polygalacturonases, endoglucanases, and pectin methylesterases. However, the promoting effects of exogenous auxin is difficult to apply for roots. It has long been explained by the lower threshold for root growth that has already be supplied by the endogenous auxin. Recent researches in root biology discloses that root growth is finely tuned by the Polar Auxin Transport (PAT) which maintains the optimum auxin in different root zones in order to keep its contrast functions, e.g. cell division in meristem, cell expansion in elongation zone.

Polar auxin transport (PAT) is the key process to regulate plant growth and morphogensis. The long distance transport of PAT generally has two directions, basipetal (rootward) and acropetal (shootward). Auxin maxima in the quiescent center origins from the basipetal auxin transport from the shoot and the auxin biosynthesis in the root. The acropetal transport of auxin from the quiescent center to elongation zone is the highlight of root growth regulation under abiotic stresses and tropism. We reviewed in this paper that the PAT controls plant root growth via maintaining apoplast alkalization in root apical part and acidification in root distal part. The abotic and biotic stresses disturbs the PAT and auxin distribution patterns in different root zones, therefore inhibits root growth.

Figure 1 shows our recent research on PAT which is afftected by B deficiency and Al toxicity. Both Al toxicity and B deficiency inhihits root growth via blockage of the root acropetal PAT that results in the disturbance of apoplast alkalization in transition zone and acidification in elongation zone.