The fate of lateral root after their initiation: a tale with intrinsic variability, environmental plasticity and many signals at play
Roots have the challenging task of providing the nutrients required by the developing plant. Root development occurs in soil where the availability of nutrients is mostly heterogeneous in space and time. The constant interaction of processes, such as weathering, atmospheric deposition, nutrient leaching, and biological cycling, results in the formation of vertical and horizontal nutrient gradients within the soil (Fig. 1)(Giehl 2014).
Which root traits for efficient nutrient uptake? (York 2013) theoretical framework of classification of root phenes in base of their function, distinguing between exploration and exploitation, and cost. Complement with different possibilities for limitations to root growth in (Postma 2014).
The formation of LRs is a complex and highly coordinated process that depends on the interplay of hormones, where auxin acts as a central component. Indeed, the deficiency of many nutrients has been reported to affect the levels of one or more hormones in plants (for review, see Rubio et al., 2009)(Giehl 2014). The response is conditionated by both the perception of the nutrient status by the plant and locally by the root (Forde 2001).
There is considerable literature concerning the localized responses of root systems to nutrient patches. (citation not found: Drew1975) reported an increase in lateral root initiation in response to phosphate, nitrate or ammonium patches. These modifications include changes, such as root hair formation, and changes, such as the release of nutrient-mobilizing root exudates or the expression of nutrient transporters (Giehl 2014).
Different strategies may be required for acquisition of different resources. (Giehl 2014)
Auxin as promoter of cell division usutaining the activity of the PIN genes; cytokynin with an antagonist role promoter of cell differenciation at the root meristem TZ. Gibellerins, an additional regulator of root growth, selectively repress expression of ARR1 cytokinin-response transcription factor, allowing cell division to prevail over cell differentiation at early stages of meristem development, and the root meristem to build up. The onset of the cell elongation provoques a significant dilution of GA levels (a slow-moving phytohorme, in contrast to AUX moving from cell to cell at 1cm/h and for which the dilution effect is predicted to be negligible), resulting in an increase in DELLA proteins, and the reduction in cell elongation at the end of the elongation zone. (Perilli 2012, Band 2012) This model is consitent with the observation of a pick of GA in the TZ and a progressive dilution in the EZ in maize leafs (Nelissen 2012) Questions: which drives cell elongation? Role of ethylen? Ethylene regulated the maximal rate of relative elongation (citation not found: Ma2003a).
The length of the unbranched apical zone is correlated to the RER (citation not found: Various from Pagès&Draye2010). More finely, the distance between the root tip and the zone of root hair development. (citation not found: (Pagès&Draye2010) Pagès L, Serra V, Draye X, Doussan C, Pierret A. 2010. Estimating root elongation rates from morphological measurements of the root tip. Plant and Soil 328, 35–44.
Using (Band 2012) framework: Interestingly, the model results suggest that the RER does not correspond to the absolute DELLA levels but appears to reflect the fold change in DELLA as cells traverse the elongation zone (Fig. 3H).
The anatomical method for inferring growth rate patterns is laborious and time-consuming relative to marking experiments. However, in cases where the marking experiment would perturb the normal growth rate pattern, the anatomical methods would facilitate reasonable estimates of growth patterns (Silk 1989)