Agriculture Reference
In-Depth Information
prematurely differentiate until total inhibition of cell elongation and loss of meristematic
activity occur (meristem exhaustion). At the end, root tips change their physiological charac‐
teristics and the exhausted meristem becomes a structure which takes part in P uptake. In this
process, root tips locally detect P deficiency, this response being mediated by at least LPR
multicopper oxidase genes [12, 131, 132]. Recently, iron (Fe) has been reported to play a role
as well in the control of these PED reprogramming [133]. This change of root architecture is
due to the fact that, in both meristematic and elongation areas, the content of ROS is reduced
as long as the determined PED goes on [134].
In the past decade the changes in RSA evocated by P availability has been widely studied,
several genes that regulates the root architectural changes has been idetified, trascription factor
such as WRKY75, ZAT6 (ZINC FINGER 6), Pi-responsive R2R3 MYB (MYB62) and BHLH32
(BASIC HELIX_LOOP_HELIX 32) [135-138] are key regulators in this response. Mutants
affected in the RSA changes induced P availability have been isolated:
pdr2
(
phosphate deficiency
response 2
),
lpi
(low phosphorus-insensitive)
siz1
[
S
AP (scaffold attachment factor, acinus,
protein inhibitor of activated signal transducer and activator of transcription) and
Miz1
(Msx2-
interacting zinc finger), SIZ] [139-141]. It has been reported that ethylene is involved in
modulating Pi-starvation-responsive root growth, it may restrict elongation of PR, but promote
elongation of LRs [142] HPS4/SABRE (important regulator of cell expansion in Arabidopsis)
antagonistically interacts with ethylene signalling to regulate plant responses to Pi starvation.
Furthermore, it is shown that Pi-starved
hps4
mutants accumulate more auxin in their root tips
than the wild type, which may explain the increased inhibition of their primary root growth
when grown under Pi deficiency [143]. Gibberellins and ROS also trigger responses involving
DELLAs proteins which control the rate and timing of cell proliferation and they will be dealt
with in further sections.
4.2.2. Nitrogen
N is fundamental for biological molecules, such as nucleotides, amino acids, and proteins. Plants
need to acquire nitrogen (N) efficiently from the soil for growth and development. In soil, nitrate
(NO
3−
) is one of the major N sources for higher plant and their concentrations vary in both time
and space. Plants are able to sensing these variations of NO
3−
, which is one of the most impor‐
tant environmental signals affecting plant physiology and development [144]. The effects of N
supply on plant development have been particularly studied in Arabidopsis. NO
3−
-free medium
drastically reduces shoot biomass production and appears to have little effect on PR length
(Figure 2). However, NO
3−
has a dual role on LRs. On one hand, the uniform exposure of RS to
high nitrate (>10 mM) inhibits lateral root growth at a specific developmental step correspond‐
ing to the activation of the meristem in LRP after their emergence[145-147]. As a high NO
3−
supply
on only one part of the RS is able to repress lateral root growth on the whole RS, it has been
proposed that nitrate accumulation in the aerial tissues is responsible for this LRP arrest,
suggesting that long-distance signals to the root are involved. On the other hand, when the entire
RS is exposed to low nitrate concentration (10 μM) and only one part of the RS is exposed to a
high nitrate, there is local proliferation of LR. NO
3−
locally promotes LR growth and increased
lateral root growth rate due to a higher cell production in the lateral root meristem [145, 146, 148].
The local stimulation of lateral root growth by nitrate-rich patches is a striking example of the
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