Root Development and Abiotic Stress Adaptation - Abiotic Stress: Plant Responses and Applications in Agriculture - page 151

for the same DNA-binding site of the DEL1 promoter and enhances the DEL1 expression,
respectively. Under light conditions, E2Fb is the preferred binding partner, enhancing DEL1
expression and consequently repressing the endoreplicative cycle [167]. In the dark E2Fb is
degraded, allowing E2Fc to bind to the DEL1 promoter, repressing DEL1 expression. Ultra‐
violet-B (UVB) radiation damages DNA molecules by forming cyclobutane pyrimidine dimers
(CPDs) which prevent DNA transcription and translation. Plants remove CPDs by photolyas‐
es, and these enzymes are encoded by a PHOTOLYASE 1 (PHR1) [168, 169]. It has been shown
that in addition to CCS52A2, a known target of DEL1, DEL1 represses the transcription of the
PHR1 gene and thereby coordinates DNA repair and endocycle triggering [167]. After UVB
treatment, DEL1 expression is strongly downregulated, permitting the upregulation of PHR1
and thus leaving the cell able to repair its DNA.
Environmental and nutrient availability condition changes affect root apical meristem
organization [170]. ROS and Reactive Nitrogen Species (RNS) have been reported to be rapidly
induced by several kinds of environmental stresses in a variety of plant species to regulate the
plant response to biotic and abiotic stresses. In particular, oxidative stress caused by drought
and salinity, has been proposed that ROS production is an obligatory element of the response
to induce an adequate acclimatization process [114]. Therefore, the degree of accumulation of
ROS is what determines whether it is a part of the signaling mechanism (low production) or
a harmful event (high production) to plants, making the control of production and degradation
of ROS the crucial element for plant resistance to stress [114, 171-173]. ROS is never completely
eliminated, as it plays an important role in signaling and growth regulation [174]; ROS
quenching inhibits the root growth [115], and overexpression in Arabidopsis of a peroxidase
localized mainly in the elongation zone stimulates root elongation [175]. This calls for redox
control of the cell cycle, which is possibly linked to A-type cyclins, shown to be differentially
expressed under oxidative stress in tobacco, resulting in cell cycle arrest [176]. It is also known
that low temperatures [177, 178], metals [179] and nutrient deficiency [180] induce the presence
of ROS and RNS in specific tissues. These forms of stress affect root morphology by reducing
primary root growth and promoting branching, but the mechanisms of the redox generation-
sensing are not well understood.
The typical response of the Arabidopsis radical system to low phosphorous (P) availability is
an example to illustrate how complex these processes are. A recent study showed that ROSs
are involved in the developmental adaptation of the RS to low P availability [181]. Rapidly
growing roots of plants within a normal P medium synthesize ROS in the elongation zone and
QC on the root, whereas seedlings within low P mediums showed a slow growth of the PR,
and the ROS normally found in the QC relocate to cortical and epidermal tissues. In a previous
study [131], it has been indicated that Arabidopsis plants under low P conditions show a
decreased number of cells in the root apical meristem, and it decreases until it is depleted. In
these roots, all root apical meristem cells differentiate and the QC is almost indistinguishable.
A possible cause of this response to P starvation could be the cell cycle arrest modulated by
ROS and CYCAs, but it is more complicated, as the response is also modulated by auxin [170,
182] and gibberellin-DELLA pathways [183]. Interestingly, DELLAs promote survival by
reducing the levels of ROS [184], suggesting a link between the gibberellin-DELLA cell cycle