Plants unlike other living forms are sessile thereby facing severe biotic and abiotic
stresses. Plants have evolved different efficient defence responses which thrive upon
a number of intrinsic factors, such as genotypic and phenotypic constitutions and
developmental circumstances, and extrinsic factors like severity and duration of the
stresses. Stress management uses molecular and biochemical level controls, the competence,
and speed, at which a stress signal is perceived and transmitted to generate stress
signal molecules and activate stress-protective mechanisms. A well-concerted action
of the plants' competence at morphological, physiological, biochemical, and molecular
strata regulates numerous adaptive responses to biotic and abiotic stresses. Genetic
manipulations of signalling networks have been widely used to improve plant productivity
under stressful conditions. Advanced biotechnological application will enable maintaining
agriculture in a sustainable manner. In this special issue, we present two reviews
and four research papers which address genomic, molecular, and physiological regulations
as well as signalling networks dealing with plant responses to abiotic and biotic
factors.
Climate change, desertification, and the rise in human population have put a severe
load on agriculture and are deteriorating crop productivity. In recent years, numerous
molecular and metabolic pathways involved in plant responses and adaptation to various
types of environmental stresses have been identified and reported. Among hundreds
of metabolic pathways identified, the role of polyamines in stress management to enhance
plant acclimation and adaptation is emerging rapidly. In this special issue, a timely
review by P. Rangan et al. (2014) summarizes our knowledge on biosynthesis and catabolism
of polyamines and highlights recent progress in elucidating the functions of polyamines
in regulation of plant responses to abiotic stresses. Given a huge genetic variation
among plant species, the authors also discuss a systematic approach based on polyamine-mediated
enhancement of stress tolerance which might be used as a potential strategy for screening
and identification of natural variants within existing crop species. The identified
genotypes that possess compatible allelic variants could be then used for the improvement
of stress tolerance.
Plant-microbe interactions are at the core of symbiotic, parasitic, or mutualistic
plant-microbe relationships. These interactions have displayed a unique way of mutualistic
communications for a resource sharing. To shed light on the topic, a detailed review
of M. Libault (2014) explains the unique mechanisms in using legumes to interact with
bacteria. Leguminous plants have developed a mutualistic symbiotic relationship with
rhizobium (a type of soil bacteria). Upon bacterial infection, a new root organ called
nodule is developed that enables the leguminous plants to access a steady source of
nitrogen through the fixation and assimilation of the atmospheric N2 by the symbiotic
bacteria. In return, the bacteria also get benefit from the symbiotic plants that
provide photosynthesis product to bacteria as source of carbon. Environmental stress
or climate change, which influences the concentration of the atmospheric carbon dioxide
(CO2), will have a significant impact on plant photosynthesis. As a consequence, this
will affect the nitrogen and carbon metabolism, leading to altered nitrogen fixation
efficiency. The key regulatory mechanisms controlling carbon/nitrogen balances with
particular attention to legume nodulation are reviewed by coeditor M. Libault in his
review article. In addition, readers can also get an overview about the effect of
the change in CO2 level on nitrogen fixation efficiency through this review, giving
rise to idea as to how we could mitigate the impact of the change in atmospheric CO2
concentration.
Besides the change in atmospheric CO2 level, water deficit is one of the major constraints
for nodulation, and, as a consequence for plant productivity. This topical issue is
presented here by a research article of S. Sulieman et al. (2014). In their study,
the authors assessed the growth and nodulation attributes of two soybean varieties
DT2008 and Williams 82 (W82), which have contrasting drought-tolerant capacity, in
a symbiotic association with Bradyrhizobium japonicum under drought and subsequent
rehydration. The authors aimed to understand the correlation between N2 fixation efficiency
and differential drought-responsive phenotypes of DT2008 and W82. Their results also
provide genetic resources and basis foundation for further genomic research that would
lead to better understanding of mechanisms involved in regulation of N2 fixation in
soybean under drought at molecular level.
Heavy metal pollution has been a matter of grave concern. Until recently, efforts
have been mainly restricted to phytoremediation of soils using plant species with
high metal uptake capacity such as Brassica species. Chromium (Cr) is a highly phytotoxic
heavy metal affecting crop productivity and human health via entering the food chain.
Phytoremediation of Cr-polluted soils has been mostly demonstrated in using herbaceous
plants, whereas use of cotton cultivars in Cr phytoremediation is least explored.
In the present issue, M. K. Daud et al. (2014) have shown the potentials of two transgenic
cotton cultivars (J208 and Z905) and their hybrid line (ZD14) in Cr phytoremediation
using a multiple biomarker approach. Their work showed that these cotton cultivars
and hybrid line could effectively uptake and sequestrate Cr in dead parts of the plants,
such as vacuole and cell wall, besides having a more highly accelerated antioxidant
system. This study thus proposes a new role of cotton cultivars in phytoremediation
of Cr-polluted soils.
The interactions between macro- and micronutrient homeostases have been reported in
many physiological and nutritional situations. N. Bouain et al. (2014) studied the
interaction between phosphate (Pi) and zinc (Zn) homeostasis in two lettuce varieties.
The results revealed that the variation in Pi and Zn supplies affects the biomass
and photosynthesis as well as the dynamics of Pi transport in a contrasting manner
between the two varieties, indicating a genetic basis for such physiological responses.
On the basis of their results, the authors proposed a working model of how Pi and
Zn signalling pathways are integrated into functional networks to control plant growth.
Salinity is a major abiotic stress worldwide claiming agriculture lands and affecting
productivity. Research paper by A. Matsui et al. (2014) investigated salt stress in
Arabidopsis. The authors reported that the tasiRNA-ARF pathway is involved in controlling
floral architecture in plants under drought and high salinity. The ta-siRNA biosynthesis
mutants and mutated ARF3-overexpressing plants showed short stamen under drought and
salt stress, which hampered self-pollination. This study reports for the first time
that a type of ta-siRNAs (tasiRNA-ARF) plays an important role in maintaining normal
stamen growth and fertilization under drought and high salinity.