Root form and function in plant as an adaptation to changing climate

Climate variables including temperature, atmosphere CO2 and precipitation are expected to change during this century. As consequence, in the short- and long-term, the increase of soil temperature, salinity, drought and waterlogging stresses could be the more exceeding problems for agricultural productivity and the functioning of the natural ecosystems. Root system represents the first and more sensitive target of the climate change, being seriously damaged in its form and function, and consequently strongly contributes to limit plant growth, development and crop productivity. This review focuses on changes of root morphology, architecture, distribution and dynamics, and on essential root physiological processes, such as water and nutrient uptake in response to soil warming, salinity, drought and waterlogging. The literature data appear sometimes to be controversial due to the complexity of root system characterized by different root types, genetically, developmentally and functionally distinct, and by diverse root morphological parameters such as total root length, biomass, specific root length (SRL) and tissue density and fineness differently involved on root stress responses. For example, the change on total root length and dry weight, the lateral root formation, the depth of rooting and the root dynamics represent the preferential strategy for plant species in water-limited environments. Whereas, the development of aerenchyma, tissue containing enlarged gas spaces with a low-resistance pathway to oxygen, often accompanied by the aerotropic and extensive lateral roots formation, the herringbone-type root architecture, the emergence of adventitious roots and the presence of anatomical barriers are expressed in flooded root. Salinity reduces plant growth and yield by two mechanisms, osmotic stress and ion cytotoxicity. It is difficult to separate the osmotic effect from specific ion effects that overlap during the development of salinity stress, thereby some uncertainty exists regarding the relative importance of both mechanisms. The responses of root cells are finalized to maintain their own correct functionality, despite of the condition of elevated Na+ concentration.The genetic diversity of the root system in the plant response to climate change was also reported. Behind the root “form” changes, many metabolic and physiological pathways are involved in the plant adaptation to climate change. The maintenance of lower respiration rate, carbohydrate metabolism and cell expansion and elongation, often mediated by hormones, are expressed in roots grown in dry soil. At molecular level, the deposition of proline, metabolite responsible of the total osmotic adjustments, the higher expansin and xyloglucan endotransglycolase/hydrolase (XTH) activities, enzymes of the cell wall extension, represent the physiological mechanisms implemented by plants for improving their drought tolerance. In waterlogging soils and also in presence of salinity, the adaptation physiological mechanisms are addressed to improve the cellular energy status and reduce the accumulation of toxic end products that acidify the cytosol or damage membrane integrity. Remarks on proteomic and molecular aspects which represent a future approach to individuate the plant strategies for their adaptation to the climate change are also included.