Abscisic Acid Transport and Homeostasis in the Context of Stomatal Regulation

The discovery of cytosolic ABA receptors is an important breakthrough in stomatal research; signaling via these receptors is involved in determining the basal stomatal conductance and stomatal responsiveness. However, the source of ABA in guard cells is still not fully understood. The level of ABA increases in guard cells by de novo synthesis, recycling from inactive conjugates via β-glucosidases BG1 and BG2 and by import, whereas it decreases by hydroxylation, conjugation, and export. ABA importers include the NRT1/PTR family protein AIT1, ATP-binding cassette protein ABCG40, and possibly ABCG22, whereas the DTX family member DTX50 and ABCG25 function as ABA exporters. Here, we review the proteins involved in ABA transport and homeostasis and their physiological role in stomatal regulation. Recent experiments suggest that functional redundancy probably exists among ABA transporters between vascu- lature and guard cells and ABA recycling proteins, as stomatal functioning remained intact in abcg22, abcg25, abcg40, aitl, and bglbg2 mutants. Only the initial response to reduced air humidity was significantly delayed in abcg22. Considering the reports showing autonomous ABA synthesis in guard cells, we discuss that rapid stomatal responses to atmospheric factors might depend primarily on guard cellsynthesized ABA, whereas in the case of long-term soil water deficit, ABA synthesized in the vasculature might have a significant role.

Different hydraulic strategies under drought stress between Fraxinus mandshurica and Larix gmelinii seedlings

Persistent and severe drought induced by global climate change causes tree mortality mainly due to the hydraulic imbalance of conduit systems,but the magnitude of injury may be species dependent.A water-exclusion experiment was carried out on seedlings of two tree species with distinct characteristics,i.e.,Fraxinus mandshurica and Larix gmelinii to examine hydraulic responses of leaf,stem,and root to drought stress.The two species displayed different hydraulic strategies and related traits in response to drought stress.L.gmelinii reduced its leaf hydraulic conductance by quick stomatal closure and a slow decline in leaf water potential,with a more isohydric stomatal regulation to maintain its water status.In contrast,F.mandshurica was more anisohydric with a negative stomatal safety margin,exhibiting strong resistance to embolism in stem and leafstem segmentation of hydraulic vulnerability to preserve the hydraulic integrity of stem.These differences in hydraulic behaviors and traits between the two species in response to drought stress provide a potential mechanism for their coexistence in temperate forests,including which in the forest modeling would improve our prediction of tree growth and distribution under future climate change.

Molecular mechanisms of salinity tolerance in rice

Salinity is one of the major abiotic stresses which impose constraints to plant growth and production.Rice(Oryza sativa L.)is one of the most important staple food crops and a model monocot plant.Its production is expanding into regions that are affected by soil salinity,requiring cultivars more tolerant to saline conditions.Understanding the molecular mechanisms of such tolerance could lay a foundation for varietal improvement of salt tolerance in rice.In spite of extensive studies exploring the mechanism of salt tolerance,there has been limited progress in breeding for increased salinity tolerance.In this review,we summarize the information about the major molecular mechanisms underlying salinity tolerance in rice and further discuss the limitations in breeding for salinity tolerance.We show that numerous gene families and interaction networks are involved in the regulation of rice responses to salinity,prompting a need for a comprehensive functional analysis.We also show that most studies are based on whole-plant level analyses with only a few reports focused on tissue-and/or cell-specific gene expression.More details of salt-responsive channel and transporter activities at tissue-and cell-specific level still need to be documented before these traits can be incorporated into elite rice germplasm.Thus,future studies should focus on diversity of available genetic resources and,particular,wild rice relatives,to reincorporate salinity tolerance traits lost during domestication.