Characterization of the mode of plant interactions with different environmental stress cues: drought and plant parasitic nematodes

Abstract

Plants are vulnerable to a variety of biotic and abiotic environmental stressors, which suppresses and limits the growth and yield potential of agricultural crops. Recently, drought has become one of most expensive abiotic stressors, needing tolerant cultivars as economically and environmentally sustainable option. However, it is not necessarily forthcoming; especially since most drought-responsive genes (DRGs) lead to stimulate stomata closures, and in turn repress plant carbon fixation and subsequent growth and development. Hence, several studies have exploited induced systemic tolerance (IST), activated by beneficial soil microbes plant growth-promoting rhizobacteria (PGPR), that promote both plant growth and defense capacity against a range of abiotic stresses including drought, excess salinity, and extreme temperatures. Recently, my research has identified two PGPR-response genes, Response to Desiccation (RD)29A and RD29B, that play essential roles in plant drought responses/tolerance, without trading off their growth and development. Here, we demonstrate that, using high-resolution RT-qPCR, RD29A and RD29B are positioned as upstream regulators, fine-tuning the temporal dynamics of stress-regulating transcription factor (TF) and their target DRG expressions in elaborating the IST activations. Indeed, the knock-out mutants of RD29A and RD29B (rd29a and rd29b) have shown enhanced susceptibility of drought stress compared to WT and confirmed these genes have played an important role in P. polymyxa induced IST. However, their modes of actions are considerably autonomous, exerting distinct roles in different ecological conditions, e.g., excess salinity. When subjected to increasing salts, T-DNA insertion KO mutant of RD29A (rd29a) displayed enhanced tolerance comparing to WT and rd29b plants, proposing that RD29A, but not RD29B, is a negative regulator of salinity responses, and further studies are needed to optimize their uses in engineering drought tolerant crops. On the other hand, a number of biotic sterssors, ii for instance, plant parasitic nematodes (PPN), cause significant yield losses in the agricultural industry, but yet failed to develop resistant cultivars because of little understanding on plant defense elements and mechanisms. In fact, the underlying modes, and mechanisms of PPN infections are still uncharacterized. For the last century, it has been proposed that chemotaxis is the primary means by which PPN locate host plant roots. However, questions remain what the identities and actions of chemoattractants are to deliver host-specific messages toward PPN. In the present study, a unique multidimensional agar-based motility assay was developed to assess the impacts of root exudates on the short-range motility and orientation of PPN. Three PPN (Rotylenchulus reniformis, Meloidogyne incognita and Heterodera glycines) and root exudates from their respective host and nonhost plants (cotton, soybean and peanut) were used to validate the assay. As predicted, R. reniformis and M. incognita were attracted to root exudates of cotton and soybean (hosts), but not to the exudates of peanut (nonhost). Likewise, H. glycines was attracted to soybean (host) root exudates. These results underpinned the intrinsic roles of root exudates in conveying the host specificity of PPN. In particular, PPN selectively identified and targeted to hydrophilic, but not hydrophobic, fractions of root exudates, indicating that groundwater should be an effective matrix for chemotaxis associated with PPN and their host plants.