Response of hrpZ_Psph-transgenic N. benthamiana plants under cadmium stress

The hrpZ_Psph gene from Pseudomonas syringae pv. phaseolicola, in its secretable form (SP/hrpZ_Psph), has previously proven capable of conferring resistance against rhizomania disease as well as abiotic stresses in Nicotiana benthamiana plants, while enhancing plant growth. This study aimed at investigating the response of SP/hrpZ_Psph-expressing plants under cadmium stress. Transgenic N. benthamiana lines, homozygous for the SP/hrpZ_Psph gene, and wild-type plants were exposed to Cd at different stress levels (0, 50, 100, 150 μΜ CdCl₂). Plants' response to stress was assessed at germination and at the whole plant level on the basis of physiological and growth parameters, including seed germination percentage, shoot and root length, total chlorophyll content, fresh and dry root weight, as well as overall symptomatology, and Cd content in leaves and roots. At germination phase, significant differences were noted in germination rates and post-germination growth among stress levels, with Cd effects being in most cases analogous to the level applied but also among plant categories. Although seedling growth was adversely affected in all plant categories, especially at high stress level, lines #6 and #9 showed the lowest decrease in root and shoot length over control. The superiority of these lines was further manifested at the whole plant level by the absence of stress-attributed symptoms and the low or zero reduction in chlorophyll content. Interestingly, a differential tissue-specific Cd accumulation pattern was observed in wt- and hrpZ_Psph-plants, with the former showing an increased Cd content in leaves and the latter retaining Cd in the roots. These data are discussed in the context of possible mechanisms underlying the hrpZ_Psph-based Cd stress resistance.