Abscisic acid and ethylene in the control of nodule-specific response on drought in yellow lupine

The ability of symbiosis with Rhizobium bacteria is an extremely important trait of Lupinus luteus L. Its cultivation increases soil fertility and reduces the necessity of using chemical fertilizers, thus makes lupine an excellent proecological and agronomical species. Nevertheless, nodule functioning associated with nitrogen fixation can be strongly affected by soil drought. On the face of changing climatic conditions, the emphasis should be given to get the knowledge concerning the phytohormonal and molecular markers of early response on water deficit in soil. In this paper, we have shown that soil drought stress, by causing water loss in nodules and affecting their structure, evokes degradation of symbiosome and leads to hormonal and molecular changes. The stress factor modulated abscisic acid (ABA) and ethylene (ET) biosynthesis pathways. It was reflected by increased expression of LlZEP (ZEAXANTHIN EPOXIDASE) involved in ABA biosynthesis and strong accumulation of this phytohormone. At the same time, the mRNA of synthase (LlACS) and oxidase (LlACO) of 1-aminocyclopropane-1-carboxylic acid (ACC) were elevated, while ACC accumulated significantly. Moreover, drought stress negatively influenced the expression of LlBOP (BLADE-ON-PETIOLE) and LlLbI (LEGHEMOGLOBIN), previously pointed out as regulators of nodule functioning. Simultaneously the iron and nitrogen content drastically decreased in the stressed nodules. Presented novel information describing phytohormonal changes during drought stress directly in the nodules can help to elucidate the hormonal and molecular mechanisms that control plant response to water loss.