Objectives Nitrogen (N) and phosphorus (P) nutrient deficiency is an important factor inhibiting crop growth and development. Microalgal biofertilizer is a novel fertilizer consisting of single-cell photosynthetic algal biomass. An increasing attention is focused on the effects of microalgal biofertilizer in promoting crop growth and improving soil fertility. This study aimed to investigate the growth-promoting effect of microalgal biofertilizer on foxtail millet (Setaria italica L.), particularly, its function in enhancing foxtail millet tolerance to N and P deficiency stresses, providing a theoretical foundation for microalgal biofertilizer application in foxtail millet and other crop production for reducing chemical fertilizer usage and promoting green sustainability.

Methods A hydroponic method was used to cultivate millet seedlings of Jingu 21 variety. The N-P treatment included normal level of N and P in Hoagland nutrient solution (N 210 mg/L and P 71 mg/L, respectively), 1/2 level of N and P in the nutrient solution (N 105 mg/L and P 35.5 mg/L, respectively), and none of N and P addition in the nutrient solution. Three microalgal biofertilizers, namely Chlorella sp. DT01 (DT), Chlorella emersonii (AS), and Chlorella pyrenoidosa (DB), were prepared, and then combined with the three N-P treatments at 1∶1 ratio, respectively, totally composing nine treatments. Microalgal biofertilizer-free control was set for each N-P treatment. The millet seedlings at age of two-open leaves and one-sprout were transferred to these 12 treatment solutions, and cultivated in a greenhouse for 9 days, with the treatment solutions replenished every 3 days. At the end of the treatments, the seedlings were harvested for measurement of seedling growth, biomass, pigment content, photosynthetic activity, N and P contents, as well as the expression profiles of nitrate transporter genes (SiNRT1.11 and SiNRT2.1) and phosphate transporter genes (SiPHT1.2 and SiPHT1.1).

Results Compared with normal N-P treatment, the NP-free and 1/2 NP treatments significantly reduced biomass, pigment (chlorophyll a, b and carotenoid) content and photosynthesis of foxtail millet seedlings. Under all the three N-P conditions, application of each of the three microalgal biofertilizers significantly increased the fresh weight and dry weight, pigment content (especially carotenoids) and three photosynthetic parameters (actual photochemical efficiency Y(II), photochemical quenching coefficient qP and non-photochemical quenching coefficient NPQ) of millet seedlings. Furthermore, the microalgae biofertilizer application largely enhanced the total N and P contents in millet seedlings. Of the three microalgal biofertilizers, DT demonstrated the most pronounced growth-promoting effect. Additionally, qRT-PCR analysis of N or P transportation related genes revealed that DT treatment significantly up-regulated the expression of nitrate transporter gene SiNRT1.11 in the shoots and the nitrate transporter gene SiNRT2.1 in the roots of foxtail millet seedlings, as well as phosphate transporter genes SiPHT1.2 and SiPHT1.1 in root tissues of millet.

Conclusions Application of the three microalgal biofertilizers, especially DT, could significantly up-regulate the expression of nitrate transporter gene SiNRT1.11 in the shoots and the nitrate transporter gene SiNRT2.1 in the roots, and phosphate transporter genes SiPHT1.2 and SiPHT1.1 in roots under N and P deficiency, thus promote foxtail millet seedling growth, photosynthetic efficiency, and resistance to nitrogen and phosphorus deficiency.