The B-box (BBX) proteins are zinc-finger transcription factors with a key role in growth and developmental regulatory networks mediated by light. AtBBX21 overexpressing (BBX21-OE) potato (Solanum tuberosum) plants, cultivated in optimal water conditions, have a higher photosynthesis rate and stomatal conductance without penalty in water use efficiency (WUE) and with a higher tuber yield. In this work, we cultivated potato plants in two water regimes: 100 and 35% field capacity of water restriction that imposed leaf water potentials between −0.3 and −1.2 MPa for vegetative and tuber growth during 14 or 28 days, respectively. We found that 42-day-old plants of BBX21-OE were more tolerant to water restriction with higher levels of chlorophylls and tuber yield than wild-type spunta (WT) plants. In addition, the BBX21-OE lines showed higher photosynthesis rates and WUE under water restriction during the morning. Mechanistically, we found that BBX21-OE lines were more tolerant to moderated drought by enhancing mesophyll conductance (g m ) and maximum capacity of electron transport (J max ), and by reducing abscisic acid (ABA) sensitivity in plant tissues. By RNA-seq analysis, we found 204 genes whose expression decreased by drought in WT plants and expressed independently of the water condition in BBX21-OE lines as SAP12, MYB73, EGYP1, TIP2-1 and DREB2A, and expressions were confirmed by quantitative polymerase chain reaction. These results suggest that BBX21 interplays with the ABA and growth signaling networks, improving the photosynthetic behavior in suboptimal water conditions with an increase in potato tuber yield.

Shade avoidance syndrome (SAS) is a strategy of major adaptive significance and typically includes elongation of the stem and petioles, leaf hyponasty, reduced branching, and phototropic orientation of the plant shoot toward canopy gaps. Cryptochrome 1 and phytochrome B (phyB) are both the major photoreceptors that sense the reduction in blue light fluence rate and the low Red:Far-Red (R:FR) ratio, respectively, both light signals associated with plant density and the resource reallocation when SAS responses are triggered. BBX24 has been implicated in the SAS as a regulator of DELLA activity, but this interaction does not explain all the observed BBX24-dependent regulation in shade light. Here, through a combination of transcriptional meta-analysis and large-scale identification of BBX24-interacting transcription factors, we found that JAZ3, a jasmonic acid signaling component, is a direct target of BBX24. Furthermore, we demonstrated that joint loss of BBX24 and JAZ3 function causes insensitivity to DELLA accumulation, and the defective shade-induced elongation in this mutant is rescued by loss of DELLA or phytochrome B functions. Therefore, we propose that JAZ3 is part of the regulatory network that controls the plant growth in response to shade, through a mechanism in which BBX24 and JAZ3 jointly regulate DELLA activity. Our results provide new insights into the participation of BBX24 and JA signaling in the hypocotyl shade avoidance response in Arabidopsis.