Interactions between zinc (Zn) and phosphate (Pi) nutrition in plants have long been recognized, but little information is available on their molecular bases and biological significance. This work aimed at examining the effects of Zn deficiency on Pi accumulation in Arabidopsis thaliana and uncovering genes involved in the Zn–Pi synergy. Wild-type plants as well as mutants affected in Pi signalling and transport genes, namely the transcription factor PHR1, the E2-conjugase PHO2, and the Pi exporter PHO1, were examined. Zn deficiency caused an increase in shoot Pi content in the wild type as well as in the pho2 mutant, but not in the phr1 or pho1 mutants. This indicated that PHR1 and PHO1 participate in the coregulation of Zn and Pi homeostasis. Zn deprivation had a very limited effect on transcript levels of Pi-starvation-responsive genes such as AT4, IPS1, and microRNA399, or on of members of the high-affinity Pi transporter family PHT1. Interestingly, one of the PHO1 homologues, PHO1;H3, was upregulated in response to Zn deficiency. The expression pattern of PHO1 and PHO1;H3 were similar, both being expressed in cells of the root vascular cylinder and both localized to the Golgi when expressed transiently in tobacco cells. When grown in Zn-free medium, pho1;h3 mutant plants displayed higher Pi contents in the shoots than wild-type plants. This was, however, not observed in a pho1 pho1;h3 double mutant, suggesting that PHO1;H3 restricts root-to-shoot Pi transfer requiring PHO1 function for Pi homeostasis in response to Zn deficiency.

Summary: Phosphate (P_i) is a major limiting factor for plant growth. Plants respond to limiting P_i supplies by inducing a suite of adaptive responses comprising altered growth behaviour, enhanced P_i acquisition and reduced P_i demand that together define a distinct physiological state. In P_i‐starved plants, continued root growth is required for P_i acquisition from new sources, yet meristem activity consumes P_i. Therefore, we analysed the relationship between organ growth, phosphate starvation‐responsive (PSR) gene expression and P_i content in Arabidopsis thaliana under growth‐promoting or inhibitory conditions. Induction of PSR gene expression after transfer of plants to P_i‐depleted conditions quantitatively reflects prior levels of P_i acquisition, and hence is sensitive to the balance of P_i supply and demand. When plants are P_i‐starved, enhanced root or shoot growth exacerbates, whereas growth inhibition suppresses, P_i starvation responses, suggesting that the magnitude of organ growth activity specifies the level of P_i demand. Inhibition of cell‐cycle activity, but not of cell expansion or cell growth, reduces P_i starvation‐responsive gene expression. Thus, the level of cell‐cycle activity specifies the magnitude of P_i demand in P_i‐starved plants. We propose that cell‐cycle activity is the ultimate arbiter for P_i demand in growing organs, and that other factors that influence levels of PSR gene expression do so by affecting growth through modulation of meristem activity.

The responses of long-term growth of plants under elevated CO₂ have been studied extensively. Comparatively, the responses of plants to subambient CO₂ concentrations have not been well studied. This study aims to investigate the responses of the model C₃ plant, Arabidopsis thaliana, to low CO₂ at the molecular level. Results showed that low CO₂ dramatically decreased biomass productivity, together with delayed flowering and increased stomatal density. Furthermore, alteration of thylakoid stacking in both bundle sheath and mesophyll cells, upregulation of PEPC and PEPC-K together with altered expression of a number of regulators known involved in photosynthesis development were observed. These responses to low CO₂ are discussed with regard to the fitness of C₃ plants under low CO₂. This work also briefly discusses the relevance of the data to C₄ photosynthesis evolution.