To cope with growth in low-phosphate (Pi) soils, plants have evolved adaptive responses that involve both developmental and metabolic changes. PHOSPHATE STARVATION RESPONSE 1 (PHR1) and related transcription factors play a central role in the control of Pi starvation responses (PSRs). How Pi levels control PHR1 activity, and thus PSRs, remains to be elucidated. Here, we identify a direct Pi-dependent inhibitor of PHR1 in Arabidopsis, SPX1, a nuclear protein that shares the SPX domain with yeast Pi sensors and with several Pi starvation signaling proteins from plants. Double mutation of SPX1 and of a related gene, SPX2, resulted in molecular and physiological changes indicative of increased PHR1 activity in plants grown in Pi-sufficient conditions or after Pi refeeding of Pi-starved plants but had only a limited effect on PHR1 activity in Pi-starved plants. These data indicate that SPX1 and SPX2 have a cellular Pi-dependent inhibitory effect on PHR1. Coimmunoprecipitation assays showed that the SPX1/PHR1 interaction in planta is highly Pi-dependent. DNA-binding and pull-down assays with bacterially expressed, affinity-purified tagged SPX1 and ΔPHR1 proteins showed that SPX1 is a competitive inhibitor of PHR1 binding to its recognition sequence, and that its efficiency is highly dependent on the presence of Pi or phosphite, a nonmetabolizable Pi analog that can repress PSRs. The relative strength of the SPX1/PHR1 interaction is thus directly influenced by Pi, providing a link between Pi perception and signaling. Significance When P levels are low, plants activate an array of adaptive responses to increase efficient acquisition and use of phosphate (Pi), the form in which P is preferentially absorbed, and to protect themselves from Pi starvation stress. Considerable progress has been made recently in dissecting the plant Pi starvation signaling pathway. Nonetheless, little is known as to how Pi levels are perceived by plants. Here, we identify the nuclear protein SPX1 as a Pi-dependent inhibitor of DNA binding by PHOSPHATE STARVATION RESPONSE 1 (PHR1), a master regulator of Pi starvation responses. We show that the Pi dependence of SPX1 inhibition of PHR1 activity can be recreated in vitro using purified proteins, which indicates that the SPX1/PHR1 module links Pi sensing and signaling.

Based on their evolutionary origin, MADS box transcription factor genes have been divided into two classes, namely, type I and II. The plant-specific type II MIKC MADS box genes have been most intensively studied and shown to be key regulators of developmental processes, such as meristem identity, flowering time, and fruit and seed development. By contrast, very little is known about type I MADS domain transcription factors, and they have not attracted interest for a long time. A number of recent studies have now indicated a key regulatory role for type I MADS box factors in plant reproduction, in particular in specifying female gametophyte, embryo, and endosperm development. These analyses have also suggested that type I MADS box factors are decisive for setting reproductive boundaries between species.

Fruits result from complex biological processes that begin soon after fertilization. Among these processes are cell division and expansion, accumulation of secondary metabolites, and an increase in carbohydrate biosynthesis. Later fruit ripening is accomplished by chlorophyll degradation and cell wall lysis. Fruit maturation is an essential step to optimize seed dispersal, and is controlled by a complex network of transcription factors and genetic regulators that are strongly influenced by phytohormones. Abscisic acid (ABA) and ethylene are the major regulators of ripening and senescence in both dry and fleshy fruits, as demonstrated by numerous ripening-defective mutants, effects of exogenous hormone application, and transcriptome analyses. While ethylene is the best characterized player in the final step of a fruit's life, ABA also has a key regulatory role, promoting ethylene production and acting as a stress-related hormone in response to drought and pathogen attack. In this review, we focus on the role of ABA and ethylene in relation to the interconnected biotic and abiotic phenomena that affect ripening and senescence. We integrate and discuss the most recent data available regarding these biological processes, which are crucial for post-harvest fruit conservation and for food safety.

In Arabidopsis thaliana and many other plant species, ovules arise from carpel tissue as new meristematic formations. Cell fate in proliferating ovule primordia is specified by particular ovule identity factors, such as the homeodomain factor BELL1 (BEL1) and MADS box family members SEEDSTICK (STK), SHATTERPROOF1 (SHP1), SHP2, and AGAMOUS. Both in the bel1 mutant and the stk shp1 shp2 triple mutant, integuments are transformed into carpelloid structures. Combining these mutants in a bel1 stk shp1 shp2 quadruple mutant, we showed that the bel1 phenotype is significantly enhanced. We also demonstrate that ovule differentiation requires the regulation of the stem cell maintenance gene WUSCHEL, repression of which is predominantly maintained by BEL1 during ovule development. Based on yeast three-hybrid assays and genetic data, we show that BEL1 interacts with the ovule identity MADS box factors when they dimerize with SEPALLATA proteins. We propose a model for ovule development that explains how the balance between carpel identity activity and ovule identity activity is established by a MADS box homeodomain protein complex.