The potato PHOTOPERIOD RESPONSIVE 1 (PHOR1) protein was the first plant Arm protein to be associated with hormone signalling and plant growth regulation, and is a U-box/Arm protein (Fig. 1). PHOR1 is a positive regulator of gibberellin (GA) signalling. GA is a key regulator of plant growth and development, including of cell elongation both in shoots and roots. Activation of an intracellular GA receptor causes the proteasomal degradation of DELLA proteins via an SCF-E3 ubiquitin ligase, and thus derepression of cellular GA responses (Hartweck and Olszewski 2006).
GA induces stem elongation in wild-type potato plants; PHOR1 antisense plants have shorter stems than wild-type and are less sensitive to exogenous GA. Conversely, PHOR1 overexpressing plants have elongated stems and show an overactive GA response (Amador et al. 2001). Interestingly, GA changes the subcellular distribution of a PHOR1-GFP (green fluorescent protein) fusion protein. PHOR1-GFP is present in both the nucleus and the cytosol, but moves transiently to the nucleus in response to GA. Nuclear localisation of PHOR1 requires the Arm repeats, whilst the U-box drives cytosolic localisation (Amador et al. 2001). Given the recent discovery of a soluble GA receptor (Hartweck and Olszewski 2006), it is intriguing to speculate whether PHOR1 (like NtPUB4 (Sect. 2.1) and ARC1 (Sect. 4.1)) binds to as-yet un-characterised membrane-associated GA receptor(s), for which physiological evidence exists (Hartweck and Olszewski 2006). It is not yet known whether PHOR1 functions as an E3 ubiquitin ligase in vivo. Discovery of PHOR1 interaction partners will determine whether PHOR1 can degrade DELLA proteins or novel targets.
Potato tuberisation requires short day conditions, which also enhance PHOR1 mRNA levels. Paradoxically, plants with reduced PHOR1 activity show earlier tuberisation and increased tuber yield in short days, suggesting that PHOR1 inhibits short day-induced tuberisation (Amador et al. 2001).
Thus, tuberisation is likely to be a complex process in which PHOR1 plays a regulatory role, integrating environmental and hormonal signals.
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